The Proterozoic in Canada 9781487595838

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THE PROTEROZOIC IN CANADA

THE ROYAL SOCIETY OF CANADA Special Publications 1. The Grenville Problem. Edited by JAMES E. THOMSON 2. The Proterozoic in Canada. Edited by JAMES E. GILL

The Proterosoic in Canada

THE ROYAL SOCIETY OF CANADA SPECIAL PUBLICATIONS, NO. 2 Edited by James E. Gill

PUBLISHED BY THE UNIVERSITY OF TORONTO PRESS IN CO-OPERATION WITH THE ROYAL SOCIETY OF CANADA

1957

COPYRIGHT ©, CANADA, 1957 PRINTED IN CANADA UNIVERSITY OF TORONTO PRESS LONDON: OXFORD UNIVERSITY PRESS SCHOLARLY REPRINT SERIES ISBN 0-8020-7015-9 LC 58-21139

PREFACE

"PROTEROZOIC" ROCK GROUPS include those Precambrian rocks that are least deformed and metamorphosed. Except for the absence of fossils in the sediments they differ little from younger rocks and geologists have felt considerable confidence in interpreting their origins and historical significance. Because of this and the occurrence in them of important economic deposits, they have been examined in great detail in some localities. The results of such studies and of others of a reconnaissance nature are summarized in the descriptive papers in this volume. Protcrozoic time produced many things of great interest to geologists and some of tremendous economic import to the people of Canada and the United States. Most of the iron deposits of the Lake Superior district, the Michigan copper deposits, the enormously important Sudbury nickel-copperplatinum deposits, the spectacular silver deposits of the Cobalt district, and all of the important uranium ores in the Shield area are considered to have formed during that time. In the west, the great Sullivan zinc-lead-silver deposit occurs in rocks classed as Proterozoic and may have formed during that era. The list includes many of our largest ore deposits and some others may well have formed during Proterozoic time. The advance of general knowledge and especially the great mass of new information bearing on the operation of geologic processes have resulted in the questioning of old interpretations of Precambrian history and the terminology that grew out of them. As is usual in such situations, various suggestions for revision have been made and a considerable amount of stimulating discussion has resulted. The Grenville symposium and this one on the Proterozoic were arranged with the idea of bringing together the views of active workers in the Precambrian in the hope that suggestions of enduring value might accrue. Each contributor of a descriptive paper in this symposium has described the rocks which he regards as "Proterozoic." Since the coverage of Precambrian areas in Canada is fairly complete, these papers together give a fair view of the meanings attached to the term. Three papers deal with problems that arise from the current use of the term Proterozoic and with possible changes in its use to bring it more into accord with the facts. Whether or not any of the suggestions are adopted, nothing but good can come from such a comprehensive review and airing of problems. v

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PREFACE

The symposium was approved by the programme committee of Section IV and was organized by Dr. J. E. Hawley. In addition to those contributing directly, Dr. I. W. Jones assisted in arranging for the Quebec papers and J. F. Henderson and H. S. Bostock had a hand in the original planning. James E. GUI Editor

CONTENTS

Preface Proterozoic in Canada

v j. M. HARRISON, F.R.S.C., and K. E. EADE

Discussion of "Proterozoic in Canada"

3

j. T. WILSON, F.R.S.C.

10

ALICE E. WILSON, F.R.S.C.

18

Dating the Proterozoic of Canada R. M. FARQUHAR and R. D. RUSSELL

28

Life in the Proterozoic

THE CANADIAN SHIELD Proterozoic Rocks of the Southern Part of the Canadian Shield: Summary JAMES E. THOMSON, F.R.S.C.

33

Proterozoic Rocks of Northwestern Quebec and Larder Lake, Ontario JAMES E. THOMSON, F.R.S.C.

38

The Proterozoic of the Cobalt Area

ROBERT THOMSON

40

The Proterozoic of the Matachewan-Wanapitei-Temagami Area JAMES E. THOMSON, F.R.S.C.

46

The Questionable Proterozoic Rocks of the Sudbury-Espanola Area JAMES E. THOMSON, F.R.S.C.

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Stratigraphy, Quirke Lake-Elliot Lake Sector, Blind River Area, Ontario s. M. ROSCOE

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The North Shore of Lake Huron from Gladstone to Spragge Townships E. M. ABRAHAM

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The Proterozoic of the Original Huronian

JAMES E. THOMSON, F.R.S.C.

The Proterozoic of the Mamainse Point Area

63

JAMES E. THOMSON, F.R.S.C.

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The Proterozoic of the Port Arthur and Lake Nipigon Regions, Ontario w. w. MOORHOUSE

67

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CONTENTS

Questionable Proterozoic Rocks of Manitoba G. H. CHARLEWOOD and J. F. DAVTES

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Proterozoic Rocks of the Northwest Territories and Saskatchewan i. c. BROWN and G. M. WRIGHT

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The Proterozoic Stratigraphy of the Canadian Arctic Archipelago and Northwestern Greenland R. G. BLACKADAR

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Proterozoic Rocks of the Northern Part of the Labrador Geosyncline, the Cape Smith Belt, and the Richmond Gulf Area ROBERT BERGERON

101

Geology of Certain Proterozoic Rocks in Quebec and Labrador w. F. FAHRIG

112

Late Precambrian Rocks of the North Shore of the St. Lawrence River and of the Mistassini and Otish Mountains Areas, Quebec ROBERT BERGERON

124

The Grenville Province

132

D. F. HEWITT, F.R.S.C.

The Proterozoic of Eastern Canadian Appalachia L. J. WEEKS, F.R.S.C.

141

The Proterozoic of the Cordillera in Southeastern British Columbia and Southwestern Alberta j. E. REESOR

150

Possible Proterozoic Occurrences in British Columbia, the Yukon and Northwest Territories H. c. GUNNING, F.R.S.C.

178

Summary and Discussion

j. E. GILL, F.R.S.C. 183

CONTRIBUTORS

E. M. ABRAHAM, Ontario Department of Mines, Toronto, Ont. ROBERT BERGERON, Quebec Department of Mines, Québec, Que. R. G. BLACKADAR, Geological Survey of Canada, Ottawa, Ont. i. c. BROWN, Geological Survey of Canada, Ottawa, Ont. G. H. CHARLEWOOD, formerly with the Manitoba Mines Branch, Department of Mines and Natural Resources, Winnipeg, Man., now with Heath & Sherwood Drilling, Kirkland Lake, Ont. j. F. DAVIES, Manitoba Mines Branch, Department of Mines and Natural Resources, Winnipeg, Man. K. E. BADE, Geological Survey of Canada, Ottawa, Ont. w. F. FAHRIG, Geological Survey of Canada, Ottawa, Ont. R. M. FARQUHAR, Department of Physics, University of Toronto, Toronto, Ont. JAMES E. GILL, McGill University, Montreal, Que. H. c. GUNNING, University of British Columbia, Vancouver, B.C. j. M. HARRISON, Geological Survey of Canada, Ottawa, Ont. D. F. HEWITT, Ontario Department of Mines, Toronto, Ont. w. w. MOORHOUSE, University of Toronto, Toronto, Ont. j. E. REESOR, Geological Survey of Canada, Ottawa, Ont. s. M. ROSCOE, Geological Survey of Canada, Ottawa, Ont. R. D. RUSSELL, Department of Physics, University of Toronto, Toronto, Ont. ix

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CONTRIBUTORS

JAMES E. THOMSON, Ontario Department of Mines, Toronto, Ont. ROBERT THOMSON, Ontario Department of Mines, Cobalt, Ont. L. j. WEEKS, Geological Survey of Canada, Ottawa, Ont. ALICE E. WILSON, retired from the Geological Survey of Canada, Ottawa, Ont. j. T. WILSON, University of Toronto, Toronto, Ont. G. M. WRIGHT, Geological Survey of Canada, Ottawa, Ont.

THE PROTEROZOIC IN CANADA

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PROTEROZOIC IN CANADA* J. M. Harrison, F.R.S.C., and K. E. Bade IN RECENT YEARS, several geologists have been arguing that the term Proterozoic is at least needless and that it may actually be an impediment to understanding the Precambrian. This paper is an attempt to learn whether there is any justification for using the term, and if so, to what rocks it refers. In 1934 the Committee on Stratigraphie Nomenclature, established by the Royal Society of Canada, issued its report through the chairman, F. J. Alcock ( 1 ). The committee stated that "it was proposed to define the younger Precambrian era as beginning with the commencement of the Bruce Scries. Decisions in other fields as to which of the two Precambrian eras a particular formation belongs will have to be made by the individual worker." This is an ideal, of course, and was based on the assumption that Collins' work in the Lake Huron-Sudbury districts was sound. Actually, it seems that the fundamental idea behind the division was broader—that in many areas of Precambrian rocks two assemblages are separated by a profound unconformity. The older assemblage is highly deformed and complexly intruded, whereas the younger, Proterozoic, is usually much less altered and commonly dips at low angles over wide areas. Thus, although it is not explicitly stated, the Proterozoic units depend for their definition on their relationship to underlying rocks, the Archean. There is, of course, a refinement to this concept when it is applied to Precambrian rocks outside the Canadian Shield. In the Appalachians and Cordillera, the Proterozoic rocks are those that underlie the Cambrian, commonly with little or no evidence of unconformable relations (7, 28, 29, 38, 39). It is just possible that the Harbour Main volcanic rocks of Newfoundland, which are unconformably beneath the younger Proterozoic (38), are Archean in age, but, if so, these are the only Archean rocks in contact with Proterozoic in the Canadian Appalachians and Cordillera. The Archean had also been known as the Archeozoic, or time of ancient life, so the fact that the "zoic" ending was dropped at least implies that the committee did not consider the evidence of life from these rocks particularly impressive. That the younger era was called Proterozoic, or time of first life, correspondingly implies that the committee considered that the evidence from the rocks indicated life did exist when they were deposited. •Published by permission of the Acting Deputy Minister, Department of Mines and Technical Surveys, Ottawa. 3

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The definition of Proterozoic rocks according to this concept, then, appears to be about as follows: "Those Precambrian rocks that contain evidence of life and which are separated from the underlying rocks by a profound unconformity, or which are assumed to have formed in late Precambrian time because of their relations to Paleozoic rocks." However, we doubt that the evidence of life has ever been a serious consideration in classifying the rocks, partly because many so-called Proterozoic rocks are of a type in which fossiliferous material would be rare at best, but mainly because there is still controversy on what constitutes evidence of life. Nevertheless, the Proterozoic was assigned a connotation of time. This was implicit in the definition of the Alcock committee and, in fact, Proterozoic time has commonly been considered to have begun 1,000 million years ago. Just how this figure came to be chosen is not clear, but it is probably based on the fact that Paleozoic time began about 500 million years ago and Proterozoic, for a first approximation, was assumed to have lasted as long as all subsequent time. Another line of reasoning that gives an age of not less than 1,000 million years for the beginning of Proterozoic conies from paleontological concepts. It has been stated that it took at least as long for life to evolve to the point exemplified by Olenellus as it did to evolve from Olenellus to man. If this is a valid hypothesis, then the Proterozoic began not less than 1,000 million years ago, and possibly began much earlier. Evidence that it did begin much earlier is provided by radioactivity dating. For many years the origin of collenia, or "cup-and-ball" structures so characteristic of Precambrian limestones associated with iron-formation, has been disputed. In recent years evidence for organic origin has been multiplying (7) and the data presented by Tyler and Barghoorn (37) are powerful evidence for organic origin of such structures in the Gunflint iron-formation of western Ontario. Similar structures have been reported from other so-called Proterozoic rocks nearly everywhere in the Canadian Shield, including the Ungava iron ranges. Veins cutting the Ungava rocks contain galena on which determinations made in the laboratories of the Geological Survey gave ages based on Pb 206/204 ranging from 1,630 million years to 2,320 million years for five different veins. The average of these is about 1,900 million years, which should be a first approximation of the age of the veins and indicates an age of 2,000 million years or more for the rocks. Not only was life far enough advanced to leave identifiable remains in both iron-formation and dolomite of Ungava, it was sufficiently abundant to leave shaly beds so rich in carbon that the material has been used for fuel in camp stoves. Tests run at the Mines Branch, Department of Mines and Technical Surveys, on grab samples of this rock indicated 5,250 B.T.U. per pound, or roughly the same as lignite. Thus, it has taken 1.500 million years for life to evolve from a primitive, but still fairly complex, organism to Olenellus. And according to Holmes (20), rocks containing algal structures in Africa are more than 2,650 million years old.

PROTEROZOIC IN CANADA

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Thus the beginnings of life go back a very long way in the earth's history, and the term "Proterozoic," on the basis of life, may apply to more than 2,000 million years. A couple of years ago this age would have taken us back to the dawn of earth history, but now we are told the earth is 4,800 million years old. Parenthetically, it may be observed that the age of the earth is increasing at such a rate it seems as if the theory of relativity applies. At any rate, there still seems to be room for Archean time. Some years ago, Gill (11, p. 25) pointed out that there seemed to be important differences between rocks called Archean and those called Proterozoic. The Archean consist mainly of lavas and greywacke-conglomerate sedimentary rocks, whereas the Proterozoic contain abundant clean quartzites and, commonly, limestones. On this basis, and on that of radioactivity datings, the Grenville was assigned to Proterozoic. Probably also the Green Head rocks of New Brunswick, which are very similar to Grenville and which contain Collenia-typc structures, should also be classified as Proterozoic rather than Archean (38). The aggregate thickness of such rocks in the southern Cordillera is more than 50,000 feet (28) and, according to age determinations on the Sullivan ores, they are over 1,100 million years old. The differences between these Proterozoic successions and the Archean were considered by Gill to be significant and he suggested (13) that it was not until certain conditions became established on the earth that the clean sands and limestones could form. More recently, he has concluded that what is called Proterozoic is simply a lithologie type, and therefore, as a term of stratigraphie significance, it is a misnomer and should be abandoned ( 15 ). At about the same time J. T. Wilson came to somewhat similar conclusions (40, 41, 42), and in his more recent papers has argued that many Proterozoic rocks form secondary mountains on the continental sides of the Archean or primary mountains. In other words, the terms apply equally well to any mountains of whatever age. This argument is, in essence, an extension of Gill's remarks. Both authors considered that the terms Archean and Proterozoic should be abandoned because "Archean" rocks of one area may actually be younger than "Proterozoic" of another. So far as lithology is concerned, there is, apparently, a distinction; but if we consider that the distinction is due to some effect dependent on the history of the earth, then it is significant from the viewpoint of time. This was at least implied in the papers Gill presented in 1948 and 1949 (13, 14) and perhaps the idea merits serious consideration. So far as radioactivity age determinations are available to us at the moment of writing, no Archean rocks have been shown to be younger than any Proterozoic, although far too few determinations have yet been made to be sure this will remain true. Perhaps some combination of tectonics, atmosphere, and temperature were needed before clean sands and limestones could be deposited in quantity and, concomitantly, before life could evolve (10, 33, 34, 35). Although the lithology of Proterozoic rocks is distinctive in some aspects, it is by no means absolute. The rocks of the Ungava belt contain the

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J. M. HARRISON & K. E. EADE

"typical" Proterozoic rocks, but they also contain a much thicker succession of volcanic material indistinguishable from Archean rocks. The basic lavas of the Coppermine group probably do not differ greatly from Archean, nor those in the belt that crosses the northern tip of Ungava and those in the Proterozoic successions of the Cordillera and Appalachians. The chief distinction of these lavas is that those of the Shield do lie with great unconformity on Archean rocks and that all are associated with rocks that contain evidence of life. The Missi series, in the Flin Flon district, lies with great unconformity on the Amisk lavas. It is characterized by roundstone conglomerates, contains pebbles of granite, greenstone, and so on and probably fits the concept of secondary deposits suggested by J. T. Wilson (42). However, the Missi is considered to be Archean, as well as its presumed correlative farther north, the Sickle series (18). One of the main arguments against the terms Archean and Proterozoic has been the belief that there arc, accordingly, only one or two major orogenies in Precambrian time. This concept has been criticized in one form or another by .several authors (1, 16, 25, 31, 36, 40, 43) of whom M. E. Wilson (43) appears to have been the first. However, we doubt if this is a valid criticism. It is true that several geologists so regarded the breakdown, including M. E. Wilson in later publications (44, 45), but many geologists considered that there could be several orogenies in each era. This has been tacitly assumed on several correlational charts, for example Lord's (26, p. 30), where Proterozoic groups make an impressive list of superimposed units. Gill has pointed out (11, 12) that truncation of belts of rocks by others indicates that the truncating belt is younger, a principle given a good illustration by Holmes for the Shield of India (21). If we apply this to the Proterozoic rocks of the Northwest Territories, some interesting results appear. Minerals from pegmatite dykes that are overlain by the rocks of the Proterozoic Great Slave group in the east arm of Great Slave Lake are about 1,850 years old, and the Great Slave rocks are apparently cut by mineralized faults about 1,700 million years old (3). Great Slave rocks, then, are more than 1,700 million years old and younger than about 1,850 million years, or in other words the Great Slave tectonic cycle lasted about 150 million years. In the past, the Snare rocks north of Great Slave Lake and the Great Slave group have been correlated, chiefly on the basis of lithology, but Snare rocks have been traced directly, or nearly so, into Echo Bay rocks of Great Bear Lake.1 Hence, they are considered to be of the same general age and are treated as a unit. The trend of this unit is north to northeast, considerably different from the Great Slave unit. The Snare-Echo Bay groups have been folded and intruded, the uranium minerals at Great Bear Lake accompanying the intrusion so that the mineralization more or less marks the closing stages of the orogeny that followed the end of sedimentation. Age determinations indicate the sedimentary rocks are older than 1,400 million years. It appears, therefore, that the Echo Bay-Snare rocks are 'See illustration for paper on "Proterozoic Rocks of the Northwest Territories and Saskatchewan," p. 82.

PROTEROZOIG IN CANADA

7

younger than the Great Slave rocks (at least 1,700 million years) for, by analogy with Great Slave rocks, it seems unlikely that approximately 300 million years elapsed from the time of deposition of Echo Bay rocks to their mineralization. The Bathurst group of sediments apparently underlie the Coppermine, but together they form a unit trending about west-northwest and apparently cutting off the belt of Snare—Echo Bay rocks. The rocks represented are platform types although the Coppermine in general is characterized by tremendous thicknesses of basic volcanic rocks. No age determinations are available for this group, but it is suggested that, as this belt of rocks cuts off the Snare-Echo Bay belt, the Coppermine-Bathurst are younger, and perhaps much younger. As a result of recent reconnaissance studies (46), we know that a major fault trending south-southeast along the Bathurst Inlet-Western River lineament cuts the Bathurst group and probably cuts the Coppermine. To the south, the extension of the fault is covered by undisturbed Dubawnt sandstone, and isolated patches of rocks identical with the Dubawnt lie in the valley of the fault well north of the Dubawnt exposures. It is suggested that these rocks are the youngest of the Protcrozoic that occur in the mainland of the Northwest Territories. It is interesting to note that the Dubawnt rocks heretofore have always been correlated with the Athabasca group. If our reasoning is correct, the Dubawnt group is much younger than 1,400 million years and the latest data on radioactivity ages indicate the Athabasca rocks of the north shore of the lake are nearly 2,000 million years old (9), although those to the south of the lake could be much younger. We fail to understand, therefore, why the classification of rocks as Proterozoic indicates that there have been only one or two orogenies in the Proterozoic "era." Nor can we agree that calling rocks Proterozoic implies they are the same age, any more than calling both Cambrian and Permian strata Paleozoic implies that they are the same age. Largely at the instigation of Harrison, the American Commission on Stratigraphie Nomenclature recommended that the terms Archean and Proterozoic be abandoned and that Early and Late be substituted in the relative sense (2). In a critical comment of the recommendation Stockwell (2) pointed out that Archean and Proterozoic were well-entrenched terms and that they meant essentially what was recommended. This was implied by M. E. Wilson many years ago (43) and was succinctly stated by Jolliffe (23) who indicated that Proterozoic was used by him in the sense of relatively late Precambrian. Furthermore, the use of these terms avoids awkward statements such as "early Early Precambrian" or "later Earlier Precambrian." Finally, we do not think it particularly serious if units have to be transposed from Proterozoic to Archean, or vice versa—our palaeontological friends are continually rearranging strata on the basis of better data and dating. In summary then, we consider that Proterozoic is a valid and useful term, even though it has been badly abused at times. We do not consider that it imposes any restrictions on erogenic cycles, or that it is simply a lithologie

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term, or that its use implies correlation in absolute age. Proterozoic rocks of the Canadian Shield are probably as old as 2,000 million years, based on present concepts of absolute age from ratios of radioactive isotopes, and as more data accumulate we should soon be able to assign the beginning of Proterozoic an absolute age. Lang ( 24 ) has already suggested that Proterozoic time began more than 1,800 million years ago, so we are simply extending the time a little, although, on the precepts suggested, Holmes would advance it much more. We strongly recommend that local names be used for local successions of rocks, as has been done everywhere in the Canadian Shield except in Ontario and adjacent parts of Quebec, but we think that the term "Proterozoic" and its companion "Archean" will continue to provide useful frames for reference. REFERENCES (1) ALCOCK, F. J. (1934). Report of the National Committee on Stratigraphical Nomenclature; Trans. Roy. Soc. Can., Ser. Ill, vol. 28, Sec. IV, pp. 113-121. (2) AMERICAN COMMISSION ON STRATIGRAPHIC NOMENCLATURE (1955). Kept. 3; in Am. Assoc. Pet. Geol., vol. 39, pp. 1859-1861. (3) COLI.INS, C. B., LANG, A. H., ROBINSON, S. C., and FARQUHAR, R. M. (1952). Age determinations for some uranium deposits in the Canadian Shield; Proc. Geol. Assoc. Can., vol. 5, pp. 15-41. (4) COLLINS, W. H. (1925). North shore of Lake Huron; Geol. Surv., Can., Mom. 143. (5) COOKE, H. C. (1947). The Canadian Shield; in Geol. and Econ. Mins. of Canada, 3rd edit., Geol. Surv., Can., Econ. Geol. Ser. no. 1, pp. 11-97. (6) GUMMING, G. L., WILSON, J. T., FARQUHAR, R. M., and RUSSELL, R. D. (1955). Some dates and subdivisions of the Canadian Shield; Proc. Geol. Assoc. Can.. vol. 7, pt. 2, pp. 27-79. (7) DOUGLAS, R. J. W. (in press). Proterozoic, eastern Cordillera; in Geol. and Econ. Mins. of Canada, 4th edit., Geol. Surv., Can., Econ. Geol. Ser. no. 1. (8) EADE, K. E. (1949). Sims Lake area, Labrador; private rept. (9) ECKELMANN, W. R., and KUI.P, J. L. (1956). Uranium-lead method of age determination, pt. 1; Lake Athabasca problem; Bull. Geol. Soc. Am., vol. 67, pp. 35-54. (10) FRITZ, MADELEINE A. (1949). Life before the Cambrian; Proc. Geol. Assoc. Can., vol. 2, pp. 37-42. (11) GILL, J. E. (1948). The Canadian Precambrian Shield; in Struct. Geol. Canadian Ore Dcps; Can. Inst. Min. Met., Anniv. vol., pp. 20-48. (12) (1949). Natural divisions of the Canadian Shield; Trans. Roy. Soc. Can., Ser. Ill, vol. 43, Sec. IV, pp. 61-69. (13) (1952). Mountain building in the Canadian pre-Cambrian Shield; Intl. Geol. Cong., Rept. 18th Sess., pt. 13, pp. 97-104. (14) (1952). Early history of the Canadian Precambrian Shield; Proc. Geol. Assoc. Can., vol. 5, pp. 57-68. (15) (1955). Precambrian nomenclature in Canada; Trans. Roy. Soc. Can., Ser. Ill, vol. 49, Sec. IV, pp. 25-29. (16) GROUT, F. F., SCHWARTZ, E. M., and THIEL, G. A. (1951). Precambrian stratigraphy of Minnesota; Bull. Geol. Soc. Am., vol. 62, pp. 1017-1078. (17) GUNNING, H. C., and AMBROSE, J. W. (1939). The Timiskaming-Keewatin problem in the Rouyn-Harricanaw region, northwestern Quebec; Trans. Roy. Soc. Can., Ser. Ill, vol. 33, Sec. IV, pp. 19-49. (18) HARRISON, J. M. (1951). Precambrian correlation and nomenclature, and problems of the Kisseynew gneisses, in Manitoba; Geol. Surv., Can., Bull. 20. (19) (in press). The Canadian Shield; in Geol. and Econ. Mins. of Canada, 4th edit., Geol. Surv., Can., Econ. Geol. Ser. no. 1. (20) HOLMES, A. (1946). Principles of Physical Geology; Thos. Nelson and Sons, New York. (21) (1955). Dating the Precambrian of peninsular India and Ceylon; Proc. Geol. Assoc. Can., vol. 7, pt. 2, pp. 81-106.

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(22) JOHNSTON, W. G. Q. (1954). Geology of the Tcmiskaming-Grenville contact southeast of Lake Temagami, Northern Ontario, Canada; Bull. Geol. Soc. Am., vol. 65, pp. 1047-1074. (23) JOLLIFFE, A. W. (1952). The northwestern part of the Canadian Shield; Intl. Geol. Cong., Kept. 18th Sess., pt. 13, pp. 141-149. (24) LANG, A. H. (1952). Canadian deposits of uranium and thorium; Geol. Surv., Can., Econ. Geol. Ser. no. 16, pp. 22-24. (25) LEITH, C. K., LUND, R. J., and LEITII, A. (1935). Precambrian rocks of the Lake Superior region; U.S. Geol. Surv., Prof. Paper 184. (26) LORD, C. S. (1951). Mineral industry of district of Mackenzie, Northwest Territories; Geol. Surv., Can., Mem. 261, pp. 30-38. (27) MARSDEN, R. W. (1955). Precambrian correlations in the Lake Superior region in Michigan, Wisconsin, and Minnesota; Proc. Geol. Assoc. Can., vol. 7, pt. 2, pp. 107-116. (28) MULLIGAN, R. (in press). Precambrian of southwest British Columbia; in Geol. and Econ. Mins. of Canada, 4th edit., Geol. Surv., Can., Econ. Geol. Ser. no. 1. (29) (in press). Precambrian of northern British Columbia and Yukon; in Geol. and Econ. Mins. of Canada, 4th edit., Geol. Surv., Can., Econ. Geol. Ser. no. 1. (30) NORMAN, G. W. H. (1940). Thrust faulting of Grenvillc gneisses northwestward against the Mistassini series of Mistassini Lake, Quebec; Jour. Geol., vol. 48, pp. 512-525. ( 3 1 ) PETTIJOHN, F. J. (1937). Early prc-Cambrian geology and correlational problems of northern subprovincc of the Lake Superior region; Bull. Geol. Soc. Am., vol. 48. pp. 153-202. (32) ( 1 9 4 3 ) . Archean sedimentation; Bull. Geol. Soc. Am., vol. 54, pp. 925-972. (33) R A N K A M A , K. (1955). Geological evidence of chemical composition of the Prccninbrinn atmosphere; Geol. Soc. Am., Spec. Paper bl>, pp. 651—664. (34) RUBEY. W. W. (1955). Development of the hydrosphere and atmosphere; Geo!. Soc. Am., Spec. Paper 62, pp. 631-650. (35) THODE, H. G., MACNAMARA, J., and FLEMING, W. H. (1953). Sulphur isotope fractionation in nature, and geological and biological time scales; Geochim. Cosmochim. Acta. vol. 3, pp. 235-243. (36) THOMSON, J. E. (1953). Problems of Precambrian stratigraphy west of Sudbury; Trans. Roy. Soc. Can., Ser. Ill, vol. 47, Sec. IV, pp. 61-70. (37) TYLER, S. A., and BARGHOORX, E. S. (1954). Occurrence of structurally preserved plants in pre-Cambrian rocks of the Canadian Shield; Science, vol. 119, pp. 606-608. (38) WEEKS. L. J. (in press). Precambrian in Appalachian region; in Geol. and Econ. Mins. of Canada, 4th edit., Geol. Surv., Can., Econ. Geol. Ser. no. 1. (39) WHEKLER. H. E. (1947). Base of the Cambrian system; Jour. Geol., vol. 55, pp. 153-159. (40) WILSON. J. Tuzo (1949). The origin of continents and Precambrian history; Trans. Roy. Soc. Can., Ser. III. vol. 43, Sec. IV, pp. 173-182. (41) (1952). Geochronology in Precambrian time; Trans. Am. Geophys. Union, vol. 5. pp. 196-203. (42) ( 1954). Development and structure of the crust; in The Earth as a Planet. vol. 2, pp. 138-214. (43) WILSON. M. E. (1918). Subprovincial limitations of pre-Cambrian nomenclature in the St. Lawrence basin; Jour. Geol., vol. 26, pp. 325-333. (44) (1939). The Canadian Shield; in Geologic der Erde, Geology of North America, Bd. 1, pp. 232-311. (45) (1941). Pre-Cambrian; in Geology, Fiftieth Anniv. vol., Geol. Soc. Am., pp. 271-305. (46) WRIGHT, G. M. (in press). Geological notes on eastern district of Mackenzie, Northwest Territories; Geol. Surv., Can., Paper.

DISCUSSION OF "PROTEROZOIC IN CANADA" J. T. Wilson, F.R.S.C. That was indeed a fair and sunlit earth which our predecessors, the first geologists, had presented to them for study. The uniform strata of the newer periods of our earth's history in their succession, well exposed, and following one another in due and regular order, everywhere contained abundant fossil remains which afforded a certain clue by which correlation could be made even in widely separated areas. We, their unfortunate successors, in pursuing our studies are obliged to descend into the deeper parts of the earth where the light begins to fail and when once we pass through that last grim portal into the drear pre-Cambrian world, we enter into what these earlier geologists regarded as a hopeless chaos. Here we lose the guiding thread of life, and the darkness deepens. At first we could dimly descry but the outlines of the vast indeterminate ruins of former worlds, but as our eyes become accustomed to the darkness these become somewhat more distinct and we recognize succession even in this ruined waste. [FRANK D. ADAMS (1909), p. 105.] DR. J. M. HARRISON has been so kind as to invite me to discuss his paper on "Proterozoic in Canada," and particularly the question of whether the word "Proterozoic" is still a useful one. I am grateful to him, especially since this has given me the opportunity to clarify my own knowledge. I should also like to thank R. M. Farquhar and M. R. Dence for their comments and assistance. In writing this paper I have consulted a few of the chief papers in which the present usages of the terms Proterozoic and Archean were fixed. From these, some of which I shall quote, it is clear that there was never any agreement among geologists as to what precisely the terms meant and which groups of rocks they included, but at least the men who established their usage all had a common philosophy about the manner of development of the earth. They thought that the continents had been permanent features of an earth which had cooled rapidly. It is important to recognize that philosophy; to see to what extent it has been changed and to discover whether the changes invalidate ideas once held about the Proterozoic and Archean. The first attempts to give the geological record absolute dates in numbers of years were based upon denudational methods which have been reviewed by Jeffreys (1952) and Zuener ( 1952). Although not precise, these methods suggested that the earth was at least several hundreds of millions of years old. The development of the theory of evolution led to fresh estimates which also supported the idea of an old earth. Towards the end of the last century 10

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and just before the discovery of the existence of radioctivity, Helmholtz and Kelvin pointed out that all the sources of energy then known would not suffice to keep the sun and the earth hot for long. Kelvin considered that the earth had had a hot origin only a few tens of millions of years ago and that it had been cooling rapidly. His views were widely publicized. It was in 1896, while Kelvin was working on this problem, that radioactivity was discovered, but a few years elapsed before it was realized that radioactivity could provide enough heat to keep the earth warm and before the first age determinations made in 1906 confirmed the fact that some rocks had been formed many hundreds of millions of years ago. This completely invalidated Kelvin's arguments, but in the meantime many geologists who doubted Kelvin's verdict had been influenced by his opinions. By an unfortunate coincidence it was during this short period of controversy, when it was widely held that the earth was cooling rapidly, that our present time scale was fixed. For it seems fairly certain that the basis of subdivision of Precambrian time taught to the present generation of geologists and adopted by geological surveys throughout the world was largely established as a result of the endeavours of the "International Committee on Geological Nomenclature, representing the Geological Surveys of the United States and Canada" which first met in 1903. During the last half of the nineteenth century, as the Canadian Shield was explored, many arguments arose about correlation and nomenclature. G. A. Young (1932 and 1933) has given a detailed account of the opinions and arguments of this confusing period, so that it is sufficient to say that it was in an endeavour to arrive at some agreement that the International Committee was established. It included the most distinguished Precambrian geologists on both sides of the border, and produced two reports. The Special Committee for the Lake Superior region did not use the terms Archean and Protcrozoic but they "recognized and adopted . . . the following succession and nomenclature" (Van Hise, 1905). CAMBRIAN PRECAMBRIAN

Unconformity

Keweenawan (Nipigon) Unconformity Upper

Huronian

Middle Lower

Keewatin

Unconformity Unconformity

Unconformity

Eruptive contact Laurentian

The Special Committee on the Correlation of the Precambrian rocks of the Adirondack Mountains, the "original Laurentian area" of Canada, and

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Western Ontario, did not use the terms Archean and Proterozoic either, but divided the whole succession as follows : CAMBRIAN

Unconformity PREC AM BRIAN

Grenville series Intrusive contact Laurentian

In the report, Adams et al. ( 1907) point out that the Laurentian is the same as in the Lake Superior region and cuts both Keewatin and Grenville rocks, but they did not "attempt any correlation of the Grenville series with the Huronian or Keewatin." While these two regional committees were deliberating, Chamberlain and Salisbury published their textbook ( 1905 ) which was to become the standard text in North American colleges for a number of years. They were undoubtedly in touch with the committees for they adopted the terms later published by the Lake Superior committee and divided them into two eras, arriving at this time scale : PALEOZOIC ALGONKIAN Or PROTEROZOIC

ARCHEOZOIC Or ARCHEAN

Cambrian Unconformity Keweenawan Unconformity Huronian

Unconformity Keewatin Eruptive contact Laurentian

Of the older Archean era they wrote, reflecting the current ideas about a rapidly cooling earth, "the rocks of no later era are so largely igneous, so notably deformed or so highly metamorphosed." The younger Proterozoic era they described as "a time when igneous activity was still rather pronounced, though by no means so overwhelmingly as in Archean time. Sedimentation had become, for the first time, the leading process in the formation of the geological record." In 1908 Van Hise, who had been an active member of the Lake Superior Special Committee, devoted his presidential address to the Geological Society of America to "The problem of the pre-Cambrian." He was fully in accord with Chamberlin and Salisbury and used the same divisions. Although he admitted that this classification had been developed for the Lake Superior region, he considered that it had world-wide application, and correlated the successions in France, Scotland, Finland and China with that at Lake Superior. He admitted that the position of the Grenville rocks was uncertain. He favoured placing them in the Archean, but in the follow-

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ing year in another paper he seems to have taken the opposite view (Van Rise, 1909). He found five general distinctions between Archean and Proterozoic rocks : ( 1 ) An unconformity between them was general and "believed to be the most distinctive and widespread exhibited by the pre-Cambrian formations." (2) "The Archean is a series dominantly composed of igneous rocks, largely volcanic and for extensive areas probably submarine. Sediments are subordinate. The Algonkian is a series of rocks which is mainly sedimentary. Volcanic rocks are subordinate." (3) "When the Algonkian rocks were laid down, essentially the present conditions prevailed on the earth. The Archean rocks, on the other hand, indicate that during this era the physical conditions had not yet become such as to lead to the orderly succession of sedimentary rocks like those being formed today." (4) "The folding and metamorphism of the Archean area on the whole is very much further advanced than the Algonkian." (5) Areas of the two may often be separated on physiographic grounds. It seems that these views expressed by Van Hise, by Chamberlin and Salisbury and by the Lake Superior Special Committee were accepted as authoritative and were widely adopted. In particular they form the basis, with minor changes, for the 1934 report of the Canadian National Committee on Stratigraphie Nomenclature (Alcock, 1936) and for the usage prevailing in the Geological Survey of Canada. It is therefore of some interest to note that this usage was that adopted in the Lake Superior region of the United States, where it is satisfactory. The mistake was made in trying to make this usage universal. Its application to the Grenville region was never clear. It does not apply to all parts of the Canadian Shield and two of the most distinguished Canadian geologists of the time both disagreed with it. Adams (1909) in a most eloquent paper stresses the large number of unconformities in the Precambrian. He concludes that "it seems that the division of the pre-Cambrian rocks of Laurentia into two great major divisions—Archean and Algonkian—is not supported by the facts in our possession." He settled for two major breaks and three major periods, which seems also to have been A. C. Lawson's view. Canadian geology owes much to American graduate schools, but it was natural that those schools taught what was correct for the United States without realizing that their views on this subject were not sufficiently general to apply to the whole Canadian Shield. It was a natural consequence of immaturity that Canadians, finding so much that was sound in that teaching, should not be critical enough nor strong enough to insist on a broader interpretation. Most of our troubles with a cramped and telescoped Precambrian classification stem from the insistence of such dominant personalities as Van Hise, Chamberlin and Salisbury that the whole Precambrian succession of the

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world be fitted to the few representatives of that succession present in Minnesota and Wisconsin. In support of those distinguished geologists it is worth pointing out that the picture of a cooling earth whose history was divided by revolutions into eras each more settled than the preceding was more tenable then than now. The idea was widely held that the earth was generally quiescent but that at long intervals world-wide revolutions occurred, accompanied by mountain building and igneous intrusion. This view has now been pretty thoroughly demolished, for example by Knopf (1948), Gilluly (1949) and Rutten (1949). It was also easier to think of the later eras as quiet periods because large areas of metamorphic rocks now known to be Paleozoic or Mesozoic in age were at that time considered to be Archean. The Piedmont district of the eastern United States and the Shuswap terrain of British Columbia are examples. Although the first age determinations were made by Boltwood in 1907 and although Ellsworth in 1932 showed that the pegmatites of southeastern Manitoba were about two billion years old and twice the age of the Grenville pegmatites, few ages were available and few geologists paid any attention to them until recently. Since 1938 and especially since 1951 more and better age determinations have become available. The age of the earth has been established at about four and a half billion years and that of the oldest pegmatites at three billion years. Thus the Precambrian represents most of geological time, and Holmes (1948) has urged that the time has come to "liberate Precambrian geology from a telescoped classification." Grout agrees, and writes, "In the absence of fossil evidence of age, radioactive methods are the only real methods of estimating time" (Grout, Gruner, Schwartz and Thiel, 1951 ). There are not yet enough age determinations to do this satisfactorily, but we can clear the way by removing misconceptions. I should like to suggest that the root of the trouble lies in the fact that Van Hise gave several separate definitions of the distinction between Archean and Proterozoic. They all applied equally well in the Lake Superior region, but they led to contradictions elsewhere, and because different geologists have used different distinctions, endless confusion has resulted. One distinction which has been widely followed depends upon points ( 1 ) and (4) of Van Hise. It is indeed true that in many places strata that are but slightly altered rest with a profound unconformity upon a metamorphosed and distorted basement, but as Leith, Lund and Leith pointed out in 1935 the two kinds of rocks and the divisions between them are not everywhere of the same ages. If Proterozoic and Archean are used for these two classes of rocks, the names imply types of rocks and should not be used for periods of time. For example, the ages of veins in Proterozoic types of rock at Martin Lake, Saskatchewan, and of the gneisses bracket an unconformity that is between 16 and 19 x 108 years old (Robinson, 1955; Eckelmann and Kulp, 1956; Collins, Farquhar and Russell, 1954). On the other hand the pegmatites in the Lewisian gneisses which underly the Torridonian

DISCUSSION OF "PROTEROZOIC IN CANADA"

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sandstone in Scotland show that the great unconformity there is less than 10 x 108 years old (Holmes, Shillibeer and Wilson, 1955). That distinction in rock types is a very real and useful one, but not a division in time. These rock types need names, but the words Proterozoictype and Archean-type are not good, though I admit to having used them. Basement rocks and cover rocks seem to be better terms and to have the advantage of applying equally well, as they should, to Paleozoic and subsequent rocks as well as to Precambrian rocks. Another distinction which has been used is based upon points (2) and (3) of Van Hise. Gill ( 1952) has revived the idea that the oldest rocks in the Canadian Shield are predominantly volcanic, and MacGregor (1951) has done the same in Africa and McKinstry (1945) in Australia. Recently Wilson, Russell and Farquhar ( 1956a) have given a list of these old volcanic areas which they call continental nuclei, of their properties and of the ages of their pegmatites. All seem to be older than two billion years and it is possible that all are about two and a half billion years old. Here is one definition of the Archean that may be good. This was the meaning which Van Hise attached to the Archean. These rocks, which are everywhere of the same types as the Keewatin and Timiskaming, can be recognized in the field and probably represent a definite period of time. It is of interest to note that this was the definition of Archean recommended by M. Hurst and other Ontario Department of Mines geologists in a minority report to the 1934 Committee on Stratigraphie Nomenclature (Alcock, 1934). Even if this definition of the Archean were to be accepted, it would not be essential to use Proterozoic for all subsequent Precambrian rocks, though some might recommend doing so. Many rocks such as the Grenville and other predominantly sedimentary gneisses would then be placed in the Proterozoic, which would be contrary to present usage. A new term would seem to be needed. A third distinction can be made. The boundary between the Archean and Proterozoic might be placed at some other arbitrary date, such as one billion years ago. This has little to recommend it. No other event of any great significance is known and the field geologist could not recognize Archean from Proterozoic rocks. Until abundant age determinations are available any such arbitrary divisions are impossible: when age determinations are available such distinctions will be unnecessary because the absolute ages can be stated. One other point must be made. It seems to have been tacitly assumed that some special significance attaches to the beginning of the Cambrian in addition to the sudden emergence of fossils of animals with hard shells. It is doubtful if this is justified. Would we not do better to have one general time scale and another quite separate one based on fossils? To any individual the events of his own life are important, but one keeps one's own private chronology of events that happened when one was 21 years old or in the year that one got married separate from the universal calendar. On that

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basis, there seems to be little logic in defining a Proterozoic period with a petrologically determined beginning and a biologically determined end. It might be better to carry the major division of post-Archean time right down to the present and keep the biological time scale separate. Proterozoic could certainly not be used for this or endless confusion would result. In conclusion, I think that there is a need to distinguish the two types of rocks which occur above and below major unconformities, and that the words basement and cover might be appropriate, without any connotation of age, Precambrian or subsequent. The term Archean originally used by Dana in 1872 for the whole of the Precambrian has been considerably restricted and abused already. A case can be made for restricting it to the rocks of continental nuclei only and for considering that it represents the dominantly volcanic and igneous rocks of the period from about two or two and a half to three billion years ago. Even this would be confusing in view of the widespread use of Archean for any Precambrian basement rocks. To apply the term Proterozoic to all Precambrian rocks younger than continental nuclei and regardless of metamorphism seems to be also possible, but even less desirable than the use of Archean. To take any data other than the close of the predominantly volcanic period of continental nuclei as the division between Archean and Proterozoic seems to me to be most unsound and not useful. The fact of the matter is that the words Archean and Proterozoic were introduced when the philosophy of geological history was quite different from what it is today. Fifty years ago the earth was believed to have had a short and rapidly cooling history. The full length of Precambrian time was certainly not realized. Continental blocks were believed to have been permanent features of the earth's crust since its origin. Widely separated parts of the earth were considered to have been subjected to simultaneous, great, but infrequent revolutions. In contrast to this it is now realized that the earth's history and particularly the Precambrian part of it has been vastly long. If, as seems likely, the earth has been cooling it has been doing so very slowly. Continents have not been permanent. They have grown from nothing. Revolutions have not been world wide and separated by great intervals of quiescence; rather there has been continuous activity, first in one mobile belt, then in another. This is not the place to expatiate upon this new philosophy. The writer's views are given elsewhere (Wilson, 1954, Wilson, Russell and Farquhar, 1956a and b, Wilson, 1957) ; but it is evident that we invite confusion if we bolster up terms invented to describe situations in one philosophy and transfer them to describe events in another quite different view of history. The continuing use of the word Archean can be justified on historical grounds for the earliest predominantly volcanic rocks and their associated intrusives. One can sympathize with the 1934 committee who stated, "It is, moreover, such a fine word and so widely used that the committee felt it should be retained."

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The use of Proterozoic for the rest of Precambrian time seems to the author to require much greater distortion and to be less justified. The original meaning has been so much changed and used for such a variety of concepts that its continued use would only provide a quagmire of uncertainty as a foundation for future chronology. Justification on HumptyDumpty's grounds that "When I use a word it means just what I choose it to mean" seems to have been overextended already. In the present time of transition it is best to use local names. The subject of this symposium might have been made clearer if the title had been "Precambrian cover rocks in Canada." REFERENCES ADAMS, F. D. (1909). Jour. Geol., vol. 17, p. 105. ADAMS, F. D., BARLOW, A. E., COLEMAN, A. P., GUSHING, H. P., KEMP, J. F.. and VAN HISE, C. R. (1907). Jour. Geol.. vol. 15, p. 191. ALCOCK, F. J. (1934). Trans. Roy. Soc. Can., Srr. Ill, vol. 28, Sec. IV. p. 113. BOLTWOOD, B. B. (1907). Amor. Jour. Sci., vol. 23, p. 77. COLLINS. C. B.. FARQUHAR, R. M., and RUSSELL, R. D. (1954). Bull. Cool. Soc. Am.. vol. 65. p. 1. CHAMBERI.IN, T. C., and SALISBURY, R. D. (1906). Geology; Henry Holt. vol. 2, p. 1. ECKEI.MANN, W. R., and KULP, J. L. (1956). Bull. Geol. Soc. Am., vol. 67, p. 35. ELLSWORTH. H. V. (1932). Geol. Surv., Can., Econ. Geol. Ser. no. 11. GILL, J. E. (1952). Proc. Geol. Assoc. Can., vol. 5, p. 57. GILLULY, J. (1949). Bull. Geol. Soc. Am., vol. 60, p. 561. GROUT, F. F., GRUNER, J. W., SCHWARTZ, G. M., and THIEL, G. A. (1951). Bull. Geol. Soc. Am., vol. 62, p. 1017. HOLMES, A. (1948). Intl. Geol. Cong., Rept. 18th Sess., pt. 14, p. 254. HOLMES, A.. SHILLIBEER. H. A., and WILSON, J. T. (1955). Nature, vol. 176, p. 390. JEFFREYS, H. (1952). The Earth; 3rd edit.. Cambridge, p. 250. KNOPF, A. (1948). Bull. Geol. Soc. Am., vol. 59, p. 649. LEITH, C. K., LUND, R. J., and LEITH, A. (1935). U.S. Geol. Surv., Prof. Paper 184, p. 10. McKiNSTRY, H. E. (1945). Amer. Jour. Sci., vol. 243-A, Daly vol., p. 448. MACGREGOR, A. M. (1951). Geol. Soc. South Africa, Trans., vol. 54, p. xxvii. ROBINSON, S. C. (1955). Geol. Surv.. Can., Bull. 31. RUTTEN, L. M. R. (1949). Bull. Geol. Soc. Am., vol. 60, p. 1755. VAN HISE, C. R. (1905). Jour. Geol., vol. 13, p. 89. (1908). Bull. Geol. Soc. Am., vol. 19, p. 1. • (1909). Jour. Geol.. vol. 17, p. 97. WILSON, J. T. (1954). The Earth as a Planet; Univ. of Chicago Press, chap. 4. (1957). Nature, vol. 179. pp. 228-230. WILSON, J. T.. RUSSELL, R. D.. and FARQUHAR, R. M. (1956a). Radioactivity and age of minerals; in Handbuch der Physik, Springer-Verlag, vol. 47, p. 351. (1956b). Trans. Can. Inst. Min. Met., vol. 59, pp. 310-318. YOUNG, G. A. (1932). Trans. Roy. Soc. Can., Ser. III. vol. 26, Sec. IV, p. 341. (1933). Trans. Roy. Soc. Can., Ser. Ill, vol. 27, Sec. IV, p. 67. ZUENER, F. E. (1952). Dating the Past; 3rd edit., Methuen.

LIFE IN THE PROTEROZOIC Alice E. Wilson, F.R.S.C. EVER SINCE the realization that life moved on from the simple to the complex, geologists and palaeontologists have been searching for life in the "Proterozoic," arguing that the advanced life of the Cambrian presupposes life in the late Precambrian at least. What form or forms of life existed then and why more of it is not evident has been a question in men's minds for at least a century. There are two major phases in the story of that quest: the history of the finding and descriptions of such forms as there are; and the advance in the methods of proving or disproving the organic nature of the forms found. In passing it might be mentioned that of late years there has been a tendency to define the era between the very ancient, oldest known Precambrian rocks by rock terms or time terms, rather than by life terms such as the somewhat indefinite term "Proterozoic," which by its origin signifies "earlier life." Its meaning is ambiguous and clashes with the word "Archeozoic"—ancient life. Some authors have used the term "Infracambrian" or Walcott's ( 1914) term "Lipalian." It is from these rocks, immediately underlying the Cambrian, that most of the "Proterozoic" fossils have been found and described. The majority of the forms are considered to be algae, though there have been a few crustaceans, worms, and so on, and a very few brachiopods. In modern seas calcareous algal deposits are formed in two ways. Some types, like blue-green algae, from their nature, deposit calcareous crusts externally. Other algae, not forming calcareous crusts, may, within their trailing fingers, gather to themselves sediments bearing carbonaceous matter. The latter method may well have been operative from the beginning of life. One reservation, however, must be borne in mind—throughout the thousands of millions of years of Precambrian time, and the hundreds of millions of years since the end of the era, many thousands of feet of calcareous deposits have been brought into solution and redeposited and are now being worn away and redeposited. Hence vastly more carbonaceous organic material is loose to be caught with the sediments in the algal masses of today.

SOME RECORDED PROTEROZOIC "FOSSILS" The dramatic interest in finding and in describing Proterozoic "fossils" is best illustrated by a somewhat detailed story of the first discovery. Later when peculiar forms were found in the Precambrian a tendency grew to give 18

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them names according to form and without further investigation, and to let it go at that. Recently two new methods of testing specimens have been discovered: the ratio of the carbon isotopes C12 and C13 in the limestone, and the content of stable organic acids. In 1858, in several localities of the Grenville province of Quebec and Ontario, Logan found the puzzling forms which were later called Eozoon canadense. In 1859 he exhibited them at a meeting of the American Association for the Advancement of Science. In November of that year Darwin's Origin of Species was published. In 1862 Logan took his specimens to England and showed them to a number of British geologists. It is probable that the furore over Darwin's theories stimulated him in the desire to establish the possible organic nature of his find. In 1865 he sent his specimens to Sir William Dawson (1865) who, with the corroboration of Dr. W. B. Carpenter, and Dr. Sterry Hunt of the Geological Survey of Canada, pronounced them the earliest known form of life. The specimens showed interlayered calcium carbonate and serpentine. It was believed that the calcium carbonate layer was the result of organic deposition and that the serpentine filled the cavities. Dawson considered the organic material to be Foraminifera, one-celled animal life. He held that belief as late as 1895 when he published the Dawn of Life, though he states he did not exclude the possibility of previous life. It has since been stated, particularly by Osann (1902) that the structures of banded calcium carbonate and serpentine called Eozoon canadense are the result of two periods of alteration, the serpentine due to the alteration of diopside and tremolite which in turn are products from impure limestone previously intruded by an igneous mass. Eozoon canadense may not prove to be of organic origin but the impetus it gave to the search for such life in the "Proterozoic" can hardly be overestimated now. Billings (1873, 1874) described and illustrated by drawings (not photographs) some forms from the "Huronian" near St. John's, Newfoundland. He named them Aspidella terranovica. The writer remembers these specimens in the Museum of the Geological Survey. After the burning of the Parliament Buildings, when Parliament was moved to the Museum Building, the exhibits were packed and stored. Aspidella terranovica is still in storage. No tests were ever made upon the specimens. Matthew (1898) in a letter to A. S. Packard states that "Aspidella terranovica appears to be a slickensided mud concretion striated by pressure." Walcott (1899) figured a specimen as questionably Aspidella though elsewhere he cites Matthew's opinion, and suggested that "they may be spherulitic concretions." The form has not been mentioned in the literature since. A year before Dawson published the Dawn of Life, Cayeux (1894) described and figured a number of forms as Precambrian organic remains. Great doubt has been thrown upon the authenticity of his interpretation by Rauff (1895) and Raymond (1935, p. 380). Nicholson and Etheridge (1878, 1888) tentatively compared some types of Proterozoic "algae" to Girvanella from the Ordovician and Carboniferous rocks of Scotland.

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Walcott (1899) published a report on the Precambrian Belt series of Montana, including some fragments considered to be of organic origin. Some forms were interpreted as worms, and some as crustaceans. The organic origin of most of the fragments is very doubtful. Raymond (1935, p. 382) suggests that a few, particularly some of the crustaceans, may be authentic. In 1911 Dr. Andrew C. Lawson visited the Steeprock Lake area, west of Port Arthur, Ontario. The geology had previously been described by H. L. Smyth (1891). Lawson found what he took to be fossils in the lower limestone of the Atikokan series above the conglomerate. They were studied and described by Walcott (1912) as two species of a new genus Atikokania. Walcott considered them to be related to sponges or to the Cambrian Archaeocyathinae. The form is somewhat similar, a cross section showing a central cavity with radially arranged pillars; but, though the relationship was never established, they in turn were widely greeted as the oldest known fossils, related to the sponges found in Huronian rocks. But Abbott ( 1903 ) had already published some similar forms in England proving them to be inorganic concretions of Permian age. Later, after referring to Walcott's "coralloid masses," Abbott (1914) states, "It seems clear that changes in rock after deposition lead sometimes to the formation of tubes . . . the most banded formation (probably allied to the septa of Walcott's specimens) is seen in weathered mortar found in the shady parts of old buildings especially those situated near the coast." Raymond (1935, p. 381 ) noted similar forms in Mount Edith Cavell apparently originating by the infiltration of silica solutions which have silicified most of the rock of the Lower Cambrian in the area. In 1915 Walcott (1914-16) described and figured a number of forms under the title "Precambrian Algonkian Algal Flora," from the American Cordilleran area. In this article he refigured the somewhat problematic illustrations from the Beltian series of Montana, as associated forms. The algal forms he likened to the modern "water biscuits" deposited in fresh water by lime-secreting blue-green algae. These forms have later been described variously, principally by Raymond (1935, p. 376) as septarian concretions, ripple marks, shrinkage cracks and "what seems to be a calcareous tufa (Cottenia ? frequens}." Considerably later Fenton and Fenton (1931) described and figured a number of species of "Callenta" at some length. A few years ago a boulder of typical "Collenia" was brought to the office of the writer. It was turned up by a plough in a field near Matawatchan, Renfrew county, Ontario. The specimen exhibits a combination of "Collenia columnaria" Fenton and Fenton and "Collenia symmetrica" Fenton and Fenton, depending upon the lateral crowding. The specimen was examined optically by several Precambrian geologists who reported irregular banding of limestone and dolomite coloured a little by iron. Some minute round particles scattered throughout were examined by the palaeobotanist for possible spores. The result was negative. Later one of the Precambrian geologists

LIFE IN THE PROTEROZOIC

21

PLATE I 1. Top of Collenia-liie model. 2. Side view of the same, showing the Collenia-lïke laminations, the surface excrescence being flattened by the imposition of later material. 3. The same, slightly tilted to show the position of the nut of the wheel. 4. "Collenia" from a boulder near Matawatchan, Ontario. 5. "Collenia" from south and west of Lake Mistassini, Quebec. The ruler is one foot plus one-quarter inch at each end.

22

A. E. WILSON

brought in a white "Collenia"-like, specimen (Plate I, figs. 1 and 2). The form is similar to parts of the first specimen but finer. The small "Collenia" model came from the refining plant of the Brucite Mine at Farm Point, Quebec, north of Ottawa. In the process of separating the brucite from the Grenville limestone and dolomite the soft residual mass settled on the nut of a revolving wheel. The nut of the wheel is imprinted on the model. This form of ''Collenia" at least, then, can be the result of mechanical action, probably due to current and density or even to some chemical action. Fenton and Fenton (1936) described the usual group of algal (?) forms but also some linguloids of Algonkian age from the Beltian series. The latter certainly look authentic. They occurred in beds of shaly limestone. The beds above and below are described as shaly limestones having shaly partings and with sandy cross laminae. Modern linguloids inhabit a similar environment in the sands or muds of subtropical seas, sometimes above low tidemark in inlets or estuaries.1 But not all later reports of life in the Precambrian come from the "Proterozoic" of North America. In Australia in the second decade of the twentieth century "Cryptozoon australicum" was discovered and described by Howchin (1914) in rocks which he believed to be of Cambrian or Precambrian age. The age was questioned. Later, in 1928, Sir T. W. David (1928) of Australia clarified the history of the Precambrian and supposed Precambrian, comparing them to occurrences elsewhere. He lists fossils described from outside Australia. Many of those have since been discarded as fossils. Others occur in rocks of which the exact age is not known. He also described and figured forms in the Australian rocks of the Adelaide series. Some of the forms illustrated appear to be true fossil remains, some of them resembling Walcott's crustaceans. The author, however, suggested tentatively that the rocks "may be Lipalian in age, that is, belonging to the time represented in North America by the unconformity between the top of the Keweenawan and the base of the Cambrian. On the other hand it may be 'Proterozoic' (Algonkian), possibly homotaxial with the Keweenawan, but unlike it the Adelaide series appears to be conformable with the lower Cambrian." Both age and organic origin of these two Australian occurrences are still in doubt but it might well be that the crustacean-like forms were organic. "Collenia" and "Eozoon" have also been cited from Australian rocks by Mawson (1925). Pierre Hupé (1952) described as "Problemática" a number of forms found by MM. Fauvelet and H. Carpentier near Timdrhas in Morocco, North Africa. The "Problemática" occurred beneath the Lower Cambrian, in what has been called the Infracambrian. Within the series is a lower and upper detrital phase separated by an andésite flow. The forms described occurred in the upper phase. Hupé states that the specimens were exhibited iSince this paper was completed a second mechanically moulded specimen of "Collenia" was brought by a geological engineer to the office of the writer. It was formed at the turn of a hot water pipe with a pressure of 10 Ibs. to the inch.

LIFE IN THE PROTEROZOIC

23

in the Sorbonne in 1952, and were examined by geologists both French and from other countries. Opinions differed greatly as to their origin. The majority considered them organic, probably belonging to the Arthropoda. Hupé himself held that opinion. To the writer, in the illustrations they appear more like concretions, though some other test such as the ratio of the carbon isotopes would be needed to establish their origin. They are mentioned here to show how widespread is the interest in possible organic forms in the late Precambrian, and how problematic their origins. Again from Africa, this time from Tanganyika and Uganda in East Africa, come reports of algal forms in Precambrian rocks. In 1955 Pallister (1955) cites possible algal forms. He states that "in places large spheroidal blocks of very compact white and red-brown cherty material occur; elsewhere angular flinty masses are cemented together with open cavities which frequently contain clusters of quartz prisms. Many fragments display irregular banding. The bands are often arranged haphazard but occasionally show concentric arrangement. . . . Some individuals are circular in cross section with a long axis very many times greater than the diameter; other groups have a flat oval cross-section.'' He refers these to the two genera Collenia and Oncolithes Pia. As has been shown above some forms resembling Collenia can originate mechanically. Johnson (1945) has found specimens of Mississippian age made up of a number of filaments of different kinds in a single nodule, showing that more than the shape is necessary for determination. Describing some Carboniferous reef-building algae from northern England and southern Scotland in 1950 Anderson (1950) drew attention to the fact that the classification of living algae is not based upon the growth habit of the thallus, but upon structures rarely preserved in fossil forms. He concludes that the form of growth may have been the result of physical environment and studied the occurrence of fossil algae by what he calls ''form genera" instead of making numerous species of various forms. Some forms such as Collenia described from the Precambrian persist to the Carboniferous. It was also noted that some growth forms are more common at certain horizons. Fenton and Fenton (1937, 1938) had drawn attention to this earlier. The same method might well be applied, if possible, to the Precambrian forms, always making allowance for changing environmental conditions. A year or two ago a small group of algae and fungi were found near Schreiber, Ontario, north of Lake Superior, in dense black cherts of Proterozoic age. Protected by the silica they have preserved some filaments. From them Tyler (1954) has distinguished two algae, two fungi and a calcareous flagellate. On the other hand it has been shown by J. E. Hawley (1926) that ferrous silicate growth in 25 per cent concentrated water glass can produce long filaments very similar to algal filaments. The form and shape of the growth varies with the strength of the concentration of the water glass. Hawley was dealing with the possibilities of life in the Archean rather than in the

24

A. E. WILSON

Proterozoic but the principle is the same. Many of the forms described as algal in the Proterozoic contain silica. The oldest form described as a fossil alga is Corycium enigmaticum Sederholm from the Archean rocks of Sweden. His interpretation was not supported by all his contemporaries. Later Rankama (1948) accepted the biogenic origin of the form because of the C12/C13 ratio. Craig (1954) questions the conclusion stating that conditions during Archean time were not the same as in the later Precambrian and subsequent eras, and the enrichment of the C12 in the C ia /C 13 combination could be from other sources which were not considered by Rankama. The occurrence is mentioned here but not discussed further because the form was found in the Archean rocks not in rocks of Proterozoic age. There are numerous other references to the occurrence of possible algal forms from various Proterozoic areas in Canada and elsewhere. The Precambrian forms mentioned above have been more thoroughly investigated than most. The organic origin of many of them apparently has proved to be unlikely, and in time they have been discarded as fossils. It is rather curious that the few least suspect forms remaining have been identified as crustaceans and a very few linguloid brachiopods, both forms having the dark horny shell that has usually been considered as chitinous. Some trilobites and linguloid brachiopods of the Lower Cambrian and later have the same sheath. Various reasons have been suggested for the scarcity of fossils in the Precambrian sedimentary rocks. Daly's theory that the sea-water had not at the time dissolved sufficient calcium hardly holds if algal reefs were from lime-secreting forms, or, if the algae, knocked about by currents, caught material in their filaments as they do today in the Florida Keys. The plant forms have usually been referred to the lime-secreting blue-green algae or to a replacement of that secretion by silica. If enough calcium was present in the sea-water to provide material for such primitive lime-depositing organic matter as algae why did not other organisms make use of it? Some gastropods, primitive cephalopods and lime-secreting brachiopods more advanced than the chitinous-sheathed linguloids existed in the Cambrian seas. They must have lived before. Why did they not make use of the calcium? Another theory is that the presence of acid in the water prevented the formation of calcium shells. That suggestion partly agrees with Daly's theory that there was not yet enough calcium to form skeletons. It also is partly borne out by the preservation of some more acid-resisting chitinous-shelled forms and the lack of calcium-shelled forms, but, if so, how did the algae make use of it? One theory is that life began on land in the soil and migrated to the ocean—except for some fish, the reversal of the order as seen in the evolution of later life which developed in the sea and then migrated to the land. Another theory is that life began in fresh water with little calcium. A still

LIFE IN THE PROTEROZOIC

25

further suggestion is that life at first was plankton and would have been retarded by a heavy shell, that the shell forms were the result of a later sessile life. DEVELOPMENT OF RESEARCH METHOD The first forms considered to be organic were judged by their appearance and shape. Superficial resemblances to organic shapes and structures such as those in the "water biscuit" of today led to the identification of some Precambrian "fossils." The next step was examination of the mineral content, and the succession of deposition and metamorphism, as in the Eozoon canadense. Then, a wider knowledge of the inorganic processes, such as the growth of concretions, to some extent gave pause to the habit of considering everything of unusual form to have an organic origin. The application of optical determination of minerals further strengthened such doubts. The finding of forms produced today by mechanical means, such as that described in this paper, resembling Cállenla, and those known to be due to the disintegration of mortar under weathering still further casts doubt on many Precambrian "fossils" described as algae. Recently two more effective methods of establishing organic and inorganic origin of algae, or supposed algae, are being developed: the ratio of C12/C13 carbon isotopes in imprisoned material, and the presence of the more stable organic acids. All limestones contain C12 and C'a isotopes, but tests of limestones of known origin have shown that the ratio of C12 to C13 is greater where the limestone contains organic matter, whether it be original or organic matter caught in the masses by wave action. Biochemical analysis, also, has begun to open up further possibilities of establishing the organic origin of specimens. It has been found that some proteins are contained in living shells. Tests show that proteins are still preserved in fossil shells protected between laminations of calcium carbonates. A curve of the rate of breaking down of the proteins has been postulated by plotting the results obtained from exposing them to a series of closely controlled tests of great heat. Some of the acids break down under the process but some of the more stable ones remain. By the curves thus established it is considered that some stable acids can remain for several billion years. Some of the algal structure preserving the stable acids have been found embedded in silica and thus not exposed to disintegration, just as today algae in hot springs are embedded in silica from the hot silicabearing springs of Yellowstone Park. Such organic matter has been found, also, in Precambrian shales of Michigan and in Precambrian sediments of Sweden and Finland. Several tests of both carbon isotopes and stable organic acids are being carried out upon algal-like forms of the Canadian "Proterozoic" but are not yet completed.

26

A. E. WILSON

CONCLUSION There can be little doubt that life existed in the Precambrian, especially in the later Precambrian. A few of the chitinous forms seem undoubtedly to be of organic origin. But the present writer agrees with those authors who suggest caution in publishing genera and species by form alone when dealing with algal-like forms. Currents, density, chemical action in the process of forming, and, later, erosion can produce strange forms. Specimens preserving filaments mingled with carbon and chains of cells, such as are known in living organisms, may be accepted as organic in origin, but other algal-like forms require confirmation by such methods as the ratio of C12/C13, or the content of the more stable organic acids. REFERENCES ABBOTT, G. (1903). The cellular magnesium limestone of Durham; Geol. Soc. London, Quart. Jour., vol. 5, Scr. 59, p. 51. (1914). Is Atikokanin lawsoni a concretion? Nature, vol. 94, pp. 477-478. ANDERSON, F. W. (1950. Some reef-building calcareous algae from the Carboniferous rocks of northern England and southern Scotland; Yorkshire Gcol. Soc. Proc., vol. 28, pt. 1, p. 5. BILLINGS, E. (1873). Fossils in the Huronian rocks; Can. Nat., N.S., vol. 6, p. 478. (1874). Palaeozoic fossils; Geol. Surv., Can., Pal. Foss. II, p. 77. CAYEUX, L. (1894). Les preuves de l'existence d'organismes dans le terrain précambrien: Geol. France, Bull. Ser. 3, vol. 22, pp. 197-229. CRAIG, H. (1954). Geochemical implications of the isotopic composition of carbon; Gcochim. Cosmochim. Acta, vol. 6, pp. 186—196. DAVID, SIR T. W. E. (1928). Notes on newly discovered fossils in the Adelaide series (Lipalian?), South Australia; Trans. Roy. Soc. S. Autralia, vol. 52, pp. 191-209, pis. 13-18. DAWSON, SIR WM. (1865). On the structure of certain organic remains in the Laurentian limestones of Canada; Geol. Soc. London, Quart. Jour., vol. 21, pp. 51-59, pis. 6 and 7. (1865). Can. Nat., N.S., vol. 2, pp. 99-127, figs. 1-3. FENTON, C. L., and FENTON, M. A. (1931). Algae in Glacier National Park; Jour. Geol., vol. 39, pp. 682-685. (1936). Walcott's "Pre-Cambrian Algonkian Flora" and associated animals; Bull. Geol. Soc. Am., vol. 47, p. 616, pi. 3, figs. 4 and 5. (1937). Belt series of the north: stratigraphy, sedimentation, paleontology; Bull. Geol. Soc. Am., vol. 48, p. 1873. (1938). Primitive algae as environment indicators; Pan Amer. Geol., vol. 70, p. 1. HAWLEY, J. E. (1926). An evaluation of the evidence of life in the Archean; Jour. Geol., vol. 34, pp. 441-461, figs. 1-6. HOWCHIN, W. (1914). The occurrence of the genus Cryptozoon in ( ? ) Cambrian of Australia; Trans. Roy. Soc. S. Australia, vol. 38, pp. 1-10, pis. 1-5. HUPÉ, R. (1952). Sur des Problemática du Précambrien III; Protectorat de la Republique Française au Maroc, Service Géologique, Mem. 103, pp. 297-318. JOHNSON, J. H. (1945). Calcareous algae of the upper Leadville limestone near Glenwood Springs, Colorado; Bull. Geol. Soc. Am., vol. 56, p. 829. MATTHEW, G. F. (1898). Letter to A. S. Packard; Proc. Amej. Assoc. Adv. Sci., vol. 47, p. 323. MAWSON, K. T. (1925). Evidence and indications of algal contributions in the Cambrian and pre-Cambrian limestones of South Australia; Trans. Roy. Soc. S. Australia, vol. 49, pp. 186-190. NICHOLSON, H. A., and ETHERIDGE, R. (1878). Girvanella; Mon. Sil. Foss. Girvan Dist., p. 23. (1888). Organisms in Palaeozoic limestones; Geol. Mag., Dec. 3, vol. 5, p. 22. OSANN, C. A. (1902). On the Eozoon limestone of Côte St. Pierre; Can. Geol. Surv., Ann. Rept. 12, sec. 0, p. 60. PALLISTER, J. W. (1955). The occurrence and significance of pre-Cambrian algal forms at Kabwer Hill, Bukoba district, Tanganyika and in Singo, Uganda; Geol. Mag., vol. 92, no. 6, p. 500.

LIFE IN THE PROTEROZOIG

27

RANKAMA. K. (1948). New evidence of the origin of prc-Cambrian carbon; Bull. Geol. Soc. Amer., vol. 59, pp. 389-416. RAUFF, H. (1896). Uber angebliche Organismenreste aus Prâcambrischen Schichten der Bretagne; Neues Jahr. Min., Bd. 1, pp. 117-138. RAYMOND, P. E. (1935). Pre-Cambrian life; Bull. Geol. Soc. Am., vol. 46. SMYTH, H. L. (1891). Structural geology of Steeprock Lake, Ontario; Am. Jour. Sci., vol. 42, pp. 317-331. TYLF.R, S. A., and BAROHORN, E. S. (1954). Occurrence of structurally preserved plants in pre-Cambrian rocks of the Canadian Shield; Science, vol. 119, pp. 606—608. WAI.COTT, C. D. (1899). Precambrian fossiliferous formations; Bull. Geol. Soc. Am.. vol. 10, p. 231, pi. 27, figs. 14 and 15, pp. 234-239, pis. 23-26. (1912). Geology of Steeprock Lake, Ontario; Geol. Surv., Can., Mem. 28, pp. 17-19. (1914). Abrupt appearance of the Cambrian fauna on the North American continent; Smithsonian Misc. Coll., vol. 57, p. 14. (1914-16). Pre-Cambrian Algonkian algal flora; Smithsonian Misc. Coll., vol. 64, no. 2, pp. 100-117.

DATING THE PROTEROZOIC OF CANADA R. M. Farquhar and R. D. Russell ONE OF THE MAJOR PROBLEMS that at present faces those geologists studying Precambrian geology is the lack of any well-defined time scale into which the events of Precambrian time may be fitted. There is as yet no Precambrian counterpart to the index fossil time scale which provides such an excellent and orderly framework for the geological history since the beginning of the Cambrian. The best time scale at present available is one based on absolute ages determined from the decay of naturally existing radioactive isotopes. In the short but active history of absolute age determinations most of the basic research has been directed toward the dating of the highly radioactive vein minerals and pegmatites associated with igneous activity, because it was felt that in such materials the basic assumptions in these methods of age determination would be most nearly fulfilled. These determinations have already had considerable bearing on ideas about Precambrian time in general and Precambrian nomenclature in particular. It is evident that the Precambrian era covers a time span far longer than was suspected a few years ago, in fact five-sixths of all geological time. This fact alone suggests that the Precambrian was an era which included many periods of geological evolution similar to those which have occurred since the beginning of the Cambrian. It has been suggested that the cramped classification which has been used to describe events in the Precambrian should be amended (Holmes, 1948). Wilson (supra) has summarized the situation with respect to this terminology. The present lack of age data for Precambrian sediments prevents any definite assignment of age to such rocks. The only "ages" which can be presented are limiting ones, determined by ages for igneous bodies that underlie or intrude the sediments. One example of how such crosscutting relationships may be used to place limits on the ages of Proterozoic-type rocks, is found in the age determinations made on pitchblendes and monazites from the Lake Athabasca region. Robinson (1955) has reported ages of 1,700 to 1,800 million years for primary monazites in the Archean-type Tazin series. The earliest epigenetic uranium deposits of the area are about 1,500 to 1,600 million years in age, and since the deposition of pitchblendes appears to be connected with the areas overlain by Proterozoic-type Athabasca sediments, the age of these sediments probably lies between 1,500 and 28

29

DATING THE PROTEROZOIC OF CANADA

1,800 million years. An age range of 300 million years is not particularly satisfying to the geologist, but it is the best than can be done under the circumstances. In most areas of Proterozoic-type rocks where any attempt at dating has been made, the age given is an upper or a lower limit, simply because the sediments overlie an older formation which has been dated, or are cut by veins whose age can in some way be estimated. The only vein minerals which to the present have been used for the purpose of obtaining these limiting ages are pitchblendes and galenas. Of these two minerals, pitchblendes in general give more accurate and dependable ages than do galenas. The problem of dating pitchblendes is often complicated by the discordant ages based on the ratios Pb 200 /U 238 , Pb 207 /U 233 and Pb 20T /Pb 20G , obtained for a given sample, and further obscured by the results of reworking and redeposition of deposits in situ. The particular area which has been most fully investigated, the Lake Athabasca region, has required many age determinations together with much careful geological and mineralogical investigation to unravel even the general history of the pitchblende deposits. In many instances, the galena method (Russell et al., 1954) appears to provide age determinations which agree well with other methods. Table I lists a number of ages determined from the Archean-type rocks of the "Keewatin" province. The ages are all very similar, and although some TABLK I "KKIÎWATIX" LKAD ORKS

No. GG1A 059 4G3 627 525 041 077 039 472 524 404

582 149 516 515 519 584 583 575

Locality Golden Manitou Geco Chicobi Lake Madsen Red Lake Hollinger Barvnc Elder Golden Manitou Timmins Montbeillard Twp. Steeprock Lake New Norzone Sioux Lookout Geneva Lake Geneva Lake Lake Shore McElroy Twp. (in Timiskaming seds.) Hearst Twp. McKellar Harbour

Isotopic ratios

Average indicated Ace (XlO'yrs)

20G/204

207/204

208/204

13.44 13.42 13.40 13.47 13.40 13.50 13.07 13.70 13.74

13.91 13.94 13.95 13.88 14.11 14.23 14.37

14.08 14.05 14 07 14 02 14.75 14.74 14.84 14 99 14.87

15.00 14.83 14.98 14.76 15.10 15.16 15.02

33.49 33.51 33 . 40 33.13 33.71 33.01 33 . 70 34.38 33 . 72 33.80 33 . 75 33.90 33.54 34.31 34.34 33.99

24 ±2 24±2 24±2 24±2 23±2 22±2 23 ±2

15.02 15.33 15.67

15.26 15.41 15.43

34.59 35.09 36.36

20±2 18±2 15±2

20 ±2 20 ±2 20 ±2 27±2 25 ±2

2.5 ±2 20 ±2 23 ±2 25 ±2

younger ages of deposition are indicated by a few samples, they represent a small minority of those analysed. Galenas deposited in sedimentary rocks, particularly when they occur as minor constituents of an ore body, may be subject to considerable reworking and suffer additions of both older and

30

R. M. FARQUHAR & R. D. RUSSELL

younger leads. Examples of this type of reworking are found in samples from the Proterozoic-type rocks near Cobalt, Ontario, as shown in Table II. TABLE II COBALT LEAD ORES No. 585 529 4G6 005 001 002 004 GOO 003

Locality Ingram Twp. in Xipissing diabase Badger Silver, in Huronian conglomerate La \vson Kcrr Lake, in Cobalt quartzitc Kcrr Lake, at base of Cobalt sediments Silver Miller, in Xipissing diabase Delhi Timagami in Xipissing diabase Gillies Loi, in Xipissing diabase Cobalt lode, in Keewatin above Xipissing diabase sill

206/204

207/204

208/204

Average indicated ages (108 yrs)

14.78

14.99

34.08

20±2

14.89 14.94

15.30

15.39

34.71 34.84

20±2 20±2

15.02

15.44

34.84

19±2

15.10 15 .21

15.50 15.47

35 . 00 34.97

19±2

15.90

15.05

35.47

Hi ±2

10.08

J5 70

30.15

13 ±2

18.51

15.96

38.50

2±3

Isotopic ratios

1!)±2

The "ages" calculated from the measured isotopic abundances range from less than 800 million years to 2,000 million years within a relatively small geographical area. The variations are considerable, and most regions where this sort of reworking has gone on can be recognized readily, if a sufficient number of samples are analysed. In an area where such additions of older and younger leads have taken place, the calculated "ages" are obviously meaningless, insofar as dating the containing rocks is concerned. Age determinations based on the isotopic constitution of a single galena sample from any area should be used with caution, and should not be considered reliable until supported by further analyses and geological evidence. The sketch map shown in Figure 1 summarizes our present knowledge of the absolute ages of the "Proterozoic" and the "Archean" rocks of the Canadian Shield. The "Proterozoic"-type sedimentary rocks which have been best dated are the Athabasca sediments, described previously. The apparent lack of uranium mineralization in other areas of "Proterozoic" rocks permits only the vaguest of limiting ages to be ascribed to other areas in the Shield. The lead mineralization in the Precambrian sedimentary formations of the western Cordillera has been estimated to have taken place approximately 1,100 million years ago (Gumming et al., 1955). This figure provides a lower limit for the age of the sediments themselves. In a similar manner, a lower limit of about 1,600 million years has been placed on the age of the Labrador trough rocks (Gumming et al., 1955). These determinations, based on the galena method, are subject to all the previously mentioned sources of error. The lowest age limit placed on the Huronian has

DATING THE PROTEROZOIC OF CANADA

31

FIGURE 1

been estimated from ages of both galenas and pitchblendes deposited close to the Grenville-Keewatin boundary. The age relationship between the Grenville and the Huronian has been previously examined by Shillibeer and Gumming (1956). These authors also describe recent age determinations on the Grenville and Keewatin basement rocks; the age limits which they report are included in Figure 1. The Yellowknife Archean complex has also been dated from age determination by the potassium-argon method (Shillibeer and Russell, 1954), the thorium-lead method (Folinsbee et al., 1955) and the galena method (Gumming et al., 1955). The range of ages found for this area has also been included in Figure 1. The only other basement rocks of Precambrian age on which an age limit can be placed are those in the Great Bear province, cut by the well-dated pitchblende veins known to be 1,400 million years in age (Gumming et al., 1955). It is only within the last year that our knowledge of both the technical problems and the reliability of the basic assumptions underlying age determinations has motivated any direct extension of absolute dating methods to sedimentary rocks. The work which has been done, however, has been confined to samples from fossiliferous strata (Lipson, 1956; Cormier, Pinson and Herzog, 1956). Most of the ages have been determined by the potassium-argon and the rubidium-strontium methods, and while there are still a few questions about these methods, the results agree fairly well with the presently accepted "B" time scale put forward by Holmes (1947). Most of the analyses have been made on glauconites (potassium-rich micas found in sediments) and extension to Proterozoic-type rocks should be simple. If this is done, absolute ages for the Proterozoic rocks of Canada will supersede the inadequate, limiting data which we have at present, and give us a much surer basis on which to build a Precambrian time scale.

32

R. M. FARQUHAR & R. D. RUSSELL REFERENCES

CORMIER, R.. PINSON, W. H.. and HERZOG, L. F. (1956). X.Y.O., vol. 3936, pt. 5, p. 25. GUMMING, G. L., WILSON, J. T., FARQUHAR, R. M., and RUSSELL, R. D. (1955). Proc. Geol. Assoc. Can., vol. 7, pt. 2, p. 27. FOLINSBEE. R. E.. LIPSON, J., and REYNOLDS, J. H. (1956). Geochim. Cosmochim. Acta, vol. 10, p. 60. HOLMES, A. (1947). Trans. Geol. Soc. Glasgow, vol. 21, p. 117. (1948). Intl. Geol. Cong., Rept. 18th Sess., pt. 14, p. 254. LIPSON, J. (1956). Geochim. Cosmochim. Acta, vol. 10, p. 149. ROBINSON. S. C. (1955). Geol. Surv., Can., Bull. 31. RUSSELL. R. D., FARQUHAR, R. M., GUMMING, G. L., and WILSON, J. T. (1954). Trans. Am. Geophys. Union, vol. 35, p. 301. SHILLIBEER, H. A., and RUSSELL, R. D. (1954). Can. J. Phys., vol. 32, p. 681. SHILLIBEER. H. A., and GUMMING, G. L. (1956). The Grenville Problem, Roy. Soc. Can., Special Publications, no. 1.

PROTEROZOIC ROCKS OF THE SOUTHERN PART OF THE CANADIAN SHIELD: SUMMARY James E. Thomson, F.R.S.C. THIS PAPER gives a brief introductory summary of papers, by various authors, on the following areas in northwestern Quebec, Ontario, and Manitoba: ( 1 ) northwestern Quebec and Larder Lake, Ontario, (2) Cobalt, (3) Matachewan-Wanapitei-Temagami, (4) Sudbury-Espanola, (5) Quirke LakeElliot Lake, (6) north shore of Lake Huron (Gladstone to Spragge townships), (7) the Original Huronian, (8) Mamainse Point, Lake Superior, (9) Port Arthur-Lake Nipigon region, and (10) Manitoba. These areas are indicated on Figure 1. Throughout much of this vast expanse of country the Proterozoic age of certain strata is generally accepted, but there are also areas of considerable size in which the Proterozoic age of certain formations is questionable. The rocks described as Proterozoic in this group of papers have the following features in common : ( 1 ) the volcanic and sedimentary formations rest with profound unconformity upon the Archean (Early Precambrian) rocks; (2) these formations are generally undeformed or gently deformed, except in the vicinity of faults; (3) the grade of metamorphism is low; (4) the strata are intersected by basic intrusives in the form of sills or dykes. The largest areas (basins, or portions thereof) covered by strata of generally accepted Proterozoic age are : ( 1 ) from Beauchastel township, northwestern Quebec, through Larder Lake, Cobalt, Matachewan, and Temagami to Lake Wanapitei; ( 2 ) Quirke Lake—Elliot Lake—Blind River—Bruce Mines, that is, the Original Huronian; and (3) the north shore of Lake Superior. Of course, there are localities and erosion remnants in and adjacent to these larger basins where the relationships may be doubtful, but, in general, the presence of a great unconformity below the sediments has been accepted by the geologists who have mapped the areas concerned. The questionable Proterozoic areas fall into two categories. These are ( 1 ) very isolated erosion remnants far from well-established Proterozoic rocks (the two areas in Manitoba are examples): and (2) a large area where the criteria listed above for the recognition of Proterozoic strata cannot be found although it lies between the two main areas of undisputed Proterozoic rocks. This is described as the Sudbury-Espanola area for the 33

FIGURE 1. Sketch map showing the main Proterozoic areas of the southern part of the Canadian Shield. (Modified after the Geological Map of Canada, Map 820A, Geol. Surv., Can., 1945.)

PROTEROZOIC ROCKS OF SOUTHERN CANADIAN SHIELD

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symposium although its boundaries may extend far beyond these geographic localities (see Fig. 1 of Sudbury-Espanola paper). Inasmuch as the strata of questionable Proterozoic age in the Sudbury-Espanola area appear to be intruded by granitic rocks of batholithic dimensions, although the contacts are sometimes faulted, there is a geological problem of great regional extent and of utmost importance involved here. If our attention is first directed to sedimentary formations of unquestioned Proterozoic age, we find them assigned to two main time-stratigraphic units, namely the Huronian of the north shore of Lake Huron, and the AnimikieKeweenawan of the north shore of Lake Superior. Contacts of Huronian and Animikie-Keweenawan rocks are not found in the Canadian portion of the Lake Superior basin so their mutual age relationships must be deduced from geological information derived south of the International Boundary. If we consider first the regional correlation of the two main Huronian basins it becomes a matter of comparing the stratigraphy of the type locality (Bruce Mines area ), with that of the larger basin extending east from Lake Wanapitei. We see that the Bruce and Cobalt series appear to be common to both districts but the situation at Lake Wanapitei is obscured by lack of good evidence of the unconformity and by local deformation of the Bruce series.1 It is possible that future comparative studies of both basins may lead to a harmonious stratigraphie interpretation, possibly by the transfer of certain formations in the Lake Wanapitei area, now classified as Huronian on published maps, to the pre-Huronian. At any rate, type sections could be established in both areas for purposes of comparison. A start has already been made in that direction in the Quirke Lake-Elliot Lake area, where mineral exploration has provided much new information, and at Cobalt, where detailed studies have been in progress for some time. The stratigraphy of these areas is described by Roscoe and Robert Thomson. In the Quirke Lake trough, Roscoe shows that the direction of transport of clastic sediments was towards the southeast. His studies indicate rapid facies changes at several stratigraphie horizons. This will make it difficult to project the detailed stratigraphy of a type section for any appreciable distance. Future detailed mapping of the area will show whether or not detailed stratigraphie correlation is possible over large areas. The questionable Proterozoic rocks of the Sudbury-Espanola area have been discussed in a symposium paper by J. E. Thomson. From the Cutler granitic batholith on the north shore of Lake Huron eastward to the vicinity of Lake Wanapitei there is an apparent upward succession from volcanic rocks through the entire sedimentary sequence without a major unconformity being recognized. In addition, the deformation and metamorphism of this belt is much greater than that of the typical Proterozoic areas. The stratigraphie interpretation of this belt by W. H. Collins and his associates, ^Preliminary field work by the writer in 1956, following the preparation of this paper, indicated that the great unconformity at the base of the Huronian may occur south of Lake Wanapitei, near Skead. Further detailed studies are planned here.

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as given on Map 155A of the Geological Survey of Canada, has been in general use for many years. Collins subdivided the sedimentary formations into pre-Huronian and Huronian although he recognized that the great unconformity required by his definition of the Huronian could not be found. In recent years, geological data obtained from extensive mineral exploration and from a few detailed government surveys throws doubt on Collins' regional correlation of this geological complex. It is now recognized that deformation throughout the belt is much greater than could be assessed by reconnaissance-type surveys. This, together with greater knowledge of rapid faciès changes, means that long-range correlation of lithologically similar formations is not justified. Thomson has recommended a moratorium on the use of time-stratigraphic terminology throughout the questionable Proterozoic belt until additional field work throws more light on the structure and stratigraphy. Possible alternative solutions of the correlation and stratigraphy to Collins' concept are : ( 1 ) that the sedimentary formations of the Sudbury-Espanola area are entirely pre-Huronian; (2) that elements of pre-Huronian and Huronian are brought into juxtaposition by regional thrust faulting; or (3) that a hitherto unrecognized age group is present. The Animikie and Keweenawan rocks north of Lake Superior are described by Moorhouse. This area is isolated from other major Proterozoic basins of Ontario and its sedimentary rocks have little in common with them. The sedimentary suite is characterized by iron-formation, sandstone, cherts, carbonates, argillaceous beds and thin conglomeratic horizons. The uppermost Proterozoic unit (Osier formation) consists of amygdaloidal lavas, estimated to be from 6,000 to 10,000 feet in thickness. These appear to be devoid of noteworthy conglomerate interbeds and the stratigraphy is thus quite different from that at Mamainse Point where the other thick succession of Keweenawan lavas occurs. The great unconformity between Archean and Proterozoic rocks is everywhere well marked ; both sedimentary and volcanic rocks have suffered only mild warping and nowhere exhibit important folding. The Animikie and basal sediments of the Keweenawan have the same gentle dip throughout the region but locally there is considerable angular discordance at the contact. Faulting is widespread around Port Arthur; the faults appear to be steeply dipping, normal type and locally suggest a block fault pattern. Correlation of the Animikie-Keweenawan rocks of the region with their prototypes elsewhere is uncertain. Moorhouse points out that the Animikie sediments consist of precipitated shallow- and deep-water faciès and a terrigenous clastic faciès. The sedimentary features and structure of the region are typical of foreland deposition. The intrusives of assumed Proterozoic age deserve much more attention. Robert Thomson points out the likelihood of local orifices that fed diabase magma to various parts of the Nipissing diabase at Cobalt rather than magmatic migration from one source over long distances. This raises the question as to the relative ages of the various sheets of gabbro and diabase that cut Proterozoic rocks. The Sudbury nickel irruptive and the so-called Killarney

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granite intrude the questionable Proterozoic belt and are therefore in the same questionable category so far as age is concerned. However, there are acid intrusives of definite Late Precambrian age. These are the felsites that cut the Keweenawan lava flows at Point Mamainse and elsewhere around the Lake Superior basin. CONCLUSIONS Study of the Proterozoic rocks of the central part of the Canadian Shield during the past century has served to locate the main basins and stratigraphic units, has suggested tentative correlations, and has indicated innumerable problems for future investigation. In the same period mineral exploration has shown that the Proterozoic rocks are the storehouse of vast quantities of metalliferous deposits. In future geological studies the economic urge will be an important factor in promoting the scientific research necessary for the solution of these problems and it will provide much of the new information. To date, only a few Proterozoic areas have been examined in detail and the results of these investigations have weakened our confidence in conclusions based on earlier reconnaissance surveys. Some of the present geological concepts will stand the test of time, others will be swept away. It has been established to the satisfaction of most geologists that the Proterozoic is a definite and valuable major division of Precambrian time. The fact that the Archean-Proterozoic boundary is poorly defined or questionable in some districts does not detract from the value of these major divisions in other areas where the relationships are well marked. Long-range correlation of major Proterozoic groups and formations lying within the boundary of the central Canada unit are of doubtful accuracy. They may sometimes be useful as working hypotheses but too dogmatic adherence to previous concepts of the stratigraphie succession and correlation may be more harmful than helpful. Within the last decade field geologists have shown that deformation of Proterozoic strata is locally more complex than previously realized. Isotope dating research now in progress may soon help in the study of age relationships of intrusive rocks and eventually serve the same purpose for other rock types. While this and other scientific approaches are being perfected and tested against known field relationships it may be wise to restrict new time-stratigraphic terminology to a minimum and avoid the use of current terminology whenever possible. This symposium study indicates that, while considerable progress has been made to date, concentrated field and laboratory research will be necessary in the Proterozoic areas of central Canada for many decades to come.

PROTEROZOIC ROCKS OF NORTHWESTERN QUEBEC AND LARDER LAKE, ONTARIO James E. Thomson, F.R.S.C. PROTEROZOIC ROCKS occur in Beauchastel and Dasserat townships and the Ville Marie area in northwestern Quebec and in a belt extending south from Larder Lake towards Englehart in northeastern Ontario. Except for a few diabase dykes the rocks are all of sedimentary origin and belong to the Cobalt series of Huronian age. The writer is indebted to W. S. Savage, Resident Geologist, Swastika, and J. Dugas, Resident Geologist, Rouyn, for drilling information recorded here. BEAUCHASTEL AND DASSERAT TOWNSHIPS Considerable mineral exploration has been done along the belt of Cobalt sediments both here and in the adjoining Larder Lake area of Ontario because the Cobalt rocks overlie the assumed location of the Larder LakeCadillac fault zone. Ambrose and Ferguson (1945, pp. 14, 20) have stated that deep drilling through the Cobalt series south of Arntfield in Beauchastel township indicated a pre-Cobalt valley about 1,000 feet deep that may mark the regional fault. The rocks of the Cobalt series are generally conceded to be of glacial origin and consist of poorly sorted conglomerate with interbedded grit, greywacke, and argillite. They are generally flat to gently dipping but in a few places they stand almost vertically. Apparently the Cobalt series was deposited upon an erosion surface with sub-mountainous topography and had a relief of as much as 2,000 feet. Stockwell (1949-25) mentions a drill hole that passed through 1,700 feet of covering Cobalt rocks before entering a zone of talcose schists that may represent the major fault. He states that the inward dip of the argillaceous beds indicates that the strata were synclinally folded. Post-Cobalt faulting is believed to be a recurrence of movement along pre-Cobalt faults.

VILLE MARIE AREA In the Ville Marie area, Henderson (1936, pp. 19-26) has described the Gowganda and Lorrain formations of Huronian age. Both formations are flat lying, relatively unmetamorphosed, and rest unconformably on the preHuronian basement. However, the Gowganda lies on a clean unweathered 38

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surface while the Lorrain quartzite rests on a decomposed regolith. There is evidence that an erosional unconformity of some importance exists between the two formations. LARDER LAKE AREA Thomson (1941, p. 28) has reproduced a vertical section drilled through the Cobalt series east of the Kerr-Addison mine. It indicates the marked unconformity between the Timiskaming and Cobalt series, and the irregularity of the pre-Cobalt surface with the fault zones underlying the valleys. The Cobalt sediments of the Larder Lake area are relatively flat lying; the beds rarely show dips of more than 30 degrees, except near faults where the dip steepens considerably. Drilling east of Larder Lake has revealed that the Cobalt series becomes thick; a hole entered the basement rocks after cutting a 1,375-foot section of interbedded conglomerate, arkose, greywacke, siltstone and shale of the Cobalt series. Another hole on the east shore of Larder Lake a short distance north of the mouth of Milky Creek was still in Cobalt sediments when stopped at a vertical depth of 1,400 feet. Geologists of the Kerr-Addison mine (1951, p. 69) have suggested that the Cobalt .scries are involved in an underthrust graben bounded by the Milky Creek fault on the south and a fault along the north side of the Larder Lake "break" in the vicinity of the mine. The long finger of Cobalt sediments extending approximately five miles northeast in Dasserat township, Quebec, is believed to be bounded by these faults and therefore part of the downfaulted block. The downfaulting is thought to explain the great thickness of Cobalt sediments revealed by drilling within the assumed graben on either side of the interprovincial boundary. However, a map of southwestern Dasserat township by Stockwell (1949-23) shows no evidence of a graben and locates the Milky Creek fault within the finger of Cobalt sediments rather than at the margin. Also, southwest of the Kerr-Addison mine, Thomson (1947) found no evidence of a graben along its assumed strike in Hearst township. REFERENCES AMBROSE, J. W., and FERGUSON, S. A. (1945). Geology and mining properties of part of the west half of Beauchastel township; Geol. Surv., Can., Paper 45-17. HENDERSON, J. F. (1936). Geology and mineral deposits of Ville Marie and Guillet Lake map areas, Quebec; Geol. Surv., Can., Mem. 201. KERR-ADDISON STAFF, KERR-ADDIEON GOLD MINES, LTD. (1951). Can. Min. Jour., April 1951, pp. 68-75. STOCKWELL, C. H. (1949). Preliminary map, southeast Dasserat; Geol. Surv., Can., Paper 49-25. (1949). Preliminary map, southeast Dasserat; Geol. Surv., Can., Paper 49-23. THOMSON, J. E. ( 1941 ). Geology of McGarry and McVittie townships; Ont. Dept. Mines, vol. 50, pt. 7. (1947). Geology of Hearst and McFadden townships; Ont. Dept. Mines, vol. 56, pt. 8.

THE PROTEROZOIC OF THE COBALT AREA Robert Thomson AT COBALT, the Proterozoic rocks consist of pre-Paleozoic sediments and ¡ntrusives. The sediments are in angular unconformity with the older Precambrian rocks and have retained their original attitudes to a large extent, although 25 to 50 miles south of Cobalt the original attitudes have been disturbed. The Proterozoic rocks in the vicinity of Cobalt may be taken as representative of the Timiskaming Silver-Cobalt area, a term used by Reid (1943) to denote the area extending from the Ontario-Quebec boundary west to the Canadian National Railway between the stations of Spaidal and Felix, and north from near Sudbury to the Matachewan area. In the Timiskaming Silver-Cobalt area most of the rock exposures are Proterozoic, either sediments of the Cobalt series or basic intrusives (Nipissing diabase) ; this is shown on Map 155A of the Geological Survey of Canada. STRATIGRAPHY A stratigraphie column for the Proterozoic rocks in the vicinity of Cobalt is as follows : KEWEENAWAN ( ? ) . Olivine and quartz-diabase dykes Intrusive contact NIPISSING DIABASE. Basic intrusives, usually gently dipping Intrusive contact HURONIAN (Cobalt scries) Lorrain formation Arkose and quartzite Firstbrook formation Bedded greywacke (Gow g a nda formation, Coleman formation Conglomerate, bedded grcywackc and W. H. Collins, 191/) quartzite Angular unconformity

The unconformity at the base of the Cobalt series is very well marked; indeed, the nearly horizontal attitude of the Cobalt sediments suffices to distinguish them from underlying Timiskaming sediments which are otherwise somewhat similar. The profound character of this unconformity is shown not only by the difference in dips (approximating a right angle) of the rocks above and below it, but also by the occurrence of boulders of gneiss, of deepseated origin, in the basal conglomerate of the Cobalt series. 40

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COBALT SERIES The gently dipping and unaltered sediments in the vicinity of Cobalt were described as Huronian by Miller (1904). In 1905 he used the term Cobalt series in the legend on his maps as corresponding to the time term Lower Huronian, and the term Lorrain as corresponding to Middle Huronian. In 1913 he redefined the Cobalt series to include the Lorrain rocks, and this is present usage. The name Gowganda was suggested by Collins (1917) for the lower part of the Cobalt series in the Onaping map area, which is 30 to 60 miles west of Cobalt ; he stated that the rock types were too intricately associated to be resolved into smaller mapping units and moreover all types had a common mode of origin. In the vicinity of Cobalt the part of the Cobalt series that lies below the Lorrain formation has been divided into two units by most geologists who have mapped in the area. The origin of these two units is very different; the lower is related to glaciation while the upper contains normal water-lain sediments. For these two mappable units of the Cobalt series below the Lorrain formation the present writer proposes two new formational names. The lower is named Coleman formation, the upper, Firstbrook formation. Each of these is of the same order of importance as the Lorrain formation. Subdivision of the Cobalt series in this manner is believed to represent the situation much more accurately than the previous classification. The names of these three formations are those of townships near Cobalt where the characteristic rock types are well exposed. COLEMAN FORMATION The Coleman formation is composed mainly of conglomerate, but contains argillite, bedded greywacke, quartzite and arkose in varying amounts. The maximum thickness in the vicinity of Cobalt is about 500 feet; 3 or 4 miles to the southeast this formation is absent and Lorrain quartzite rests directly on older granite. The maximum thickness of the formation might be 1,000 feet. The contact between the Coleman and overlying Firstbrook formation is not well exposed. No marked unconformity is believed to occur. The Coleman conglomerate shows great local variations in regard to the size, shape, and relative amount of the boulders, as well as in the nature of the matrix. The presence of a well-marked, but thin, regolith instead of a smoothly glaciated basement is characteristic for this formation in most places. Nevertheless, the glacial origin, originally advocated by A. P. Coleman, appears to be the best explanation of its somewhat unusual nature. FIRSTBROOK FORMATION The Firstbrook formation is composed of well-bedded greywacke, with reddish, greenish, and greyish beds from 1/10 to 1/2 inch in thickness. A vertical thickness of 950 feet of this greywacke was intersected in a diamond drill hole in lot 3, concession III, Firstbrook township. In concession III, lot 4, Henwood township, a diamond drill hole, after passing through a

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vertical thickness of 1,050 feet of Lorrain quartzite and arkose, intersected 750 feet of Firstbrook bedded greywacke ; the hole was stopped before entering the underlying Coleman formation. In Gillies Limit, some 3/4 of a mile east of Latchford, a drill hole passed through 720 feet of Firstbrook bedded greywacke before reaching the underlying Coleman conglomerate. The Firstbrook formation is well exposed in the townships southwest and northwest of Cobalt, an over-all distance of at least 40 miles; the extension in an east-west direction is not known. This indicates that the sediments were laid down in a large body of water. A gradational contact with the overlying Lorrain formation is shown by some interbedding of quartzite and arkose with the bedded greywacke. East and southeast of Cobalt the Firstbrook formation is absent ; the Lorrain, where present, rests either on the Coleman or directly on the basement rocks. LORRAIN FORMATION The Lorrain formation consists of quartzite and arkose, varying from white to sea green in colour and commonly showing crossbedding. Formations of the Cobalt series overlying the Lorrain, as described by Ouirke and Collins (1930) on the north shore of Lake Huron, are not known in the vicinity of Cobalt. The maximum thickness of the Lorrain proved near Cobalt is 1,050 feet in Kenwood township; the original thickness is unknown. NIPISSING DIABASE The name Nipissing diabase was given by Miller (1913) to the massive unaltered basic intrusive which generally exhibits gently dipping contacts in the vicinity of Cobalt. Map 155A of the Geological Survey of Canada shows that it is widespread throughout the Timiskaming Silver-Cobalt area. Largely because of the gentle dips, which are usually under 30 degrees, the intrusive has been called a sill or sill-like body. However, the concordance of the contacts with bedding surfaces or a surface of unconformity is not even approximate, and it does not seem possible that such surfaces could have been important in guiding the intrusive into its present position near Cobalt. Intrusion would seem to have taken place within a range of some 1,000 feet of the Cobalt series-Basement rock contact. It seems reasonable to assume that the relationship between the position of the sill and this contact is due to its depth from surface at the time of intrusion. In other words, the depth from surface at which the diabase intrusive spread laterally was where its hydrostatic pressure was approximately equal to the pressure due to the overlying column of rock. The regularity of dip of the diabase contacts is in places changed to steep or vertical attitudes. If those occurrences where this is due to post-diabase faulting are disregarded, these interruptions, locally known as "rolls," are believed to be due to diabase magma having followed pre-diabase faults. At

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the Beaver and Timiskaming mines, concession III, lot 1, Coleman township, such a "roll" has effected the displacement of the upper contact by some 300 feet. In places, the whole diabase body has a nearly vertical attitude, which is accompanied by a diminution in thickness ; that is, the Nipissing diabase assumes a dyke-like form. Such departures from the usual sill-like form were probably influenced by pre-Nipissing faults. There is an absence of igneous breccias near Cobalt such as occur near Sudbury in association with the norite. The original shape of the diabase, if reasonable assumptions of the positions of those parts removed by erosion are made, appears to have been one sheet which covered the whole area. This is shown best by the areal mapping. One objective of a diamond drill hole, 1,500 feet long, put down in 1920 from the 500-foot level of the Crown Reserve Mines, was to test the possibility of another sheet. This hole is mentioned by Knight (1924). The contacts of the diabase at surface are somewhat elliptical in shape and can be related to what may be termed basins and arches. In some of the older reconnaissance mapping, as, for example, in Lorrain township, the elliptical outlines of basin and arch structures were not completely determined but it seems likely to the writer that these shapes will be found to be characteristic in the whole Timiskaming Silver-Cobalt area. The sizes of the basin areas show great variation. The New Lake Basin, 4 miles southeast of Cobalt, is 4 miles by 2 miles in surface dimension within the inner and upper contact of the diabase; the vertical depth to the diabase at the deepest part of the basin is some 800 feet. The origin of these basinand-dome shapes is of interest and importance. The hypothesis that these were formed by deformation after solidification of the sill does not seem to be correct; at least the nearly vertical parts of the basins could not have been formed in this way. To date the orifices from which the diabase magma spread have not been located. It does not appear to be feasible to regard the diabase of the Timiskaming Silver-Cobalt area as originating at one place and travelling the very long distances required to reach the periphery of the area. Local orifices for each basin is a much more plausible explanation of the basin shapes and of the distribution of the varieties of diabase throughout the intrusive. It has been known for a long time that the Nipissing diabase is not of uniform mineralogical or chemical composition. Studies of the mineralogical variation have been made by Satterly (1928), Phemister (1928), and Hriskevich (1952). There is a range in composition from varieties made up largely of olivine, hypersthene, augite, and labadorite to others of quartz, albite, biotite, and hornblende with plentiful quartz-feldspar intergrowths. The variation is not uniform and takes place both laterally and vertically. Differences in the composition of the original magma would seem to have been of greater importance than differentiation after injection. More study is required before an answer can be given to the question of the causes of this variation.

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KEWEENAWAN DIABASE The youngest group of Proterozoic rocks in the vicinity of Cobalt are a few dykes which cut through, and have chilled edges against, the Nipissing diabase. Some have the composition of olivine-diabase, others are quartzdiabase. STRUCTURE In the immediate vicinity of Cobalt there is no definite evidence of any folding in the Cobalt series. Disturbance in the attitude of the beds seems to be largely due to faulting. The surface of the basement complex on which the Cobalt series was laid down had broad, trough-like, depressions. Miller's (1913) interpretation of these as ancient erosional features appears to be correct, although Whitman (1922) regarded them as synclines due to folding. Cobalt lies in the Timiskaming subprovince of the Canadian Shield and the adjacent Grenville subprovince is about 25 miles to the southeast. The general strike of the boundary between them is northeasterly. Johnston ( 1954) published a careful study of a part of this boundary and concluded that it is an eastward-dipping fault zone, due to northwesterly-southeasterly directed forces. Miller, in his early work at Cobalt, noted the linear elongation of many of the water courses, some in a northeasterly others in a northwesterly direction. He suggested that these were due to erosion along faults. Johnston (1954) relates these faults to the northeasterly-southwesterly directed forces which gave rise to the faulting at the boundary of the Grenville and Timiskaming subprovinces. Some of the faults with northeasterly trend in the vicinity of Cobalt are in sets which are arcuate upwards in section and arcuate in plan, with the concave side towards the southeast. Examples of this type are the related Cobalt Lake and Contact faults, and some in the east side of the New Lake basin. The occurrence of this kind of fault suggests the action of forces directed from the boundary between the subprovinces. REFERENCES COLLINS. W. H. (1917). Onaping map area; Geol. Surv., Can., Mem. 95, p. 63. HRISKEVICH, M. E. (1952). Petrology of the Nipissing diabase sheet of the Cobalt area, Ontario; unpublished Ph.D. dissertation, Princeton Univ. JOHNSTON, W. G. Q. (1954). Geology of the Temiskaming-Grenville contact southeast of Lake Temagami, Northern Ontario, Canada; Bull. Geol. Soc. Am., vol. 65. pp. 1048-1074. KNIGHT, C. W. (1924). Cobalt and South Lorrain silver areas; Ont. Bur. Mines, vol. 31, pt. 2, p. 107. MILLER, W. G. (1904). Cobalt nickel arsenides and silver; Ont. Bur. Mines, vol. 13, pt. 1, pp. 96-103. (1905). The cobalt nickel arsenides and silver deposits of Temiskaming; Ont. Bur. Mines, vol. 14, pt. 2. (1913). The cobalt-nickel arsenides and silver deposits of Temiskaming, 4th edit.; Ont. Bur. Mines, vol. 19, pt. 2. PHEMISTER. T. C. (1928). A comparison of the Keweenawan sill rocks of Sudbury and Cobalt, Ontario; Trans. Roy. Soc. Can., Ser. III. vol. 22, Sec. IV, pt. 11, pp. 121-198.

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QUIRKE, T. T., and COLLINS, W. H. (1930). The disappearance of the Huronian; Geol. Surv., Canada., Mem. 160, p. 21. REID, J. A. (1943). Mineral resources of the Temiskaming Silver-Cobalt area; Ont. Dept. Mines, Bull. no. 134. SATTERLY, JACK (1928). The Nipissing diabase of Cobalt, South Lorrain and Gowganda, Ontario; unpublished M.A. thesis, Univ. of Toronto. WHITMAN, A. R. (1922). Genesis of the ores of the Cobalt district, Ontario, Canada; Univ. of California Publications, Bull. Dept. Geological Sciences, vol. 13, no. 7, p. 257.

THE PROTERO2OIC OF THE MATACHEWANWANAPITEI-TEMAGAMI AREA James E. Thomson, F.R.S.C. AN AREA of about 4,000 square miles, bounded roughly by Matachevvan, Wanapitei Lake, Temagami Lake, and Lake Timiskaming, is covered by rocks of Proterozoic age. Our geological information concerning much of this country comes from reconnaissance surveys; more detailed studies have been made in the vicinity of mining operations such as those at Cobalt, South Lorraine, Gowganda, and Matachewan. The sedimentary rocks belong entirely to the Cobalt series and are thus of Huronian age. The lower division is the Gowganda formation, consisting of conglomerate, greywacke, and quartzite. It is considered by Collins (1917, p. 10) to be of glacial or frigid-climate origin. The Gowganda formation is overlain by the Lorrain quartzite and arkose which has interbeds of quartz and jasper conglomerate. It is believed to be a normal subaqueous deposit. The Lorrain is overlain by banded cherty quartzite and white quartzite. In the Onaping map-area Collins (1917, p. 10) estimated the total thickness of the series to vary from 2,000 to 6,000 feet. In some parts of the country only a few hundred feet have escaped erosion. Everywhere the Cobalt series lies with profound unconformity upon the basement complex. In some localities the Gowganda formation was deposited on a surface of considerable relief (Hopper, 1942, p. 389), so that erosion has produced outliers of Cobalt sediments and exposed patches of basement rocks within the boundaries of the main basin. The strata are only gently folded and little metamorphosed except in the vicinity of strong faults. Dips of beds rarely exceed 15 degrees in some areas and 30 degrees in others. However, in the Onaping map-area Collins (1917, p. 10) noted that folding increased steadily in intensity going in a southwesterly direction until 70- to 90-degree dips are found along a northwest-southeast axis of folding. He suspects the locus of this disturbance lies to the southwest. In 1956, the writer found slightly overturned strata (probably Cobalt series) at the former Milnet nickel mine in Parkin township. These are along a "front" adjacent to a large area of granitization. Strike faulting, probably thrustal, is found at the mine. Detailed geological studies along this "front" are urgently needed. 46

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Both Cobalt series and pre-Huronian rocks are intruded by quartz-diabase sills and dykes, generally believed to be Keweenawan in age. The dykes are narrow but the sills sometimes outcrop over large areas. The intrusion seems to have taken place tranquilly and was followed by slight faulting. The general contact zone of the Cobalt series with the great terrain of granite and gneiss of the Grenville subprovince, often referred to as the "Grenville front," appears to have been the locus of great overthrust faulting. This was first noted by Todd (1925, pp. 29-30) in the Matabitchuan area. Recently, more detailed studies by Johnston (1954) have confirmed the presence of a great fault system along the Grenville front southeast of Lake Temagami. This may be a northeastern extension of a fault system along and near the Grenville front reported by Quirke and Collins (1930, pp. 32, 105) near Killarney on Georgian Bay and by Cooke (1946, pp. 13-18) south of Conisten. The Grenville front may be, as Johnston (1954, p. 1047) states, "a major tectonic feature of the earth's crust," and deserves a great deal of additional study. Studies by T. C. Phemister along the "Grenville front" southeast of Sudbury in 1956 for the Ontario Department of Mines showed that it is a well-defined intrusive and metamorphic contact followed by faulting that crosses it at a low angle. Certain parts of the "front" are thus intrusive, others, for example at Wanapitei, are faulted contacts. The percentage of true granite on the Grenville side is very small; the bulk of the rock is a metamorphic complex. A detailed report is being prepared. REFERENCES COOKE, H. C. (1946). Problems of Sudbury geology; Gcol. Surv.. Can., Bull. no. 3. COLLINS. W. H. (1917). Onaping map area: Gcol. Surv., Can.. Mem. 95. HOPPER. C. H. (1942). Geology of the Matachewan Consolidated Mine; Can. Inst. Min. Met., vol. 45. JOHNSTON, W. G. Q. (1954). Geology of the Temiskaming-Grenville contact southeast of Lake Temagami, Northern Ontario, Canada; Bull. Geol. Soc. Am., vol. 65, no. 11, pp. 1047-1073. QUIRKE, T. T., and COLLINS, W. H. (1930). The disappearance of the Huronian; Geol. Surv., Can., Mem. 160. TODD, E. W. (1925). The Matabitchuan area; Ont. Dept. Mines, vol. 34, pt. 3.

THE QUESTIONABLE PROTEROZOIC ROCKS OF THE SUDBURY-ESPANOLA AREA James E. Thomson, F.R.S.C. ACCORDING TO THE LAKE HURON SHEET (Map 155A) of the Geological Survey of Canada, rock formations of Proterozoic age in the general vicinity of Sudbury and Española are represented by the Huronian (Bruce and Cobalt series), Whitewater series, acid intrusives of Killarnean age, and basic intrusives of Keweenawan age. This is the interpretation of W. H. Collins and his associates based upon very extensive field work. However, investigations by other geologists indicate a considerable divergence of opinion on many phases of the structure, stratigraphy, and correlation. The various issues concern the rocks classified by Collins as Huronian, Whitewater series, and intrusives that cut these strata. The problems of correlation in each of Collins' age groups are summarized below. HURONIAN In the Blind River map-area the Lower Huronian (Bruce series) was found to lie with profound angular unconformity upon the basement complex (Collins, 1925, pp. 15-17). Going eastward towards Sudbury, Collins ( 1925, p. 57) correlated a part of the sedimentary sequence with the Bruce series although he admitted that the great unconformity with the preHuronian could not be found. All other investigators are in agreement that no unconformity can be seen in the volcanic-sedimentary sequence throughout the Sudbury-Espanola area. Instead there is an upward transition from volcanic rocks, correlated with the Keewatin or Archean by most geologists, including Collins, through the Sudbury series, Bruce series, and Cobalt series. The Sudbury series has been interpreted as pre-Huronian by Collins (1936, pp. 1680-81), Coleman (1914, pp. 205, 211), Knight (1917, pp. 105-106) and Burrows and Rickaby (1934, pp. 12-13), and as Huronian by Fairbairn (1941, p. 7) and Cooke (1946, p. 33). Cooke explained the absence of the great unconformity between pre-Huronian and Huronian rocks by placing a major fault along the contact. However, recent detailed studies by Phemister (1956) and the writer (Thomson, 1957) have shown that this assumed faulting is non-existent. It therefore remains an established fact that the great unconformity which separates the Early and Late Pre48

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FIOURE 1. Sketch map showing the extent of the questionable Proterozoic rocks (stippled) in the Sudbury-Espanola area.

Cambrian rocks at Blind River and Cobalt has not been found at Sudbury. Studies near Española by the writer (Thomson, 1952, pp. 13-18) have shown the situation to be similar to that at Sudbury and here, too, no unconformity has been found. If the volcanics of the Sudbury and Española areas formed during the Keewatin period, then the sedimentary sequence must be entirely pre-Huronian. This is because Collins (1925, p. 15) confined the Huronian to strata lying above a profound unconformity and intrusives into them. Much of our confusion arises from Collins' failure to apply his own definition as he worked eastward from the "Original Huronian" towards Sudbury. Interest in Precambrian stratigraphy has been greatly accelerated by the discovery of large tonnages of uranium mineralization in a well-defined stratigraphie horizon near the base of the Huronian at Blind River and in the Quirke Lake trough. It is now known that the detailed stratigraphy and structure in these localities is different in many respects from that in the Sudbury-Espanola district. In addition, great zones of thrust faulting occur along the north shore of Lake Huron and continue eastward through Sudbury. A great deal of study will be necessary here before the effect of complex faulting and folding upon stratigraphy is known. From personal observations at Sudbury, Española, Blind River and Quirke Lake, the writer suspects that two great sedimentary groups of entirely different age could occur along this belt, the older being in the Sudbury-Espanola district. East of Sudbury a similar problem arises. From Cobalt westward towards Wanapitei Lake the Cobalt series is found to lie with great angular unconformity upon the pre-Huronian volcanics, sediments, granites, and so on. Somewhere southeast of Wanapitei Lake a sudden change to Sudbury type of stratigraphy takes place. Collins (1937, p. 1444) was aware of this and stated that a large angular unconformity, probably 45 to 50 degrees,

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occurred east of Wanapitei Lake between the Cobalt series and Bruce series. Fairbairn (1939, p. 7) could only find a minor unconformity in the Ashigami Lake area. However, in Pardo and Janes townships, Bruce (1932, p. 13 ) found a marked unconformity between the Cobalt series and quartzite which he thought might be Sudburian in age although Collins classified it as Mississagi on Map 155A of the Geological Survey of Canada. It is now known that Mississagi and Sudburian quartzite are interbedded and indistinguishable elsewhere so that it does not really matter to which period the quartzite is assigned. Here, then, may be the great unconformity separating the Huronian to the north and east from the pre-Huronian to the west. The geology of Wanapitei area deserves further detailed study because it is another key locality in Prccambrian stratigraphy and structure. This paragraph is appended following field work near Lake Wanapitei by the writer in 1956. Immediately north and east of Skead station an extensive area of greatly deformed volcanic and sedimentary rocks, intruded by granite, was found. Lying on its northern flank and extending around the shores of Lake Wanapitci are undefonned quartzite and grit similar lithologically to the Mississagi quartzite of the Blind River district. This may mark the great unconformity. Attempts to trace it southeasterly to the "Grcnville front" and find a visible unconformity with older sediments are, so far, unsatisfactory, due to basic intrusives and drift cover. Probable northwesterly-striking faults may also complicate the tracing of the unconformity. Further work is planned. If this should be the long-missing unconformity, the bulk of the sedimentary formations in the SudburyEspanola area would be pre-Huronian and outside the scope of this symposium. WHITEWATER SERIES This consists of a volcanic group (Onaping formation) and sedimentary group (Onwatin and Chelmsford formations). These are completely isolated by the Sudbury nickel irruptive, so correlation has always been little more than a guess. Coleman (1905, pp. 10, 94), Collins (1937, p. 1433) and Cooke (1946, p. 50) assign the Whitewater series to the Late Precambrian and separate it from the Archean by an assumed unconformity of great magnitude along which the nickel irruptive was injected as a sill. Although Cooke (1946, p. 68) estimates the erosion of 30,000 feet of rocks before the Whitewater series was laid down, there is no visible evidence of this unconformity. The assumption that the sill would always follow the unconformity is quite unwarranted. For example, at Cobalt where the unconformity between Early and Late Precambrian rocks is well marked, the diabase sill follows a completely independent course. Also at Sudbury it is now known that the concept of a spoon-shaped sill is incorrect insofar as the southeast corner of the basin is concerned. In the vicinity of the Falconbridge mine it is a vertical composite dyke and this is not an abnormal condition due to marginal faulting of the irruptive.

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Lithologically the Whitewater series has no possible correlative among the formations that are of definite Proterozoic age such as the Animikean and Keweenawan of Lake Superior and the Huronian of Blind River or Cobalt. On the other hand, it has many of the features of volcanism, lithology and structure in common with the volcanics and sediments found immediately outside the irruptive. The writer and his associates, T. C. Phemister and Howel Williams, have recently restudied the Whitewater series in relation to the extra-irruptive strata. In forthcoming reports of the Ontario Department of Mines the Onaping tuffs of the Whitewater series and the Stobie formation (Keewatin of W. H. Collins) are correlated and assigned to the base of the stratigraphie succession at Sudbury. These volcanics have the lithological characteristics of rocks classified as Keewatin in age elsewhere in the Precambrian Shield. It is believed that the Sudbury basin was formed as a large volcano-tectonic depression during one period of volcanism and that the sedimentary rocks of the Whitewater series were formed in a closed basin, thus accounting for their differences in lithology from their extra-irruptive time equivalents. A more detailed account of the Whitewater series is published elsewhere (Thomson, 1956). ACID AND BASIC INTRUSIVES Acid intrusives in the Sudbury area consist of very large batholiths of granite and "granitic complex," and smaller bodies such as the Creighton and Murray granite, the micropegmatite of the nickel irruptive and the so-called Copper Cliff rhyolite. Heretofore a large part of the granitic intrusives has been dated as Killarnean. Basic intrusives are dykes and sills of diabase, gabbro and norite; these are cut by late olivine diabase dykes. The diabase, gabbro and norite have been dated as late Keweenawan on the assumption that they intrude Huronian strata. Lack of proof that the Bruce and Whitewater series are Proterozoic in age weakens this correlation and opens the possibility that the basic intrusives and large granitic areas may be Archean rather than Keweenawan or Killarnean. Of course, basic intrusives (Nipissing diabase) cut the Cobalt series from Wanapitei Lake to Cobalt, and if the basic intrusives are intruded by the Killarney metamorphic complex, as Collins (1937, p. 1445) states, then all the basic intrusives and metamorphic complex are post-Cobalt series and are Proterozoic in age. Studies by Phemister southeast of Sudbury in 1956 would indicate that this is correct. However, "older gabbro" (Haileyburian) occurs in northeastern Ontario (Miller and Knight, 1920, p. 235) and there is no valid reason why it could not occur in the Sudbury country if pre-Huronian rocks exist there. A granite intruding "older gabbro" could still be Archean in age. Accurate radioactivity dating seems to offer the only possibility for the final solution of these problems, providing it can be demonstrated that this dating gives the age of the rock rather then the age of its metamorphism.

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STRUCTURE It is generally admitted that rock deformation is a poor and often dangerous criterion for determining age relationships. However, the contrast in deformation between the relatively undeformed strata of the wellestablished Proterozoic areas and all the formations at Sudbury is very striking. The Proterozoic strata may become steeply inclined and sheared adjacent to major thrust faults but this deformation disappears outside of their zone of influence. On the other hand the strata in the SudburyEspanola area are everywhere complexly folded and faulted. Their orogeny closely resembles the Archean (Keewatin-Timiskaming) type found farther north in the Shield. Also the over-all grade of metamorphism of the true Proterozoic rocks nowhere approaches that of the Sudbury-Espanola strata. No large bodies of granite intrude the unquestioned Proterozoic rocks of Cobalt and Blind River as they do in the case of the area under discussion. Although these criteria, taken separately or collectively, do not constitute proof, they do harmonize with other data pointing to the Early Precambrian age of the volcanics and sediments along the Sudbury-Espanola belt. CONCLUSIONS The absence of a great unconformity, such as occurs at Blind River and Cobalt, makes it impossible to establish a satisfactory division of the formations in the Sudbury-Espanola area into an Early and Late Precambrian sequence. No solution, short of accurate rock dating, is possible until a time-stratigraphic horizon common to all the areas concerned is found and agreed upon. Using the volcanics as a common denominator, and arbitrarily calling them Keewatin, would automatically throw all the sedimentary group at Sudbury and Española into the Early Precambrian. A Keewatin age for such volcanics is the assumption often made elsewhere in the past whenever time-stratigraphic terms, such as Keewatin and Huronian, have been applied throughout the Precambrian Shield, although it is not scientifically sound. Therefore, it is important to point out that there is no field evidence to prove Proterozoic age of any rock formation in the Sudbury-Espanola area. In this respect it differs from Cobalt, Blind River, north shore of Lake Superior, and other localities where the unconformity is unquestioned. Because of these problems, the writer (Thomson, 1953, p. 66) has advocated a moratorium on current time-stratigraphic terminology throughout the Sudbury-Espanola area until detailed restudies have been made over a considerable part of the country. New maps of the Geological Branch of the Ontario Department of Mines show the fundamental relationships of rock units but avoid questionable time-stratigraphic names that must be constantly redefined as new data are accumulated. When the picture is sufficiently clarified, compilation maps can be issued to show the timestratigraphic interpretation. The confusion in Precambrian correlation brings out the necessity of

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finding a more exact method of age determination for igneous and sedimentary rocks in the district. Dating research is now being done on some of the rock types at Sudbury by H. W. Fairbairn and his associates at the Massachusetts Institute of Technology but no results have yet been reported. In 1956, L. T. Aldrich, G. L. Davis and G. W. Weatherill of the Carnegie Institution of Washington collected rock samples at Sudbury for age determination research. The dating of sulphides or pegmatites will not provide a solution to most of these problems because these were introduced much too late in the geological chronology. However, it is important that fundamental dating research on rocks be encouraged and intensified. The most obvious conclusion is the necessity for systematic and detailed field study of the vast terrain from Cobalt through Sudbury and westward towards Sault Ste. Marie. It must be remembered that most of our geological information comes from reconnaissance-type surveys. The solution of many of the problems of stratigraphy and correlation will conic from this field work although others of considerable magnitude will probably arise. The fact that this is by far the most important mineral-producing belt in Canada, and that it will hold this position far into the future, makes it imperative that this work be given a high priority and that it be carried on as expeditiously as possible. REFERENCES BRUCE, E. L. (1932). Geology of the townships of Janes, McNish, Pardo, and Dana; Ont. Dcpt. Mines, vol. 41. pt. 4, pp. 1-26. BURROWS. A. G., and RICKAIJY. H. C. (1934). Sudbury nickel field restudicd; Ont. Dept. Mines, vol. 43, pt. 2. COLEMAN, A. P. (1905). The Sudbury nickel field: Ont. Bur. Mines, vol. 14, pt. 3. (1914). The prc-Cambrian Rocks north of Lake Huron; Ont. Bur. Mines, vol. 23, pt. 1. pp. 204-236. COLLINS. W. H. ( 1 9 2 5 ) . North shore of Lake Huron: Geol. Surv., Can., Mem. 143. (1936). The Sudbury scries; Bull. Gcol. Sor. Am., vol. 47, no. 11, pp. 1675-1684. ( 1 9 3 7 ) . Timiskarning sub-province; Bull. Gcol. Soc. Am., vol. 48, no. 10, pp. 1427-1458. COOKE, H. C. (1946). Problems of Sudbury geology; Geol. Surv., Can., Bull. no. 3. FAIRBAIRN-, H. W. (1939). Geology of the Ashigami Lake area; Ont. Dept. Mines, vol. 48, pt. 10, pp. 1-15. (1941). The relation of the Sudbury scries to the Bruce series in the vicinity of Sudbury; Ont. Dcpt. Mines, vol. 50, pt. 6. KNIGHT, C. W. ( 1 9 1 7 ) . Report of the Royal Ontario Nickel Commission, pp. 105-211. MILLER, W. G., and KNIGHT, C. W. (1920). Haileyburian intrusive rocks; Ont. Dept. Mines, vol. 29, pt. 1, pp. 235-36. PHEMISTER, T. C. (in press). The Copper Cliff rhyolite; Ont. Dept. Mines, vol. 65, pt. 3. THOMSON, J. E. (1952). Geology of Baldwin township; Ont. Dept. Mines, vol. 61, pt. 4. (1953). Problems of Precambrian stratigraphy west of Sudbury; Trans. Roy. Soc. Can., Ser. Ill, vol. 47, Sec. IV, pp. 61-70. (in press). Geology of the Sudbury basin area; Ont. Dept. Mines, vol. 65, pt. 3. (in press). Geology of Falconbridge township; Ont. Dept. Mines.

STRATIGRAPHY, QUIRKE LAKE-ELLIOT LAKE SECTOR, BLIND RIVER AREA, ONTARIO* S. M. Roscoe IN THE QUIRKE LAKE-ELLIOT LAKE SECTOR of the Blind River area, about twenty-five miles northeast of the town of Blind River, a belt of Protcrozoic rocks is preserved within a shallow syncline which plunges gently towards the west. This belt, commonly referred to as the Quirke Lake trough, is about nine miles wide and contains a thickness of about 5,000 feet of Proterozoic strata in its central part. All of the important uranium deposits discovered to date in the Blind River area, with the exception of Pronto, are in this sector. These deposits are in lenses of quartz pebble conglomerate near or at the base of the sequence of Proterozoic strata. The Proterozoic sedimentary rocks of the north shore of Lake Huron region were divided by Collins into a lower, Bruce scries and an upper series which he correlated with the Cobalt series and which he believed to overlie the Bruce series unconformably. He divided the Bruce series, from bottom to top, into : the Mississagi formation—mainly quartzite ; the Bruce boulder conglomerate; the Española formation—limestone and greywacke; and the Serpent quartzite formation. Type localities were not established for these formations, but Quirke Lake was one of the most important localities where details in the succession were studied. Numerous drill holes have now provided much more detailed information on the succession than was obtainable from the original surface mapping. It is desirable to redefine some of the stratigraphie units in the light of these new data. The importance of stratigraphie correlations in exploration for uranium ore makes it very necessary that certain natural formational units in the lower part of the sequence be formally recognized and named. Figure 1 shows the stratigraphie sequence, variations in the sequence between the north and south sides of the belt, and a proposed new system of nomenclature. The sequence shows a cyclic repetition of boulder conglomerate layers, each overlain by fine grained sedimentary rocks which are in turn overlain by coarse-grained clastic sedimentary rocks. The bases of the boulder conglomerate layers are the sharpest of sedimentary contacts 'Published by permission of the Acting Deputy Minister, Department of Mines and Technical Surveys, Ottawa. 54

Figure 1. Stratigraphie sequence and fades variations, Qulrka Lake- Elliot Lake

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and the assemblages of formations between these particular contacts form natural groups which must have genetic significance. The most important change in terminology here proposed is that the Mississagi unit be elevated from formational rank to group rank and that its base be defined to be the base of the lowermost boulder conglomerate. Newly defined formational units are tentatively given names of lakes where there are prominent outcrops of these formations. It is suggested, however, that the type sections for the various formations be taken from representative drill hole intersections and that an effort be made to preserve the type core sections. PRE-HURONIAN ROCKS The pre-Huronian rocks, where overlain by Proterozoic formations, are principally metavolcanic rocks invaded by granodiorite and other granitic rocks. These basement rocks are overlain with unquestionable unconformity by the Proterozoic sedimentary rocks. The pre-Huronian rocks immediately beneath the unconformity are altered in most places. The altered zones show gradations upwards from normal basement rocks to sericitic residual deposits or paleosols. This residuum is thickest—locally up to fifty feet or more thick —where it overlies granitic rocks. GROUP A—ELLIOT GROUP The oldest group of Proterozoic rocks, tentatively called the Elliot group, includes formations which unconformably overlie pre-Huronian rocks and underlie the lowermost boulder conglomerate. The group is up to 950 feet thick along the south flank of the syncline but thins in a northerly direction and is absent in most places along the north flank. The Matinenda formation comprises the basal and major part of the Elliot group. It is composed of coarse-grained, poorly bedded clastic rocks, including quartz grit, feldspathic quartzite, arkose, and quartz pebble conglomerate; locally, pebbles and cobbles of greenstone and, more rarely, of granite are present in quartz pebble conglomerate at or near the base of the formation, constituting a basal conglomerate. The formation shows pronounced local variations in thickness as well as a general regional thickening from north to south (0 to 700 feet). The local variations in thickness are believed to reflect the original topography of the pre-Huronian surface. The thicker parts can thus be interpreted as representing filled valleys, while adjacent thinner portions overlie hills and ridges on the buried pre-Huronian surface. Isopach maps show these valley and ridges to have a southeasterly trend. Torrential crossbedding, seen on all outcrops of the formation, shows dips which were originally southeast to east (prior to folding). The formation is evidently of fluvial origin, deposited by streams which flowed in a southeasterly direction. The Matinenda formation is distinctly radioactive. The radioactivity, apparently due mainly to monazite and zircon, is highest in coarse-grained

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pyrite-bearing beds. Closely packed quartz pebble conglomerate is particularly pyritic and radioactive with the radioactivity commonly due to disseminated high grade uranium minerals—"brannerite," uraninite, and "thucolite"—as well as to thorium in the more ubiquitous monazite and zircon. The thickest, coarsest-grained, most closely packed, and most uraniferous conglomerate beds are found in relatively thick parts of the formation, that is, within or overlying pre-Huronian "valleys." Two such "valley" structures contain most of the uranium ore deposits discovered to date in the area. The Nordic formation consists of interbedded subgreywacke, quartzite, greywacke, and argillite. The contact between it and the underlying Matinenda formation is gradational. The two formations interfinger through a transitional zone which is up to one hundred feet thick. The Nordic formation is up to 350 feet thick in the southern part of the area; it is absent in the north. It was apparently deposited in deep water at the same time as coarser clastic sediments were being laid down farther north and thus represents a deep-water faciès of the upper part of the Matinenda formation. GROUP B—MISSISSAGI GROUP Group B—the Mississagi group—includes all formations between the base of the lowermost boulder conglomerate layer and the base of the Bruce conglomerate. The group thickens from north to south from a minimum of 650 feet at Ouirke Lake to about 2,300 feet in the south part of the sector. It is subdivisible into a lower formation—the Whiskey formation of interbedded argillite, siltstone, and greywacke—and an upper formation—the Ten Mile formation, which consists mainly of feldspathic quartzite. The contact between the two is gradational. The Whiskey formation has a very distinctive and continuous basal member of conglomeratic greywacke, which contains sparsely distributed, rounded to angular pebbles, cobbles, and boulders of granite, greenstone, and other rocks. In the southern part of the area it overlies the Nordic formation; farther north it overlies the Matinenda formation; along the north shore of Ouirke Lake it directly overlies the basement rocks. This polymictic conglomerate is very similar to the Bruce conglomerate and to conglomerate in the Gowganda formation. It varies in thickness from a few inches to about 200 feet. The argillite-greywacke member of the Whiskey formation thickens southward from less than 100 feet, north of Dunlop Lake, to about 700 feet near Elliot Lake. The Ten Mile formation is very similar to the Matinenda formation but is finer grained and contains only a few thin beds of quartz pebble conglomerate near its base and also near its top. The formation thickens southward from a minimum of 550 feet at Quirke Lake to about 1,700 feet near Elliot Lake. Grain size decreases from north to south. Torrential crossbedding is abundant throughout the formation and has attitudes which

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indicate that the direction of sedimentary transport was from northwest to southeast. GROUP C—QUIRKE GROUP The Quirke group includes the Bruce conglomerate, the Española formation, and the Serpent formation. Descriptions of these formations by Collins ( 1925 ) have been verified in detail by recent work and will not be repeated. This group contains an extremely distinctive sequence of rocks. Almost identical rock sequences are found in other widely separated parts of the north shore of Lake Huron region. Lithological correlations of these rocks with the Quirke group can be made with considerable confidence. GROUP D—DUNLOP GROUP In order to maintain consistency in the system of nomenclature proposed here, the Gowganda formation together with quartzite formations that overlie the Gowganda in other areas are tentatively called the Dunlop group, rather than the Cobalt series. The Gowganda formation is a heterogenous assemblage of polymictic conglomerate, greywacke, siltstone, arkose, and quartzite. The conglomerate is characteristically poorly sorted and unstratified, like the Bruce and Whiskey conglomerates. The relations of the Gowganda formation to underlying formations of the Quirke group appear to be contradictory, in some places suggesting disconformity, elsewhere conformity. The Quirke group, where it underlies the Gowganda formation at Quirke Lake, is about 1,800 feet thick, but near Elliot Lake it is only 125 feet thick. Nearly 1,700 feet of strata present at Quirke Lake, including the Serpent quartzite and all but the basal section of the Española formation, are missing at Elliot Lake. Collins concluded that this 1,700 feet of strata were removed by erosion prior to deposition of the Gowganda formation, and that an important unconformity therefore existed between the Bruce and Cobalt series. Between Quirke Lake and Elliot Lake and in several other places, the contact between the Serpent formation and the Gowganda formation is gradational. Granite pebble conglomerate and quartzite are interlayered through a zone several hundred feet thick. It is difficult, if not impossible, to draw any definite boundary between the two formations, much less to place an unconformity between them. This relationship suggests an interfingering contact rather than an unconformity and is incompatible with the concept of a regional erosional break between the Bruce series and the Cobalt series. Uplift and erosion of the Quirke group may have taken place locally prior to deposition of the Gowganda formation, but it also seems possible that sedimentation was essentially continuous over the whole area and that

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the section apparently missing at Elliot Lake (or at least a large part of it) was never deposited there. It is hoped that further studies will resolve this problem. SUMMARY Numerous deep drill holes in townships 143, 144, 149, and 150 provide much data on the sedimentary sequence and on variations in thickness and in faciès of Huronian rocks underlying the Gowganda formation. The direction of transport of the clastic sediments, as shown by thickening of the sequence, diminution of grain size, and original dip of crossbedding, was from the northwest towards the southeast. The lowermost unit, which contains uraniferous quartz pebble conglomerate, is considered to have been laid down as an alluvial deposit. Evidence for a major erosional unconformity between the Bruce series and the Cobalt series is inconclusive. REFERENCE COLLINS, W. H. (1925). North shore of Lake Huron: Gcol. Surv., Can., Mc-m. 143, pp. 45-56.

THE NORTH SHORE OF LAKE HURON FROM GLADSTONE TO SPRAGGE TOWNSHIPS E. M. Abraham THE ORIGINAL HURONIAN DISTRICT together with the interval between it and the Sudbury district is a strip of country 125 miles long and 4,000 square miles in extent along the north shore of Lake Huron. This report deals with only a small portion of this district, and covers a length of about 40 miles from Gladstone to Spragge townships. The recent discovery of uranium-bearing minerals in a mineralized quartz pebble conglomerate at or near the base of the Bruce series has led to a renewed interest in the Original Huronian and adjacent districts. GENERAL GEOLOGY The bedrock formations of the area are all Precambrian in age and consist of sedimentary, intrusive, and metamorphic rocks. They may be subdivided into three main structural units, easily distinguished from one another. These three units are, from oldest to youngest : ( 1 ) A pre-Huronian floor or basement, consisting of granite, granite gneiss and allied types and altered basic rocks in various proportions. (2) A thick succession of Huronian sediments resting unconformably on the planated pre-Huronian basement. These sediments are mainly quartzites, conglomerates, argillites and related rock types. In this particular area, the sediments that lie on the basement rocks are all considered Huronian in age, although Collins ( 1925 ) puts a part of them into the Sudbury series of pre-Huronian age. (3) An intrusive group composed largely of basic rocks which occur as sills and dykes within the basement and sedimentary groups. The rocks within this group are considered Keweenawan in age. The age of a group of metamorphic, intrusive and sedimentary-volcanic rocks lying south of a major fault zone in Long and Spragge townships is still in doubt. They may be the metamorphic equivalents of the Huronian and Keweenawan rocks or they may be pre-Huronian basement rocks thrust upward and now lying in juxtaposition with Huronian rocks to the north. 59

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ARCHEAN The Archean rocks include all those belonging to the basement group. They are the oldest rocks in the area and consist of granite and granite gneiss with various amounts of older basic volcanics and sediments. This planated basement group represents the root of a mountain system which was worn down by erosion prior to deposition of the Huronian sediments. In Archean times batholiths of granite and allied rock types intruded a crust of volcanic and sedimentary rocks. This period of intrusion was accompanied by mountain building, and followed by a long interval of erosion during which the mountains were worn down and large areas of granite and granite gneiss exposed. All that remained of the original crust of volcanic and sedimentary rocks were infolded remnants. PROTEROZOIC The Proterozoic rocks in the area include those belonging to the Huronian and Keweenawan periods. The Huronian rocks in this area have been subdivided into the Bruce and Cobalt series, which in turn are broken down into formations: Cobalt scries HURONIAN

Lorrain formation Gowganda formation

Unconformity Bruce series

(?)

Upper Mississagi formation Middle Mississagi formation Lower Mississagi formation

With detailed mapping some of the above formations can be further subdivided into members. HURONIAN The mantle of Huronian sediments is several thousand feet thick in places and rests in profound unconformity on the planated pre-Huronian basement. In addition to this unconformity, an erosional interval of lesser magnitude exists between the Cobalt and Bruce series. The Bruce series in the area under discussion has been tentatively broken down into three formations. The Lower Mississagi formation is a feldspathic quartzite with minor amounts of grit and quartz pebble conglomerate. The quartz pebble conglomerate varies in thickness from an inch or so to several feet and occurs at varying horizons. The most important of these horizons occurs at or near the base of the formation, but it is not necessarily present. Occasionally the conglomerate is heavily pyritized and uraniferous to the degree that it makes ore. The feldspathic quartzite is often well crossbedded and ripple marked. The Middle Mississagi formation is predominantly an argillite-greywacke series of rocks with minor amounts of conglomerate near the base of the

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formation. In the western part of the area this formation is 500 feet in thickness, but thins eastward. The Upper Mississagi formation is a feldspathic quartzite and contains narrow beds of slightly radioactive pea-sized conglomerate beds of noneconomic significance. It is well crossbedded in places. The Cobalt series in the area is represented by the Gowganda formation. This formation is a heterogeneous assemblage of conglomerate, impure quartzite, shale and greywacke, and minor amounts of limestone. The various rock-types are often repeated at different horizons, and where they are steeply dipping, conglomerate and quartzite beds can be traced for several thousand feet or even miles. In Striker and Scarfe townships a limestone near the base of the Gowganda formation was traced for several miles in a northwesterly direction. This limestone was underlain by conglomerate which in turn rested on the Bruce series of rocks. The limestone and underlying conglomerate may belong to this Bruce series of rocks. No rocks belonging to the Lorrain formation occur in the area under discussion. KEWEEXAWAN The Keweenawan rocks in the area consist mainly of diabase, gabbro, and diorite, as sills, dykes and irregular bodies. Lithologically the rocks are massive, dark coloured, and medium to coarse grained, with minor amounts of primary quartz or olivine. A large diabase mass in Patton township is characterized by a granitic phase near the top. This phase may represent a differentiated part of the basaltic magma which produced the diabase. STRUCTURE The sedimentary rocks of Huronian age are flat lying to steeply dipping and represent the nose and south limb of a broad westerly-plunging anticline. The nose of this anticline lies in the northwest part of the area, and it is here that the sediments are flat lying. Moving southward and eastward the sediments steepen considerably until they are steeply dipping to vertical. East-trending faults traverse the entire area. The largest and best defined of these is known as the Murray fault. It has, associated with it, a number of smaller, somewhat parallel, thrust faults. At the west end of the area, in the vicinity of Dean and Bright lakes, the pre-Huronian basement granite and granite gneiss on the south side of the Murray fault, has been thrust over the Cobalt sediments to the north. The Cobalt sediments, as they approach the zone of faulting to the south, dip south at angles increasing from 10 degrees to vertical. These sediments, for several hundred feet north of the Murray fault, are severely crumpled into small anticlines and synclines. In contrast, the granite gneiss shows little or no apparent change towards the fault. Moving eastward, the course of the fault is indicated by the juxtaposition of Mississagi quartzite on the south with Cobalt sediments on the north. Again, the Cobalt sediments, as they approach the fault, are immensely deformed. The Murray fault has an almost vertical dip, and in

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places it is marked by a narrow depression only a few feet wide and elsewhere by a zone of intense shearing and brecciation several hundred feet wide. That there has been recurrent movement along this major fault is indicated by the development of both S- and Z-shaped drag-folds. Some movement took place in late Precambrian times because the Keweenawan diabase has been intensely sheared, in places, within the zone of faulting. Associated with the Murray fault are other smaller and somewhat parallel thrusts which are highly significant. The most important of these is a low-angle thrust on the Pronto Uranium Mines property on which the pre-Huronian basement appears to have moved from the south over the Lower Mississagi ore-bearing member. In addition to the thrust faulting, major and minor northeast- and northwest-trending faults traverse the entire area. These faults offset the thrust faults and are therefore later in age. Economic Geology Most of the prospecting in the area has been confined to the search for gold, copper and uranium. The most important deposits so far found are uranium ores in a quartz pebble conglomerate near the base of the Lower Mississagi formation. SUMMARY The area is underlain by rocks ranging in age from Keweenawan to pre-Huronian. The sedimentary rocks are flat lying to steeply dipping, and represent the nose and south limb of a broad westerly plunging anticline. Easterly trending faults traverse the area. The most important of these is the Murray fault, which in Bright and Thompson townships has faulted the pre-Huronian basement granite over the Cobalt sediments. North of the fault zone the Cobalt sediments have been intensely folded and crenulated. Similar but smaller parallel thrusts also exist. Later northeast- and northwest-trending faults transect the entire area. REFERENCE COLLINS, W. H. (1925). North shore of Lake Huron; Geol. Surv., Can., Mem. 143, p. 17.

THE PROTEROZOIC OF THE ORIGINAL HURONIAN James E. Thomson, F.R.S.C. A LOCALITY NEAR BRUCE MINES that was much visited by geologists toward the close of the last century is known as the Original Huronian area. In 1914 Collins (pp. 6 and 26) described the section at Bruce Mines and substituted the names Cobalt series and Bruce series for Upper and Lower Huronian. Collins' final ideas on the Original Huronian were published in 1937 but in this paper he carried the discussion as far east as Cobalt, thus enlarging the boundaries of the Original Huronian from a local to a regional unit. UNCONFORMITIES Apparently the first major angular and erosional unconformity between the Huronian and pre-Huronian was found on an islet in Lake Huron, 3 miles east of Thessalon. Collins (1937, p. 1437) and his associates claim to have found a conglomerate at the base of the Huronian at intervals from here to Sudbury where it was known as the Ramsay Lake conglomerate. This is the base of Collins' Bruce series, which attains a thickness of from 1,400 to over 10,000 feet. The Bruce series is overlain by the Cobalt series. From Sault Ste. Marie to Española Collins found no perceptible angular unconformity between the Bruce and Cobalt series but evidence of erosional unconformity was accumulated. East of Lake Wanapitei Collins (1937, p. 1444) estimated an angular unconformity of 45-50 degrees between the Bruce and Cobalt series but later surveys by Fairbairn (1939, p. 7) failed to confirm this although a minor unconformity is admitted. A few miles farther east Bruce (1932, p. 13) obtained some evidence of a major unconformity but thought this was between the Cobalt series and the Sudbury series (pre-Huronian). According to Collins (1937, p. 1444) the lowest member of the Cobalt series is the Gowganda formation. It is believed to be of glacial origin and from 3,000 to 4,000 feet in thickness. It is succeeded by the Lorrain quartzite, apparently 5,000-6,000 feet in thickness, then by 700 feet of finely stratified cherty quartzite. As stated by the writer elsewhere in this symposium, A. P. Coleman, W. H. Collins and H. C. Cooke believed that the Whitewater series at 63

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Sudbury overlies the Huronian with profound unconformity. Collins, then, visualized three major unconformities, namely ( 1 ) between Huronian and pre-Huronian, (2) between Huronian and Whitewater series, and (3) between Huronian and Paleozoic. A minor unconformity was thought to separate the Bruce and Cobalt series. In the writer's opinion, Collins made an unwarranted assumption that the unconformity between Huronian and pre-Huronian, which is well marked at Thessalon and northeastward to the Quirke Lake trough, continued eastward into the Espanola-Sudbury country. The unconformity near Thessalon, now found as far east as the Pronto mine at Lauzon Lake, is at least 50 miles west of the Ramsay Lake conglomerate at Española where conformable relations exist throughout the entire volcanic-sedimentary sequence. Collins (1925, p. 57) admitted the lack of unconformity in the Espanola-Sudbury country. Therefore, according to his own definition that the Huronian lies above a profound unconformity, he should have assigned the sedimentary formations to the pre-Huronian. The country between Española and Lauzon Lake is little known geologically and probably contains an answer to the conflicting relationships. However, there is no proof at present that the sediments of the Bruce Mines-Quirke Lake belt are coeval with those of the Espanola-Sudbury belt and there is some suggestion that they are of greatly different ages. The great change in geological relationships apparently lies in the 20-mile gap between the Pronto mine and the mouth of the Spanish River. In this symposium Abraham has pointed out that a major fault in Long and Spragge townships separates the Huronian from metamorphosed intrusive, sedimentary and volcanic rocks to the south. The major fault system passing through the gap may have brought strata of vastly different age into juxtaposition. Near the shaft of Pater Uranium Mines, at Spragge, there are altered amygdaloidal lavas similar to those found in Baldwin township (Española district) and in the Stobie formation at Sudbury. In this vicinity it would appear that pre-Huronian rocks have been thrust against true Huronian strata. The writer's (Thomson, 1956) views on the assumed unconformity between the Whitewater series and the so-called Huronian have been stated elsewhere. It is his belief that no major unconformity has been found anywhere in the Sudbury country. Field work at Lake Wanapitei in 1956, described elsewhere in this volume, suggests that the great unconformity at the base of the Proterozoic may occur there. CONCLUSIONS A detailed restudy of the original Huronian at Bruce Mines is the first requirement if rock formations along the north shore of Lake Huron must be correlated with it for historical reasons. At present exact time-stratigraphic correlation of other areas with the Original Huronian is almost impossible because no accurately measured type section can be referred to. It is true that Collins (1914, pp. 6-9) has described a type section at Bruce

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Mines but it should be more accurately located, the detailed structure should be obtained and the faciès changes within the various members should be studied. In short, detailed modern mapping techniques should be used. The writer's impression, gained from hasty visits to the Bruce Mines area, is that drift cover and lack of drilling information may make it a much less satisfactory standard section of the Huronian than others that might be selected. Would it not be more logical to relocate a type section of the Huronian in some such locality as the Quirke Lake-Elliot Lake trough where so much information is now being made available? REFERENCES BRUCE, E. L. (1932). Geology of the townships of Janes, McNish, Pardo, and Dana; Ont. Dept. Mines, vol. 41, pt. 4, pp. 1-28. COLEMAN, A. P. (1905). The Sudbury nickel region; Ont. Bur. Mines, vol. 14, pt. 3, pp. 1-188. COLLINS, W. H. (1914). The Huronian formations of Timiskaming region, Canada; Gcol. Surv., Can., Bull. no. 8. (1925). North shore of Lake Huron; Geol. Surv., Can., Mem. 143. (1937). Timiskaming sub-province; Bull. Geol. Soc. Am., vol. 48, no. 10, pp. 1427-1458. FAIRBAIRN, H. W. (1939). Geology of the Ashigami Lake area; Ont. Dept Mines, vol. 48, pt. 10, pp. 1-15. THOMSON, JAS. E. (in press). Geology of the Sudbury basin area; Ont. Dept. Mines, vol. 65, pt. 3.

THE PROTEROZOIC OF THE MAMAINSE POINT AREA James E. Thomson, F.R.S.C. THIS is THE LARGEST AREA of Kcwecnawan rocks on the east coast of Lake Superior. A brief description is merited because it is the only locality where a detailed study of the Keweenawan has been made to date ( 1 ). An apparent thickness of between 15,000 and 17,000 feet of interbedded lavas and sediments dips towards Lake Superior. The average inclination is about 30 degrees, from a range of 20 to 48 degrees. Volcanic rocks predominate in the stratigraphie sequence and consist of regular basaltic flows up to 200 feet in thickness. Many layers of boulder conglomerate are interbedded with the basalt; the thickest of these would have a maximum true width of 1,800 feet, although this would include some interbedded flows. The strata are intersected by dykes and small bodies of felsite and felsite breccia, some of which arc considerably deformed. Unfortunately, the base of the Keweenawan series is largely drift-covered so that the relationship to the underlying plutonic and volcanic rocks is obscure; it is thought to be unconformable. Along the shore of Lake Superior small patches of Lake Superior sandstone lie with profound unconformity on the Keweenawan flows. The sandstone has been traced at intervals around the lake to Michigan, where it is classified as Upper Cambrian by the Michigan Geological Survey ( 2 ) . The Keweenawan strata at Mamainse form a broad arcuate anticline pitching towards Lake Superior. They are intersected by a system of normal faults with a radial pattern which is related to the anticline. The faults dip about normal to the inclination of the strata. Copper mineralization occurs along some of the faults ; it consists largely of chalcocite with minor amounts of chalcopyrite and native copper. The interbedded basalt and conglomerate would correlate nicely with the middle Keweenawan of the Lake Superior basin ( 3 ). REFERENCES (1) THOMSON, J. E. (1953). Geology of the Mamainse Point copper area; Ont. Dept. Mines, vol. 62, pt. 4. (2) Geological map of Michigan; Geol. Surv., Mich., 1916, Publication no. 23. (3) LEITH, C. K., LUND, R. J., and LEITH, A. (1935). Pre-Cambrian rocks of the Lake Superior region; U.S. Geol. Surv., Prof. Paper 184, p. 10. 66

THE PROTEROZOIC

OF THE PORT ARTHUR

AND LAKE NIPIGON REGIONS, ONTARIO W. W. Moorhouse THE ROCKS which are generally recognized as Proterozoic in this area outcrop principally in a narrow belt extending from the vicinity of Schreiber to the Minnesota border, and beyond. These rocks, including sediments, intrusive sills and lavas, all are gently dipping, dips being everywhere towards the south. The diabase sills form resistant features of the landscape, appearing as cuestas and mesas. Except for some of the Keweenawan sandstones and dolomites, the sediments generally occupy the valleys between these ridges. Consequently, continuous sections of the sediments are rarely obtainable, and for much of the area, the only extensive outcrops are along stream channels. ARCHEAN ROCKS In the Port Arthur region, the pre-Animikie rocks have been termed by Tanton (1931, p. 18) the "schist complex." It consists of highly folded greenstones, with some tuffaceous sediments, which have been intruded by granites and related rocks. In the Lake Nipigon area, sediments of the Timiskaming type (Windigokan sediments) also constitute an important phase of the Archean. After mountain building and intrusion, these rocks were extensively eroded, and the Proterozoic sediments (in the Thunder Bay area) have been deposited on a surprisingly flat surface which dips gently south. It seems likely that this is a surface of marine planation resulting TABLE OF FORMATIONS PLEISTOCENE: Till, lake and stream deposits KEWEENAWAN: Diabase sills and dykes Intrusive contact Osier series (volcanics) Sibley series (sediments) Erosion interval ANIMIKIE: Rove slates Gunflint iron-formation Kakabeka conglomerate Great unconformity ARCHEAN: Granite Greenstone 67

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FIGURE 1. Sketch map of Port Arthur-Lake Nipigon area, showing distribution of the principal areas of Proterozoic rocks. Scale: 1 inch to 35 miles.

from a progressive overlap of the foreland, although Goodwin (1953, p. 11) suggests that it is a pediment-like surface. ANIMIKIE Kakabeka Formation The Kakabeka formation is for the most part a gravelly conglomerate, usually only a few inches thick, but in the type locality attaining a thickness

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of 4-5 feet. It consists predominantly of pebbles of quartz, chert, jasper, granite, and so on, with a coarse sandy matrix. For the most part it reposes on relatively unweathered granite and greenstone. North of Amethyst harbour, however, the granite basement is intensely chloritized for one-half to one inch below the thin conglomerate layer. The Kakabeka formation is usually correlated with the Pokegama quartzite on the Mesabi Range. Since it was evidently formed during a period of progressive marine overlap, it seems doubtful if the two are time-equivalent. Gunflint Iron-Formation The lithology of the Gunflint has been very fully described by Gill ( 1924) and Goodwin (1953), and only a brief summary is possible here. It is noteworthy that throughout the area there appear to be considerable changes in faciès, but owing to the scarcity of continuous outcrop, the full extent of these changes cannot be given as precisely as one would like. The rocks of the Gunflint are extremely varied, but consist of mixtures of a few fundamental types. These are listed below : 1. Chert and jasper: may occur as fairly massive, structureless beds, generally not over a foot or two thick; also as algal concretions and layers, of variable size and dimensions. 2. "Taconite" : beds, usually lenticular, composed mainly of granules, a millimeter or more in diameter, the granules consisting most commonly of chert, but also of iron oxides, carbonates, silicates, and various mixtures of these, in a matrix of chert and/or carbonate. 3. Carbonate, as distinct, fairly persistent beds, of variable grain, fine to coarse, and variable thickness. The carbonate may be siderite, ankerite, dolomite or calcite. 4. Thin-bedded carbonate-chert, consisting of alternating, generally thin layers of chert and carbonate, characterized by minute spherulitic or spheroidal structures different from the granules of the taconite. 5. Thin-bedded black argillite and tuff. 6. Intraformational breccias and conglomerates, usually found in carbonate-rich beds or layers. A thin lava flow occurs near the middle of the Gunflint on Mink Mountain, near Whitefish Lake, west of Fort William. The Gunflint in Ontario differs from many other Proterozoic iron formations in the relative insignificance of minnesotaite, stilpnomelane and other silicates. They occur locally in small amounts, but rarely if ever constitute a significant proportion of any bed or lens in the formation. In the western part of the area, the Gunflint is divided into two divisions by Goodwin (1953, p. 7), which he terms the Lower and Upper respectively. The Lower is divided by Gill ( 1924, p. 44) into three units: Division 1, which comprises algal chert, chert and taconite, is 2-25 feet thick; Division 2, the "Intermediate slate" of Gill, which is a soft, black, fissile, pyritic argillite, in part tuffaceous, 4-20 feet thick; Division 3, which con-

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sists of greenalite taconite, magnetite taconite and cherty taconite, amounting to 150-210 feet. Goodwin's Upper Gunflint comprises Gill's Division 4 (algal chert and cherty taconite and conglomerate, 8-20 feet thick) and Division 5, comprising jasper, shale, taconite and black cherty limestone to a thickness of 200-380 feet. Fairly complete sections are exposed in this part of the area, and further information is provided by a number of drill holes. The only other area where a reasonably complete section can be inferred is in the Loon Lake area, at the east end of the main Animikie belt. In this area three main divisions are evident in the Gunflint iron-formation. The lower division here comprises a complex of layers and lenticles of chert, ferruginous carbonate, cherty taconite, magnetite-hematite taconite and local thin algal layers. Drill records indicate a thickness of approximately 30^0 feet. This is overlain by argillite, with some tuffaceous and carbonate-rich beds; the argillite appears from drill records to range from 30 to 100 feet. Exposures are generally poor, so that complete sections have nowhere been measured in the outcrop. This argillite appears to have been a horizon favourable to the intrusion of diabase sills, as far west as Port Arthur. The upper part of the Gunflint is composed of thin-bedded, brown-weathering carbonate and chert. The beds are very persistent and uniform, except that locally (as described below) they have been intensely folded; in the cores of the more compressed folds the carbonate has flowed, becoming essentially massive, the chert remaining as rectangular plates and slabs randomly oriented in the carbonate. In a random 5-foot section of the bedded material, it was found that the carbonate beds averaged an inch in width, and ranged from ^4 to 5 inches, while the chert beds averaged ^4 mcn in width and ranged between l/& and 2l/2 inches. There are roughly 40 per cent chert and 60 per cent carbonate beds in this section. The individual beds are themselves delicately laminated in some cases, and a certain amount of chert is present in many carbonate layers, and vice versa. Assays indicate that in this rock the iron is rather low, ranging from 4 to 17 per cent, and that lime and magnesia are rather high. The carbonate appears therefore to be a ferrodolomite or ankerite, rather than siderite. The total thickness of this unit is not known, but from drill records and measurements in the outcrop, it is over 100 feet thick. In the vicinity of Port Arthur and Fort William, the relationships of the Animikie sediments to the above sections is not clear. According to Tanton ( 1931, p. 27), at the Shuniah mine, in the northern outskirts of Port Arthur, a nearly complete section of the Gunflint was disclosed by underground work. In the Current River to the north and east, there are extensive but interrupted outcrops of argillite and ferruginous carbonate, apparently overlying a series of cherts and carbonates. It seems likely that the argillites here exposed constitute the "Intermediate slate" and that the overlying carbonates represent the chert-carbonate facies of Loon Lake. In Mariday Park, and nearby, in Port Arthur itself, are a series of isolated outcrops of limestone, reaching an aggregate thickness of nearly 20 feet. These limestones

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are grouped by Tanton (1931, p. 30) in the Lower Cherty member of the Gunflint; Goodwin (1953, p. 60) considers them to be equivalent to the limestone at the top of the Gunflint in the western sections described above. The presence of what appears to be tuffaceous material in the upper part of the limestone sections suggests the possibility of correlating these limestones with carbonate-rich tuffs in the Kakabeka section. The relations of these different occurrences are complicated not only by their isolated character but also by the presence of many minor faults, several of which are evident in the Current River section. Unfortunately there is little clue to the magnitude or sense of the displacements along these faults. West of Port Arthur and Fort William, outcrops consist chiefly of taconites, with considerable carbonate and chert in the lower part of the section. Argillites are only locally exposed, beneath diabase sills, and in one or two quarries and streams. The most complete section is exposed at Kakabeka Falls. Here, over 100 feet of black argillite, with interbeds of ferruginous carbonate and chert, contains near the base 4 to 8 feet of tuff in a carbonate matrix. Downstream from the falls, the upper part of this argillite section appears to change gradually to a section of thin-bedded carbonate and chert with shaly interbeds. This is the most striking example of faciès change observed in the iron-formation. The argillite is believed to be equivalent to the Intermediate slate further east; the presence of tuffaceous material is the principal reason for this correlation. Between the Kaministikwia River and the western part of the Gunflint Range, the principal outcrops are greenalite and chert taconites, with some carbonate and argillaceous members. Most of the outcrops appear to belong to the Upper Gunflint (Goodwin, 1953, p. 45) ; the lower members do not outcrop until the village of Hillside is reached, near Whitefish Lake. Rove Slate The Rove slate has been examined by the writer in the Pass Lake area, at the east end of the main Gunflint area, and along Slate Creek, just south of Fort William. Extensive exposures are also visible on Mount McKay, and below many of the diabase sills which form such a prominent feature of the area. The Rove slate is a predominantly argillaceous formation, generally black and generally very thinly and evenly bedded. Some red and green shaly argillites are exposed near Pass Lake. Layers of dark-coloured limestone and greywacke are also present. Magnificent concretions varying from a few inches to 8 feet in diameter are exposed at a number of places, in road metal quarries near Pass Lake and in Slate Creek. These have been described in some detail by Tanton (1931, p. 40), and a description of very unusual and abundant concretions in the Slate Creek section is being prepared by the writer. The Rove slate may be as much as 2,000 feet thick in the Mount McKay area (Tanton, 1931, p. 36); in the Pass Lake-Loon Lake area it is overlapped by the Keweenawan sediments, having been completely removed by pre-Keweenawan erosion near the northern boundary of the

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Proterozoic. Grout and Broderick (1933) state that the Rove is not over 3,000 feet thick, in Minnesota. KEWEENAWAN Sibley Formation The Sibley formation overlaps the Animikie, and in the Loon Lake area is found overlying the Archean. It has been examined by the writer chiefly in the Pass Lake-Loon Lake area; other details are therefore taken from Tanton's report. Tanton (1931, p. 50) divides the Sibley into six members, which are briefly summarized below : A Member. This unit, which is not continuous throughout the area, is a conglomerate (0-8 feet thick). It contains pebbles, boulders and fragments of the underlying Animikie and Archean rocks, in a matrix which is locally sandy, locally carbonate. It is particularly well exposed in railroad cuts east of Loon Lake. E Member. This member, about 60 feet thick, consists predominantly of sandstone, with some limestone and red argillaceous material. The sandstone is locally rather crumbly and poorly cemented. C Member. A white quartz sandstone, rather massively bedded, which forms the crest of a prominent scarp from Silver lake to Silver Islet landing. It is similar to the sandstone phase of the B Member, and is 40 feet thick. D Member. Consisting of thin interbeds of chert and limestone, this member is said by Tanton (1931, p. 53) to be rather widespread although only about 2 feet thick. E Member. The rocks of this member seen by the writer are red to maroon coloured, massive to shaly bedded, often mottled with patches, streaks and small round spots of buff or greenish colour. They are described by Tanton (1931, p. 54) as "mudstones" but the examples examined by the writer appear to be iron-stained, fine-grained carbonates, containing varying proportions of sand and silt grains, chiefly of quartz and feldspar. They form rather high hills around Silver Lake, and are easily visible from highway 17 just southwest of the town of Nipigon. Tanton (1931, p. 54) assigns a thickness of 50-350 feet to this unit. F Member. Evidently a phase of unit E, this unit Tanton (1931, p. 55) describes as grey, gritty sandstone and red, fine-grained fragmentai rock. The Sibley formation is gently dipping to horizontal in attitude, and is separated from the underlying Animikie by a considerable erosion period, since at various places it is in contact with Rove slates, the chert-carbonate member of the Gunflint, the lower cherty member of the Gunflint and the Archean. Although regionally there is very little difference in dip between the Animikie and the Sibley, locally there is a considerable angular discordance. Sediments similar in character to the Sibley extend along the Lake Superior shore and adjacent islands as far as Schreiber, and extend inland to the south and west shores of Lake Nipigon. As described by A. W. G. Wilson (1910), the Keweenawan sediments of the Lake Nipigon area are

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very similar to those of the Sibley, comprising conglomerate (4-6 feet), grits and sandstones (150 feet) and shales and dolomite (400 feet). The shales and dolomites probably correspond to the rocks described under E Member above, except that white or light-coloured dolomites appear to be more prevalent in the northern extremities of the area than do the red- and maroon-coloured types so typical of the Sibley peninsula. Osier Formation Overlying the Sibley with disconformity is a series of conglomerates, crossbedded sandstones and mudstones, totalling about 50 feet, followed by a series of basic lavas, often vesicular, and volcanic fragmentais. Logan (1863) estimated the total thickness of these lavas to be 6,000 to 10,000 feet. This section has not been studied by the writer. INTRUSIVE ROCKS Keweenawan Intrusive Rocks Dykes, sheets and sills of diabase are widespread in the Port Arthur-Lake Nipigon area. The sills and sheets, which are commonly known as the Logan sills, form the most prominent features of the topography and exhibit a number of interesting features which have been described most recently by Blackadar ( 1954). They vary in thickness from a few feet to 200 feet in the Port Arthur region, but in the Lake Nipigon area they attain thicknesses of at least 500 feet. The thin sills, such as the 15-foot lower sill on Mount McKay, are rather dense and fine grained. The sills of intermediate thickness (e.g. up to 200 feet) are characterized by a diabasic texture, which is often rather variable within a single sill. The thick Nipigon sheets are more gabbroic in texture. Locally, as for instance in a quarry for road metal just east of Port Arthur, the diabase is porphyritic, containing large phenocrysts of plagioclase. Also, in the same area, the diabase (near the top of the sheet) has a peculiar porphyritic appearance and contains grains of quartz. This phase owes its curious aspect to assimilation of granite. The larger grains of quartz and plagioclase of the granite remain as xenocrysts in the diabase, while the rest of the quartz and feldspar has become remelted, appearing as rather abundant granophyre. Further evidence of the assimilation of granite is found in the lamprophyre-like hybrid phases developed in the diabase near a large granite block included in the diabase, on an island off Silver Harbour, and in the occurrence of fragments of granite in chilled diabase at the base of the sill along the shore line to the northeast. Similar inclusions of fragments in the base of a diabase sheet in the Beardmore area have been ascribed by Peach (personal communication) to the engulfment of unconsolidated gravel over which the sheet was moving. Locally where slates are intruded by and overlie the diabase, small amounts of granophyre have been formed, which intrude the slate in a very intricate way, and contain many partly replaced inclusions of the slate. There seems little doubt that the sheets and sills in the Port Arthur region are all intrusive. Many years ago, Wilson suggested that the large sheets in

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the Lake Nipigon area were surface flows, in view of the general lack of any overlying rock. The evidence from mining in the Beardmore area, which shows the existence of a sheet cutting the steeply folded Archean at depth indicates that intrusive sheets do exist in the area, and warrants the suspicion that all are intrusive. Dykes, which cut all Archean and Proterozoic rocks in the area, are generally normal, rather fine-grained diabase. An exception is a curious graphitic dyke on Silver Islet. STRUCTURE The Animikie and Keweenawan sediments and volcanics were evidently accumulated in a foreland area, on a platform of highly folded, intruded and subsequently peneplaned Archean rocks. In this location they have suffered only mild warping, and nowhere exhibit important folding. They generally have a gentle southerly dip. However, certain phases of the iron formation, in particular the thin-bedded carbonate-chert phase, have undergone very local folding of a rather complex kind, which, so far as evidence is available, is not shared by formations below or above them. This folding has locally been sufficiently intense to result in the destruction of bedding and the brecciation of the chert layers, as noted earlier. It appears as if this folding must have taken place after the rocks were completely consolidated, since the chert fragments are perfectly angular, and, although fragmented, are not bent or distorted. It is probable that this deformation is connected with the intrusion of a diabase sill at least 30 feet thick which underlies these outcrops. These folds cannot be tectonic, since the structures are so localized, and are restricted to this unit of the section. Faulting is widespread in the Port Arthur-Fort William area. The faults appear to be largely steep-dipping, normal faults, which locally (e.g., in the Pass Lake-Blende Lake area) suggest a block-faulted pattern. The number of faults which have been recognized in mapping suggests that, if exposures were better, a very extensive pattern of intersecting faults would be evident. The faults strike in two major directions, one east to northeast, the other north to northeast, although some northwesterly strikes were observed. The faults are often filled with breccias cemented by vein quartz, which in some cases is amethystine. Some lead-zinc mineralization locally occurs in these structures, and many prospect pits have been opened on them. In some cases, as noted by Tanton (1931, p. 43), diabase dykes have intruded these faults. Displacements can rarely be determined with any precision. A fault

FIGURE 2. Sketch of folded Animikie (chert-carbonate type) exposed in railway cut east of Port Arthur. Rectangular pattern represents brecciated phase in axes of folds. Approximate scale: 1 inch to 1 chain.

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in the Shuniah mine near Port Arthur is reported to have a displacement of about 100 feet. Gill (1924, 40c) reports a displacement of 300 feet in the North Lake area. It is doubtful if most of the faults mapped have displacements of greater magitnude. There is little evidence of horizontal displacements, and in most cases slickensides, where observable, are nearly vertical. In conclusion, it must be mentioned that the preservation of the Animikie and Keweenawan sediments and volcanics on the North Shore of Lake Superior and in the Lake Nipigon region is due to the fact that these two large bodies of water are areas of crustal subsidence. This subsidence may have been facilitated by faulting of the sort described above. A normal fault has been inferred by Martin to define the northwest shore of Lake Superior, and Keweenaw fault on the south shore is well known. Others are known or inferred, around the Lake Superior basin. CORRELATION The Animikie and Keweenawan rocks of the Lake Superior basin are entirely separated from other areas of Proterozoic formations. The Animikie of the Port Arthur-Fort William area has been traced into the Gunflint area of Minnesota, and the correlation of these rocks with the Mesabi Range seems well established. These are further correlated with the Middle and Upper Huronian of the American geologists, as represented in Michigan and Wisconsin. The usage of the term Huronian for these rocks is to be deplored, but is so thoroughly embedded in the American literature that it can now only be endured. The questionable lithological similarity of the Lower Huronian of Michigan and the Bruce series of Ontario is not supported by any direct field evidence known to the writer. It is tempting to suggest that the iron-formation of the Animikie is of the same age as that of the Belcher Islands and the Labrador Trough. However, it is rather unlikely that they were deposited in a continuous marine basin, and in the absence of direct geological evidence of contemporaneity, it is preferable to assume that they were formed in similar environments which were not necessarily contemporaneous. The dating of these various sequences of similar rocks must await geophysical evidence. ENVIRONMENT OF DEPOSITION OF PROTEROZOIC SEDIMENTS The Animikie sediments may be divided into two main faciès, a precipitated faciès, largely free of clastic terrigenous material, and a terrigenous faciès, represented by the intermediate slate and the Rove formation. The precipitated faciès, in turn, may be roughly subdivided into a shallow-water or reworked faciès, and a (relatively) deep-water faciès. The shallow-water faciès is characterized by well-developed granules textures, crossbedding, lenticular and wavy bedding, intraformational breccias; the deep-water faciès by thin-bedded chert-carbonate rocks. The waters in which these rocks were deposited appear to have been marine or brackish, the boron

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content of the argillaceous types being appropriate to sediments deposited in brackish water (B. A. Bradshaw, personal communication). The sedimentary features and structure of the area are most typical of foreland deposition; it is probable that the thicker sections of Michigan and Wisconsin more closely approach a géosynclinal environment. The Keweenawan sediments of the north shore of Lake Superior and the Nipigon basin also appear to be foreland deposits. Certain interesting conclusions arise out of the features exhibited by these rocks : ( 1 ) The presence of anthraxolite in the Gunflint formation, organic material in some of the black argillites, and algal concretions, indicate that marine conditions were favourable to the existence of organisms. (2) The occurrence in the Gunflint of primary granules of hematite and magnetite is indicative of the presence of free oxygen in the shallow-water environment. (3) The available evidence points to a composition of sea water similar to that of the present day. (4) The Gunflint iron-formation appears to have been deposited under relatively normal conditions, in the absence of abundant terrigenous material. The unusual concentrations of iron and silica have been attributed to volcanic sources (Van Rise and Leith, 1911, p. 513; Goodwin, 1953, p. 66; Aldrich, 1929) and to the products of secular weathering (Gruner, 1922; Moore and Maynard, 1929, p. 524; James, 1954, p. 276; White, 1954, p. 50). Investigations of this important question as regards the Gunflint are continuing. REFERENCES ALDRICH, H. R. (1929). The geology of the Gogebic Iron Range of Wisconsin; Wisconsin Geol. and Nat. History Surv., Bull. 71. BLACKADAR, R. G. (1954). Differentiation and assimilation in the Logan sills; unpublished Ph.D. thesis, Univ. of Toronto. GILL, J. E. (1924). Gunflint iron-bearing formation; Geol. Surv., Can., Sum. Rept., pt. C, pp. 28-88. GOODWIN, A. M. (1953). The stratigraphy of the Gunflint iron-bearing formation of Ontario; unpublished Ph.D. thesis, Univ. of Wisconsin. GROUT, F. F., and SCHWARTZ, G. M. (1933). Rove formation; Minn. Geol. Surv., Bull. 24. GRUNER, J. W. (1922). The origin of the sedimentary iron-formations: the Biwabik formation of the Mesabi Range; Econ. Geol., vol. 17, pp. 407-460. HARCOURT, G. A. (1938). The southwestern part of the Schreiber area; Ont. Dept. Mines, vol. 47, pt. 9. JAMES, HAROLD L. (1954). Sedimentary faciès of iron-formation; Econ. Geol., vol. 49, pp. 235-294. LOGAN, W. (1863). Geology of Canada, p. 70. MOORE, E. S., and MAYNARD, J. E. (1929). The solution transportation and precipitation of iron and silica; Econ. Geol., vol. 24, pp. 272-303, 365-402, 506-527. TANTON, T. L. (1931). Fort William and Port Arthur, and Thunder Cape map areas, Thunder Bay district, Ont.; Geol. Surv., Can., Mem. 167. VAN HISE, C. R., and LEITH, C. K. (1911). The geology of the Lake Superior region; U.S. Geol. Surv., Monograph, vol. 52. WILSON, A. W. G. (1910). Geology of the Nipigon basin, Ont.; Geol. Surv., Can., Mem. 1.

QUESTIONABLE PROTEROZOIC ROCKS OF MANITOBA G. H. Charlewood and J. F. Davies TWO SMALL AREAS in Manitoba contain rocks of possible Proterozoic age. These are the Churchill quartzite and the San Antonio formation of the Rice Lake area. CHURCHILL QUARTZITE In the vicinity of Churchill, Manitoba, at the mouth of the Churchill River, there are outcrops of greenish-grey quartzite. The rock is essentially made up of 70 per cent fairly well-rounded quartz grains and 30 per cent sericite. There are two main sizes of quartz grains, the larger from 0.5 mm. to 0.8 mm. in diameter and the smaller from 0.07 to 0.1 mm. The sericite is secondary and appears to have an arrangement parallel to the bedding. Well-rounded pebbles of white quartzite, up to 3 inches in diameter, are scattered irregularly through the formation. Williams (1948) substantiated J. B. TyrrelFs observations of southeast dips west of the river and found a persistent southwest dip of 70 to 80 degrees east of the river. He suggests that, if the structure is not overturned, the lagoon and harbour represent a southward-plunging syncline. The Churchill quartzite is overlain by Ordovician sediments to the south but no observed contact with older Precambrian rock is recorded. J. B. Tyrrell thought it might be an altered phase of the Athabasca sandstone. SAN ANTONIO FORMATION The San Antonio formation, composed of feldspathic quartzite with a basal conglomerate, rests unconformably upon the Rice Lake group and on quartz diorite and granite which intrude the Rice Lake group. The formation covers an area of approximately 10 square miles in the vicinity of Rice Lake, southeastern Manitoba. The total thickness of the formation is about 3,000 feet. The unconformable contact between the basal conglomerate and granite and quartz diorite can be observed in two places. At both localities the conglomerate is less than 300 feet thick. It consists of sub-angular to rounded boulders of quartz diorite, and a few of vein quartz and lavas, set in a matrix of quartz, feldspar, and chlorite. The pebbles and boulders range in size from a few inches to several feet across. The quartz diorite boulders are identical with 77

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the quartz diorite upon which the conglomerate lies. Stringers of vein quartz in the quartz diorite terminate abruptly against the conglomerate at the unconformable contact. The feldspathic quartzite which makes up the bulk of the formation consists of sub-angular quartz and common microcline and plagioclase grains in a matrix of quartz, feldspar, and sericite. A few lenses of conglomerate and a very few silty beds are present throughout the quartzite. No volcanic or calcareous rocks are present in the San Antonio formation. Normal bedding is not well developed but crossbedding is common in the quartzite. Lithology and primary structures indicate that this formation probably represents a piedmont type of deposit. The San Antonio rocks outcrop in the form of an S-shaped fold. The south half of the S represents a syncline whose north limit is overturned. Dips along the south limb range from 25 to 55 degrees north and the beds face north. On the north limb the beds face south and dip more steeply, 45 to 70 degrees north. Top, strike, and dip determinations are less plentiful for the north half of the S but limited evidence suggests that this may represent an anticline. This simple pattern of an anticline and syncline may actually be complicated by minor folds. In two places intense shearing is present along the contact between the Rice Lake group and San Antonio formation. The rocks of the San Antonio formation are not known to be invaded by any intrusive rocks. However, a few small barren or weakly pyritic quartz stringers are present. Metamorphism is weak to moderate. The Rice Lake group consists of a lower volcanic series with some tuffaceous sediments, and an upper quartzose sedimentary series. No unconformity between the two can be observed. The sequence lower Rice Lake volcanics—upper Rice Lake sedimentsintrusive granite and quartz diorite-San Antonio formation, with an unconformity between the San Antonio formation and intrusive rocks, is comparable to the sequence Keewatin-Timiskaming-Algoman-Huronian. In this sense the San Antonio formation may be considered Proterozoic. REFERENCE WILLIAMS, M. Y. (1948). The geological history of Churchill, Manitoba; Western Miner, June 1948.

PROTEROZOIC

ROCKS OF THE NORTHWEST

TERRITORIES AND SASKATCHEWAN* I. C. Brown and G. M. Wright THE DEFINITION of Proterozoic rocks used in this paper is the same as that given by Dr. Harrison in his introductory paper. The area covered is the mainland part of the Northwest Territories and Saskatchewan from the Paleozoic rocks on the west to Hudson Bay on the east, and from the Arctic Coast on the north to latitude 60 degrees and Cree Lake on the south. This area is too large to discuss as a unit and is divided into three sub-areas (see Fig. 1) : (1) Eastern Great Slave Lake to Lake Athabasca. North of Lake Athabasca this area is fairly well mapped but south of the lake it is not well known. (2) Northern Arm Great Slave Lake through Great Bear and Point lakes to the Arctic Coast as far east as Bathurst Inlet. The area from Great Slave to Great Bear Lake is well mapped, but that along the Arctic Coast is not well known. (3) Eastern Mackenzie and Keewatin area. This has been mapped recently by helicopter reconnaissance. Only the more significant features of these areas will be discussed; the details may be found in the literature. The bibliography includes only the later publications in which references to the earlier publications may be found. EASTERN GREAT SLAVE LAKE TO LAKE ATHABASCA Three areas of Proterozoic rocks lie within this sub-area. The Lake Athabasca area extends south and east of Lake Athabasca and several small areas lie on the north shore. North of Lake Athabasca lies the long narrow basin along Talston and Nonacho lakes and north of this again is a large area in the East Arm of Great Slave Lake. Athabasca Area The Athabasca series consists of sandstone, conglomerate, and arkose, with minor grit, greywacke, and shale. The conglomerate contains both angular and well-rounded fragments of the underlying rocks and locally may grade into the underlying regolith. North of the lake the rocks are predominantly red, and crossbedding, ripple-marks, and mud-cracks are com*Published by permission of Director General of Scientific Services, Department of Mines and Technical Surveys, Ottawa. 79

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mon; and there are minor interbedded basalt and andésite flows. South of the lake the predominant colours are white to buff with some red beds near the base, and the sandstones are thick massive-bedded rocks, showing some torrential crossbedding but no ripple-marks or mud-cracks except near the base. South of the lake, near Trout Lake, is an area of cream to buff limestone with some sandy beds with some concentric structures in the limestone that are believed to be concretionary, although from their description they resemble the "algal" structures found in other areas of Proterozoic rocks in the Northwest Territories. South of Lake Athabasca the sandstones are mostly flat lying with initial dips of up to 10 degrees due to deposition on an uneven floor. There are a few minor exceptions along the north edge of the sandstone area where the beds lie in minor open folds. In contrast the Trout Lake limestone is highly folded with dips to 80 degrees and some brecciated areas. All these rocks are unmetamorphosed with the exception of the folded areas where the sandstone has been altered to a quartzite. Some faulting has occurred but its importance is not known. North of Lake Athabasca the rocks have been folded and dips of 75 and 80 degrees are common, although the dip of most beds is less than 40 degrees. West of Black Bay the lower beds dip 70 degrees to vertical, but the dip of the upper part rarely exceeds 50 degrees. All the rocks are cut by faults. Some of the movement on these faults was prior to deposition of the Athabasca series. The unmetamorphosed and generally flat-lying Athabasca series is considered to be late Proterozoic but at two places there is some evidence of a break within the series. North of Lake Athabasca, west of Black Bay, the lower part is steeply folded, is cut by numerous diabase dykes, and contains stretched pebbles ; whereas the upper part nearby is less steeply folded, is cut by few diabase dykes, and contains no stretched pebbles. The top of the lower part is greywacke, a type of rock not found in the upper part. Also, mapping suggests an angular unconformity of 20 to 40 degrees. South of Lake Athabasca, near Trout Lake, the Trout Lake limestone indicates entirely different conditions of deposition to those found anywhere else in the Athabasca area. The limestone is steeply folded suggesting a considerable time break between it and the surrounding flat-lying sandstone. However, no sandstone outcrops within 16 miles of the limestone, and the folding may be local. The Athabasca series is cut by diabase dykes and near Beaverlodge Lake there are minor gabbro intrusives probably associated with the basalt flows of this area. The Athabasca series everywhere lies with profound angular unconformity on the older Precambrian rocks. In many localities there is a basal conglomerate consisting of cobbles of the underlying Tazin and granitic rocks. It is nowhere in contact with Paleozoic rocks but is cut by diabase dykes and faults believed to be of the same age as similar ones overlain by undisturbed Ordovician and younger rocks. Therefore it is considered to be pre-Paleozoic.

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Nanacho Area The Nonacho group rocks occupy a long narrow basin extending northeast through Talston and Nonacho lakes. The rocks consist of conglomerate, slate, arkose, quartzite, and greywacke; one type grades into another, and beds and lenses of conglomerate and slate occur interbedded with arkose and quartzite. In general, the conglomerate is several hundred feet thick and in places at least 2,000 feet. It commonly occurs near the base of the group and is composed almost entirely of pebbles of granite and allied rocks, and near the base consists of closely packed, angular, granite fragments from one to two feet in diameter in an arkosic matrix composed largely of small granite fragments. The slates and greywackes are fine to medium grained, dark grey to black weathering rocks, and the arkoses and quartzites are buff, yellow, and light grey weathering rocks of fine to medium grain. Crossbedding, grain gradation, ripple-marks, and mud-cracks are common. Two areas of basic volcanic schist occur east of Nonacho Lake and have been included in the Nonacho group, but the relationship is uncertain. The strata generally lie in open folds, with the dips on the limbs averaging between 45 and 60 degrees, but they have been more intensely folded near contacts with younger granitic intrusions. They are cut by faults. The Nonacho series is relatively unmetamorphosed except near contacts with granitic rocks where the slates have been converted to phyllites, and the arkoses and quartzites to fine-grained, glassy, pink rocks. The Nonacho sediments are cut by younger granitic rocks and both are cut by diabase dykes. The younger granitic rocks are light grey to pink, medium to coarse grained, and resemble the older granitic rocks so closely that in areas where their contacts with the Nonacho rocks are not seen the two cannot be distinguished lithologically. An apparent extension of the Nonacho group occurs as an elongated "sliver,"' trending northeast from Tent to Whitefish lakes. The rocks are mainly sericitized and schistose impure quartzites with some sheared to massive conglomerates. Dips are 30 degrees to vertical, and the structure appears to be a southwesterly plunging syncline. This "sliver" probably represents the "tailing out" of the Nonacho sediments into granitic gneisses. In places the bordering rocks are stratiform pink gneisses of granitic composition that appear to be conformable to the limbs of the syncline. The Nonacho rocks rest unconformably on the older granitic rocks and Tazin rocks but are nowhere in contact with Paleozoic rocks. They are cut by granitic rocks and by diabase dykes and so are considered pre-Paleozoic. East Arm of Great Slave Lake Proterozoic rocks occupy the basin of the East Arm of Great Slave Lake and outliers occur to the north and south, and to the east at Artillery Lake. These rocks are divided into two groups, the Great Slave group and the Et-then group. The Great Slave group comprises six formations named in ascending order the Sosan, Kahochella, Pethei, Stark, Tochatwi, and Pearson forma-

FIGURE 1. Areas of Proterozoic rocks, Northwest Territories and northern Saskatchewan.

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tion. The Sosan formation is about 3,000 feet thick and consists of sandstone, quartzite, arkose, and conglomerate. The Kahochella formation is composed of about 1,000 feet of shaly sediments with laminated limestone, jasper, and oolitic iron formation, and associated andésite lava flows and pyroclastic rocks. The Pethei formation comprises about 1,500 feet of limestone and dolomite, with "algal" structures at some horizons. The Stark formation is possibly 1,000 feet thick, and comprises interbedded varicoloured dolomite, shale, and limestone, some layers of which are brecciated. The Tochatwi formation is about 300 feet of shaly sediments and sandstone. The Pearson formation comprises 70 to 150 feet of andésite, basalt, and trachyte lava flows, with minor interbedded argillite. Most of the clastic members of the group are red or brown, and many beds exhibit ripple-marks, crossbedding, and mud-cracks. Concretions occur here and there in shale and argillite. The strata form an easterly trending synclinorium 150 miles long; the beds on the north limb dip 5 to 10 degrees south, whereas the strata on the south limb are folded in a series of easterly trending anticlines and synclines with limbs dipping between 30 and 70 degrees. Steeply dipping faults cut all these rocks and many have been traced for miles. The Great Slave group was originally divided into a lower and an upper part with three formations in each part, but recent work in the Snowdrift and MacLean Bay areas has shown that the unconformity suggested by the absence of the Pethei formation in the southern part of the area is actually due to a change in faciès and the assemblage is now considered to be one group. It rests on an erosion surface formed on the edges of upturned Yellowknife rocks and granitic and gneissic rocks, and where observed the basal members are composed largely of detrital material formed from the rocks below them. Along the north shore of MacLeod Bay, the Great Slave rocks lie unconformably on top of Yellowknife type rocks and on granitic rocks. To the south and east they lie unconformably on granitic rocks or are cut off by the MacDonald fault. The Et-then group of coarse, clastic, sedimentary strata was deposited on an old erosion surface developed on Great Slave and older rocks. The Murky formation of conglomerate forms the base of the group and carries closely packed, round boulders of a great variety of rocks representing almost every member of the older groups. The conglomerate varies in thickness up to, probably, several thousand feet, and is locally missing. The overlying Preble formation consists of coarse, feldspathic, crossbedded and ripplemarked sandstones and quartzite. The Et-then group is nearly flat lying except in the vicinity of faults, where dips are up to 70 degrees. These faults are of great magnitude, commonly strike northeasterly, and are mostly confined to the southern, more complexly folded part of the basin of Great Slave Lake. One of them, the MacDonald fault, forms the southern boundary of the Proterozoic area and continues for some 200 miles to the northeast. The faults have displaced the Et-then group and all older rocks but

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most of the movement took place before the diabase dykes were emplaced. Older maps show large areas of granodiorite cutting the Great Slave group, but later mapping has shown that most, if not all, of these are overlain unconformably by Great Slave rocks. They are lithologically unlike any of the granitic rocks outside the basin of Great Slave Lake. Two small areas of diorite south of Snowdrift are known to cut the Great Slave rocks. All the Great Slave and Et-then rocks are cut by lithologically identical dykes and sills of diabase and wherever observed the dykes cut the sills. To the south of the MacDonald fault, south of Stark Lake, there is a small synclinal basin of gently dipping sandstone, very similar to the Sosan and sandstone of the Nonacho rocks, some 20 miles to the south. To the east, in Artillery Lake, are a few small outcrops of fine-grained limestone similar to that of the Great Slave group. The Great Slave and Et-then groups are nowhere in contact with Paleozoic rocks but faults which cut these two groups apparently pass under the Devonian rocks to the west without disturbing them; the Great Slave and Et-then groups, therefore, are considered to be pre-Paleozoic. Correlation The rocks of the Great Slave area, the Nonacho area, and the Athabasca area are separated by areas of granitic rocks and so correlation can be based only on lithological and structural similarities. The Nonacho group is very similar to the Sosan formation of the Great Slave group and might be correlated with it, and in addition the direction of folding in the Great Slave and Nonacho areas is similar. The Athabasca series is similar to the Et-then group; both are undisturbed for the most part and composed of sandstone and conglomerate. Possibly the lower part of the Athabasca series west of Black Bay and the Trout Lake limestone are correlative with the Great Slave group, as suggested by the presence of "algal" structures in the Great Slave rocks and possibly similar structures in the Trout Lake rocks, but this is hypothetical. GREAT SLAVE LAKE TO GREAT BEAR LAKE AND ARCTIC COAST Several basins of Proterozoic sedimentary and volcanic rocks have been mapped between Great Slave and Great Bear lakes. A larger and less well known area of Proterozoic rocks extends in a broad band northwest from Point Lake and Port Epworth through Great Bear Lake and Coppermine River, to the Arctic Coast at Darnley Bay. East of this, other areas of Proterozoic rocks are found on Kent Peninsula and in a broad southerly trending belt on either side of Bathurst Inlet. Snare Area Sedimentary and volcanic rocks of the Snare group form several elongated north-trending basins extending north from the north arm of Great Slave Lake nearly to Great Bear Lake. The basa! beds are coarse-grained quartzit.es. arkoses, and pebbly quartzite or conglomerate. These are overlain by dolomite, or grade upward into a mixed series of interbedded argillaceous

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rocks, greywackes, quartzites, and dolomite or limestone. Over large areas the argillaceous rocks and greywackes, or their altered equivalents, are the most abundant strata. Massive and pillowed, andesitic, basaltic, and dacitic lavas, and minor pyroclastic rocks occur in the northern part of the area and, for the most part, probably overlie the sedimentary formations. Common structural features of the Snare group include crossbedding and ripple-marks in the quartzites and other coarse-grained formations. Some beds or groups of beds in the dolomite contain "algal" structures similar to those of the dolomites of the Great Slave group. The least altered Snare strata lie in broad, open folds, and dips of less than 20 degrees are common. The Snare strata that have been metamorphosed by granitic and porphyritic intrusions dip at steeper angles but the dips rarely exceed 65 degrees and are commonly less than 45 degrees. Many faults cut the Snare strata. Most of them strike about northwest, north, or northeast, and so far as is known, all are nearly vertical. Offsets of more than half a mile are common, and the greatest inferred offset is 7 miles. The Snare rocks are intruded by granitic rocks and by feldspar and feldspar quartz porphyries and near these intrusions some shales, slates, argillites, and greywackes had been altered to phyllite, knotted quartz mica schist, and gneiss. These altered rocks are lithologically similar to the Archean Yellowknife group but commonly have more gentle dips, are more thinly bedded and here and there contain beds of dolomite, limestone, or white quartzite. Other strata have been altered to banded black, grey, green and pink cherty argillites; or to hard dense rocks containing various proportions of quartz, garnet, pyroxene, epidote, and other minerals, interlayered with impure crystalline limestone. The altered rocks were originally called the Marian group and placed in the Archean; however, subsequent work showed them to be altered Snare rocks and the term "Marian group" has been dropped. Granitic rocks intrude Snare strata in almost all places where they have been found in contact. Scattered bodies of feldspar and feldspar quartz porphyry that are probably genetically related to the Proterozoic granitic rocks likewise cut the Snare group of rocks. A few small bodies of altered diabase, gabbro, and diorite cut the Snare sediments; all the Proterozoic rocks are cut by diabase dykes. The Snare rocks rest unconformably on older granitic intrusions or on steeply inclined Archean sedimentary and volcanic rocks. Great Bear Lake Northwest of the Snare group and separated from them by an area of granitic rocks are the rocks of the Echo Bay group. Their lower part is chiefly sedimentary and consists of massive crystalline tuff, thinly banded cherty sedimentary strata, bedded tuff and other pyroclastic rocks, minor limestone, and feldspar and hornblende feldspar porphyry that is probably mainly intrusive. Their upper part consists of porphvritic and amygdaloidal andésite lava flows separated by a massive to stratified tuff.

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Structural features of the Echo Bay group are similar to the Snare group; they include mud-cracks, ripple-marks, beds showing grain gradation within argillaceous formations, and algal-like structures within chert layers. Dips are commonly less than 45 degrees, though locally much steeper. Faults and fractures are numerous. The rocks have been altered mainly by hydrothermal processes, but the degree and character of this alteration vary widely from place to place. Perhaps the most noticeable features are the widespread red staining and the almost complete absence of schist or gneiss. The base of the Echo Bay rocks is cut off by younger granitic intrusions and, so far as known, all intrusive rocks at Great Bear Lake are of Proterozoic age. They are mainly fresh, massive granite and granodiorite, and feldspar and feldspar quartz porphyries. The Cameron Bay group lies along the east side of Great Bear Lake. Members of this group consist of loosely consolidated conglomerates, arkoses, sandstones, argillites, and tuffs, interlayered, especially in the upper part, with trachyte and andésite lava flow. The conglomerates are maroon or chocolate coloured, with ferruginous cement, and break around rather than through the pebbles and cobbles. Most of the pebbles and cobbles are of typical Echo Bay volcanic rocks. Cameron Bay formations are gently inclined to about the same degree as nearby, underlying Echo Bay strata and strike about parallel with them. They are also intruded by granitic and porphyritic rocks. Cameron Bay rocks may be only slightly younger than those of the Echo Bay group and correlative with parts of the Great Slave and Snare groups. On the other hand granitic pebbles in the Cameron Bay conglomerate resemble rocks that intrude the Echo Bay group and suggest that the Cameron Bay rocks are much younger than the Echo Bay. If so, they may be of late Proterozoic age. Small areas of chocolate brown conglomerate, arkose, slate and argillite occur within the areas of Snare rocks. The conglomerates contain pebbles characteristic of the Snare group. These brown rocks are lithologically so similar to the Cameron Bay rocks that they are correlated with them, and this suggests an erosional interval between the deposition of the Snare rocks and the Cameron Bay rocks. Hornby Bay group rocks occur near Hornby Bay on the east side of Great Bear Lake and comprise coarse, brown, crumbly-weathering conglomerate and pink to red ripple-marked and crossbedded sandstone and quartzite. The series is at least several hundred feet thick but the top and bottom have not been seen. The group is gently warped with dips averaging not more than 5 to 10 degrees. Contacts with the Cameron Bay group are not exposed but the gentler dips of the Hornby Bay group indicate an angular unconformity between them. Hornby Bay rocks are cut by faults containing giant quartz veins. They are also cut by diabase dykes and sills. Arctic Coast The Epworth formation is exposed on the Arctic Coast just east of Darnley Bay, at Port Epworth and on Bathurst Inlet; inland it outcrops between

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Dismal Lakes and Coppermine River and may extend south to join the Snare rocks south of Point Lake. It consists mainly of grey, buff or light brown concretionary dolomite with minor sandstone, quartzite and conglomerate at the base. The maximum known thickness is about 8,000 feet. The Epworth strata are gently folded along axial lines trending a little east of north and commonly dip at less than 20 degrees. The Goulburn formation outcrops on Bathurst Inlet and is composed of pink and grey crossbedded quartzites with interbedded conglomerates. Some of the conglomerate contains pebbles apparently of Epworth dolomite. The formation is more than 4,000 feet thick. It has not been seen in contact with older formations but may be cut by granitic rocks. South of Bathurst Inlet, west of the Bathurst Inlet Trench and north of the bend in Western River, rocks that are probably the southern extension of the Goulburn formation have been mapped by helicopter reconnaissance. Their lower part consists of quartzite, pebbly quartzite, argillite, slate, and dolomite or limestone; and the upper part consists of pink, relatively massive-bedded, arkosic quartzite and pebbly quartzite or conglomerate. Their true thickness is unknown, but may amount to several thousand feet. These rocks have been warped into moderate folds, with dips generally less than 45 degrees, and have not been metamorphosed. The lower and upper parts of this group, so far as is known at present, are conformable. Dykes and sills of gabbroic rocks cut the sedimentary strata, but no granitic rocks are known to intrude them. They overlie the upturned strata of the Yellowknife group unconformably in several places, but have not been seen in contact with rocks of the Dubawnt group or Coppermine River scries. Similar rocks may underlie the Peacock Hills near the north end of Contwoyto Lake. The Coppermine River series occurs on the Arctic Coast near Darnley Bay, along Coppermine River and Dismal Lakes, and in Bathurst Inlet. Its known total thickness is about 28,000 feet. The lower half of the series consists of amygdaloidal basaltic lava flows with minor interlayered conglomerate. These are overlain by dark red to brown, sandy shales, sandstone and limestone. The strata dip about 12 degrees north in the Coppermine River basin and commonly at lower angles elsewhere. The older Goulburn formation was eroded before the Coppermine rocks were deposited; and in some instances the Goulburn strata are missing and Coppermine rocks rest unconformably on Epworth strata. The Epworth formation, Goulburn formation, and Coppermine River series are all intruded by dykes and sills of diabase and cut by faults. Correlation The rocks of the Snare group, Echo Bay group, Cameron Bay group, Epworth formation, and Goulburn formation either lie unconformably on older granites, the upturned edges of Archean sedimentary and volcanic rocks, or else their base is in younger intrusive rocks. They lie near but are nowhere in contact with the Paleozoic rocks. However, porphyry intrusives

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and faults that cut the Proterozoic rocks are known to pass under Ordovician rocks without disturbing them. Because the Snare and Echo Bay rocks are quite highly folded, metamorphosed, and intruded by granitic rocks they are considered early Proterozoic. The Cameron Bay group is similarly folded and intruded by granitic rocks but it is not as well indurated, and contains pebbles of Snare and Echo Bay type rocks and of granite similar to that intruding the Echo Bay group. It is overlain by the Hornby Bay group which is only gently warped and is not known to be cut by granitic rocks. Thus the Cameron Bay group is considered early Proterozoic and the Hornby Bay group late Proterozoic. Similarly along the Arctic Coast the Epworth and Goulburn formations are only gently folded, may be cut by granite and are overlain by the almost flat-lying Coppermine River series. Therefore they are considered early Proterozoic similar to the Cameron Bay group; the Coppermine River series is considered late Proterozoic. EASTERN MACKENZIE AND KEEWATIN This section lies between latitudes 60 and 66, west of Hudson Bay to longitude 108, except that the Daly Bay-Wager Bay area is not included. Before discussing this area it should be emphasized that the mapping has been done entirely by helicopter reconnaissance and the conclusions drawn may be considerably modified by more detailed mapping. The Hurwitz group occurs in several areas from near Nueltin Lake northeast to Rankin Inlet and also between Baker and Garry lakes. The Dubawnt group underlies a very large area of the central barren grounds, from upper Thclon River to the east end of Baker Lake, and from near Yathkyed Lake to mid-way between Thelon and Back rivers. Hurwitz Group Hurwitz rocks comprise conglomerate, greywacke, white quartzite, dolomite, impure quartzite, slate, and minor schist. The most prominent rock is a pure white, medium-grained, in part ripple-marked quartzite that forms pronounced ridges. Some phases of this quartzite are less pure and may be pink, purple, or even light blue. Pebble beds are known, but are not common. Conglomerate and relatively thin-bedded greywacke are abundant in southern Keewatin and less so in central Keewatin. Buff dolomite is a conspicuous but not widespread member of the Hurwitz group ; arkosic, massive sandy quartzite is locally present. From bottom to top the general sequence within the Hurwitz group may be greywacke-conglomerate, white quartzite, dolomite and greywackeimpure quartzite, although the whole sequence has not been established in any one place. In southern Keewatin the white quartzite is ripple-marked and underlain by greywacke and conglomerate while in central Keewatin it is massive, in places rests on granite, and the underlying sedimentary rocks are apparently missing. The white quartzite may be several thousand feet

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thick but the possibility of unrecognized repetition by folding must be borne in mind. The Hurwitz group is commonly folded into basin or canoe-shaped structures and dips range from low to overturned but average about 45 degrees. The grade of metamorphism is low and some of the rocks, particularly the quartzite, have lost their original clastic fabric. Some have been converted to schists or even hornfels, but the high grade metamorphic effects so typically developed in many of the sedimentary rocks of the Archcan Yellowknife group are lacking. In central Kcewatin granite intrudes the Hurwitz quartzite, and here and elsewhere the presence of considerable potassium feldspar and white mica in the quartzite, and the fact that some of the overlying sediments have been converted to schist and hornfels, suggest a post-Hurwitz granite. The rocks of the Hurwitz group are cut by diabase dykes. In southern Kcewatin cobbles of granite and greenish volcanic rock in the Hurwitz conglomerate suggest a period of erosion and an unconformable contact between the granite-gneiss-greenstone complex and the Hurwitz sediments, and the white quartzite is known to rest unconformably against granite gneiss in places. On the other hand the white quartzite has a pronounced geographic and apparently conformable structural relationship to greenstone in several localities. The unreliability of attempted correlations between greenstones and between granites at the present stage of mapping in southern and central Keewatin does not provide a sound basis for positive statements on regional relationships. The Hurwitz rocks are overlain unconformably by the sedimentary rocks of the Dubawnt group which contain numerous white cobbles apparently of Hurwitz quartzite. It is of interest to note the relative abundance of the white Hurwitz quartzite in Keewatin district, and its almost complete absence from Dubawnt Lake west to the Yellowknife area. On the basis of composition, structure and metamorphism, and relation to the younger rocks the Hurwitz group may be the rough equivalent of the Nonacho and Great Slave groups. Dubawnt Group Many thousands of square miles of the central barren grounds are underlain by the sedimentary and igneous rocks of this group. A basal conglomerate is recognized near the east end of Baker Lake and intermittently along the southeastern contact zone. Crossbedded sandstone, of many colours, covers extensive areas and in places is at least 400 feet thick. It appears to lie against granite gneiss along the entire western and northern borders of the outcrop areas with little or no underlying conglomerate; however, this may be a fault contact. Porphyritic igneous rocks, the Dubawnt porphyries, presumably in large part flows, but in some cases probably sills and plugs, are common particularly east of a line between Dubawnt and Beverley lakes. Conglomerates are abundant in the eastern half of the Dubawnt region, and

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contain cobbles of distinctive Dubawnt porphyries as well as sandstone, and white quartzite presumably of Hurwitz age. Near Thelon River, northwest of Dubawnt Lake, unfossiliferous flat-lying dolomite occurs in the upper part of the Dubawnt group. At the east end of Baker Lake dips in Dubawnt sandstone are moderately steep but over extensive areas the sandstone, porphyry and conglomerate are gently warped to flat lying. The sandstone west and northwest of Dubawnt Lake is essentially flat lying. These rocks are unmetamorphosed ; many of the sandstones are crumbly, and all exhibit sandy textures. No unconformities have been recognized within this group. The rocks of the Dubawnt group are not known to be intruded by granites. They are cut by dykes of diabase, and other gabbroic rocks that may be related to the igneous rocks of the Dubawnt group. Red porphyry dykes also cut the Dubawnt rocks. Contacts between the rocks of the Dubawnt group and older rocks are generally concealed, although the regional extent of the gently dipping unmetarnorphosed clastic sediments, as compared with the more steeply folded Hurwitz rocks, indicates beyond any reasonable doubt that the rocks of the Dubawnt group overlie all other Precambrian rocks in the region with profound unconformity. However, it is possible that the flat-lying dolomite in the Thelon River area may be younger. Despite some contradictory suggestions provided by age determinations, the available evidence suggests a likely general correlation between the Dubawnt group, the Athabasca series, the Et-then group and the Coppermine River series. All are mainly clastic sediments with, in some cases, extensive flows or sills of mainly basic igneous rocks; all are essentially flat lying, undisturbed, and unmetamorphosed; most overlie the earlier Proterozoic strata unconformably ; and all appear to be the youngest Precambrian strata in their respective areas. PROTEROZOIC INTRUSIONS Large areas of granitic rocks are shown on maps of the Northwest Territories but small areas are shown as Proterozoic. Probably much larger areas are Proterozoic and on the accompanying map ( Fig. 1 ) an attempt has been made to show some of these areas. Granitic rocks intrude the Nonacho series and lithologically similar granitic rocks extend to the northwest as far as the East Arm of Great Slave Lake. The Proterozoic and Archean granitic rocks are lithologically similar and cannot be separated except where their relations to the Nonacho series are known. Within the basin of Great Slave Lake small areas of dioritic rocks intrude the Great Slave series and further mapping may disclose more Proterozoic intrusions. North and west of the Snare rocks all the granitic rocks and large areas of feldspar and feldspar quartz porphyry, which apparently grade into granitic rocks, are known to intrude Proterozoic rocks. Proterozoic intrusions may also lie along the Arctic Coast although as yet they have only been recognized in the Coppermine River area. In central Keewatin large

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areas of porphyry cut the Dubawnt group and in southern Keewatin a fresh coarse porphyritic granite may cut the Hurwitz group. It might be of interest to note that lithologically similar porphyritic granites have been mapped, in places, from northern Manitoba almost to Great Bear Lake and if these could be shown to be Proterozoic they would greatly extend the area of Proterozoic granitic rocks. North of Great Slave Lake several large granite batholiths may be Proterozoic, but no Proterozoic sediments are present to establish their age definitely. One of them, ten miles southeast of Gordon Lake, is a muscovite biotite granite that cuts an older biotite granite or grandodiorite. Swarms of gabbro dykes cut the older granite and are cut by the younger granite. With two granites present, the younger may be Proterozoic, the older, Archean. Granite batholiths to the southwest of Gordon Lake, lithologically similar to the younger granite, also may be Proterozoic. Along the west side of Yellowknife Bay, volcanic rocks of the Yellowknife group are cut by a swarm of gabbro dykes similar to those southeast of Gordon Lake, the granite cuts the gabbro dykes as well as the volcanic rocks. If the gabbro dykes at Yellowknife and Gordon Lake are of the same age, which is difficult to prove, the granite west of Yellowknife is presumably Protcrozoic and it is probable that the granite underlying the country between Yellowknife and the nearest belt of Snare group rocks, 100 miles to the northwest, is also largely Proterozoic. Further to the east, granites of two types with crosscutting relationships are known, and the younger is a muscovite granite similar to that south and west of Gordon Lake and hence may be Proterozoic. Thus in many areas throughout the Northwest Territories there are two ages of granite and where it is possible to date the younger it is Proterozoic. Correlation by lithology is not reliable but it must be used until a reliable method of determining the ages of these rocks is developed. CONCLUSIONS Correlations of the Proterozoic rocks from area to area have been made but without positive age determinations these correlations are tentative. Correlations based on direction of folding and on the few available radioactivity age determinations have been discussed by Dr. Harrison. Correlations in this paper are based on stratigraphie succession and do not necessarily represent a similar time succession in different areas. Throughout the Northwest Territories there seem to have been two main periods of granitic intrusions, one at the end of the Archean or the beginning of the Proterozoic and the other in the middle of the Proterozoic. The rocks deposited after the exposure by erosion of the earlier granitic intrusions were a series of coarse to fine clastic rocks, limestones, and dolomites usually containing "algal" structures, and minor volcanic rocks. These rocks were then folded to dips averaging 45 degrees, or greater in some local areas, and in places were altered by intrusion of the later granitic rocks. On the basis of these broad similarities we may include in the early Proterozoic the follow-

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ing groups: Great Slave, Nonacho, Snare, Echo Bay, Cameron Bay, Hurwitz, Goulburn and Epworth. At Great Bear Lake and along the Arctic Coast there appears to have been an erosional break of some duration in the middle of the early Proterozoic because the Cameron Bay group and Goulburn formation rest disconformably on the underlying Echo Bay group and Epworth formations. The early Proterozoic rocks were folded and they and the related intrusives were widely and deeply eroded; conglomerate, sandstone, shale and some dolomite were laid down on this erosion surface to give the Athabasca, Et-then, Hornby Bay, Coppermine River, and Dubawnt groups. In the north and east basalt and porphyry flows form part of the Coppermine River series and the Dubawnt group. These groups are mainly flat lying and unmetamorphosed. No younger Precambrian rocks are known except for the diabase dykes and sills which cut them. No correlation is attempted between the Proterozoic rocks of the Northwest Territories and those of the rest of Canada because they are separated by a very wide belt of granitic rocks, and, except in so far as they can be divided into early and late Proterozoic types, correlations at present are hardly worth making. REFERENCES BARNES, F. Q. (1951). Snowdrift map-area. N.W.T.; Geol. Surv., Can.. Paper 51-6. (1952). McLean Bay map area, district of Mackenzie; Geol. Surv.. Can., Paper 52-5. FENIAK. M. (1952). MacAlpine Channel; Geol. Surv., Can., Map 1011 A. FOI.INSBEE, R. E (1952). Walmsley Lake; Geol. Surv., Can., Map 1013A. HENDERSON, J. F. (1939). Taltson Lake, district of Mackenzie; Geol. Surv., Can., Map 525A. (1939). Nonacho Lake, district of Mackenzie; Geol. Surv., Can., Map 526A. (1948). Extent of Proterozoic granitic intrusions in the western part of the Canadian Shield: Trans. Roy. Soc. Can.. Ser. Ill, vol. 42. Sec. IV, pp. 41-54. JENNEY, C. D. (1954). The Copper Mine River area, N.W.T., Canada; Proc. Geol. Assoc. Can., vol. 6, pt. 2. pp. 11-25. LORD. C. S. (1942). Snare River and Ingray Lake map-areas, N.W.T.; Geol. Surv., Can., Mem. 235. ( 1951 ). Mineral industry of the district of Mackenzie. N.W.T.: Geol. Surv.. Can., Mem. 261. (1953). Geological notes on southern district of Keewatin; Geol. Surv., Can.. Paper 53-22. (1953). Operation Keewatin, 1952, a geological reconnaissance by helicopter; Can. Inst. Min. Met. Bull., April 1953. (1953). Camsell River, district of Mackenzie; Geol. Surv., Can., Map 1014A. (1954). Aylmer Lake, district of Mackenzie; Geol. Surv., Can., Map 1031A. WRIOHT, G. M. (1951). Christie Bay, district of Mackenzie; Geol. Surv., Can., Paper 51-25.

(1951). Reliance, district of Mackenzie; Geol. Surv., Can., Paper 51-26. (1955). Geological notes on central district of Keewatin; Geol. Surv., Can., Paper 55-17. (1955). Reconnaissance by Helicopter in the Barren Grounds, an interim report; Can. Min. Jour., vol. 76, no. 4. (1957). Geological notes on eastern district of Mackenzie; Geol. Surv., Can., Paper 56-10. (Note: Lord, C. S. (1951), Mineral industry of the district of Mackenzie, contains a complete Bibliography of all publications prior to 1951.)

THE PROTEROZOIC STRATIGRAPHY OF THE CANADIAN ARCTIC ARCHIPELAGO AND NORTHWESTERN GREENLAND* R. G. Blackadar THE EXTENSIVE MAPPING PROGRAMME carried out in the eastern Canadian Arctic by the Geological Survey of Canada in recent years has delineated new regions in which rocks of presumed Proterozoic age outcrop. Figure 1, which illustrates the approximate extent of Protcrozoic rocks in the area under discussion, includes data collected in 1955. The present symposium offers an ideal opportunity to summarize the results of recent studies and to present a summary of previous work, much of which appeared in Meddelelser om Gr0nland. NORTHWEST GREENLAND AND ELLESMERE ISLAND Although the presence in the Arctic of flat-lying to gently dipping, slightly metamorphosed, sedimentary and volcanic rocks lying above granitic or gneissic rocks was known for many years, it was in northern Greenland that it first became apparent that these rocks were probably Proterozoic in age. The Agpat formation, an assemblage of quartzites, amphibolites, marbles, dolomites, clay shales, and granitic and pegmatitic intrusions, ranging in thickness from 3,000 to 4,500 feet, outcrops extensively from the vicinity of Umanak Fiord in central west Greenland to Inglefield Land in the northwest part of that sub-continent (Krueger, 1928, p. 118). These rocks are thought to be early Proterozoic in age. A less metamorphosed and presumably younger formation outcrops in northwest Greenland around Etah, Cape Russell, and from Cape Scott to the Humboldt Glacier. This is the Etah formation (L. Koch, 1929, p. 219), composed of sandstone and limestone, and intruded by diabase. Koch considered that a long period of erosion preceded the deposition of the rocks which overlie the Etah formation. Rocks presumed to be late Proterozoic in age were first described from Greenland by L. Koch (1929, p. 220) under the name Thule formation. The type section of this formation in the Cape York region comprises coarse •Published with the permission of the Deputy Minister, Department of Mines and Technical Surveys, Ottawa.

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FIGURE 1. Areas (in black) of Protcro/oic formations in the; Arctic.

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arkose, purple sandstone containing Cryptozoon reefs, cliff-forming dolomite, and lastly dark grey, micaceous, hard sandstone or locally black shales with many ripple-marks. The latter classic unit is not found on Inglefield Land whence the Etah formation was described. The deposition of the Thule formation was followed by a period of igneous activity during which diabase sills "and dykes were emplaced. The relationships between the metamorphic Agpat formation, the Etah formation, and the Thule formation are not clear; from a study of the literature it would appear that the Etah formation is only locally developed in Greenland. It may also be present here and there on Ellesmere Island. As a result of studies made on Inglefield Land and at Bache Peninsula, Ellesmere Island, Troelsen (1950, p. 35) has subdivided the Thule formation and elevated it to group rank. He named the lower part of Koch's Thule formation the Rensselaer Bay sandstone, and the overlying yellow, finegrained dolomite, the Cape Leiper dolomite. According to Troelsen, the Cape Leiper dolomite is separated from the overlying Lower Cambrian limestone by a thin layer of crystalline dolomite which he named the Cape Ingersoll dolomite. Kurtz and Wales ( 1951 ) made a study of the geology of the Thule district and proposed several new names for units also named by Troelsen. The oldest unit in their map-area is the Agpat formation, which, as mentioned above, comprises several thousands of feet of schists, gneisses, and associated granitic and pegmatitic intrusive rocks. Unconformablv overlying the Agpat formation is a "medium to coarse, red to white, crossbedded, massive, partly conglomeratic orthoquartzite" named the Wolstenholme quartzite by Kurtz and Wales. This appears to be the equivalent of the Rensselaer Bay sandstone and has a maximum thickness of 1,600 feet. Crossbedding is common in the middle part of the formation. The Wolstenholme quartzite is overlain by a succession of sandy dolomites and buff crystalline dolomites. The contact between this unit, named the Danish Village formation, and the Wolstenholme quartzite was not seen; the upper contact is marked by a diabase sill 58 feet thick. This formation is at least 800 feet thick and is probably equivalent to the Cape Leiper dolomite of Troelsen. The Danish Village formation is overlain, apparently conformably, by a thick series of sedimentary rocks named the Narssarssuk formation (Kurtz and Wales, 1951, p. 85). It comprises more than 6,000 feet of red siltstones, coarsely crystalline grey porous dolomites, and fine-grained, shaly dolomites, an assemblage that was deposited in a cyclical fashion. Fifty-eight cycles were counted by Kurtz and Wales. The upper contact of the Narssarssuk formation was not seen nor are the relationships known between this succession and the "dark grey, micaceous, hard sandstone or locally black shales with ripple marks" which, according to L. Koch (1929, p. 220) mark the top of the Thule formation near Cape York, some 60 miles southeast of Thule, Greenland.

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Rocks similar in lithology to the Thule group, and thus presumed to be of Late Precambrian age, have been reported from southeastern Ellesmere Island from the vicinity of Fram Fiord, Grise Fiord, and Harbour Fiord (Bentham, 1941, p. 37) and at Craig Harbour by Wordie (1938, p. 399). At Copes Bay, about 45 miles north of Bache Peninsula, rocks thought to be Proterozoic in age are steeply dipping and form part of the Central Ellesmere folded belt (Thorsteinsson, 1956, manuscript report). The strike of these rocks, of which part has been correlated with the Thule group, is about east and they have been thrust southward over Silurian beds bearing Atrypella phoca (Salter). The lowest beds exposed in the sections at Copes Bay comprise about 1,000 feet of light grey to reddish-orange weathering dolomite. The base of the dolomite is the major thrust fault referred to above, whereas the upper contact is a sharp disconformity which separates the dolomite from an overlying clastic unit which has been correlated with the Ransselaer Bay sandstone. This clastic unit consists of 340 feet of conglomerate which grades into 1,090 feet of distinctly banded sedimentary rocks comprising alternating white quartzite and sandstone in part crossbedded, and greyish-red-purple conglomerate. The upper 500 feet of this unit is almost entirely white sandstone and quartzite. A sharp but apparently conformable contact separates this unit from 295 feet of greyish-black fissile shale, coarse-grained grey quartzite and grey dolomite. The shale in turn grades into 1,495 feet of dolomite which contains minor quartzite bands near the top and which is correlated with the Cape Leiper and Cape Ingersoll formations. A sharp but apparently conformable contact separates this predominantly dolomite unit from dark grey limestone; trilobites of Middle Cambrian age were found 80 feet above this contact. The lowest unit measured by Thorsteinsson, 1,000 feet of dolomite, does not appear to be present in the areas examined by Troelsen and Kurtz and Wales, but it is possibly equivalent to part of L. Koch's Etah formation described above, a formation which until now has not been found outside of the type area. The nature of the contact between the Thule group and the overlying fossiliferous Lower or Middle Cambrian limestone is obscure. Some authors, notably Wordie, held that there is no break between the Thule group and the fossiliferous Cambrian strata and considered the Thule group to be merely the basement Lower Cambrian (Wordie, 1938, p. 399). On the other hand, L. Koch and Troelsen stated that there was an erosional unconformity separating the Thule group from the Cambrian. Koch (1933, p. 23) stated that "the Cambrian beds begin with a series of conglomerates which, wherever they were observed on Inglefield Land, lay parallel to the perfectly level surface of the dolomite." Troelsen observed no trace of erosion on the upper surface of the Cape Leiper dolomite but stated that "it is quite evident, however, that the upper surface of the Cape Ingersoll dolomite was subject to a certain amount of erosion before the beginning of the Cambrian transgression" (Troelsen, 1950, p. 36).

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This then is the situation in what might be called the classical area for Proterozoic stratigraphy in the North American Arctic; an examination of Figure 2 will show the probable correlation of those units mentioned thus far.

FIGURE 2. Tentative correlation table of Protcrozoic formations in the eastern Arctic.

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BAFFIN ISLAND More than 11,450 feet of sedimentary and volcanic rocks similar in lithology to the Thule group were mapped at Admiralty Inlet, northwestern Baffin Island, by Blackadar ( 1955 ). Two groups, which were further subdivided, were recognized. The lower, the Eqalulik group, comprised 4,100 feet of quartzite with minor conglomerate and infrequent crossbedded layers, overlying 500 feet of andesitic and basaltic flow rocks, pyroclastic deposits and minor quartzites. This group is overlain, possibly disconformably, by the Uluksan group, comprising 6,850 feet of shale, dolomite, mudstone, siltstone, and sandstone. Both groups are cut by numerous dykes of gabbro and are but slightly folded. Rocks similar to the Eqalulik group have been reported from the head of Milne Inlet and Tay Sound in northeastern Baffin Island (W. L. Davison, personal communication) and from the vicinity of Fury and Hecla Strait, west central Baffin Island (Teichert, 1937). At the latter locality a study of air photographs suggests that the Proterozoic rocks outcrop between Autridge Bay and Agu Bay on the north side of the strait, are inclined gently to the south and extend 10 or 15 miles inland. Dykes of gabbro are also known to intrude these rocks. SOMERSET ISLAND AND PRINCE OF WALES ISLAND An apparently unfossiliferous succession of sedimentary and igneous rocks was examined by Woakes southeast of Aston Bay, Somerset Island (see Blackadar, 1956, manuscript report). Two units were mapped; the lower comprises 7,000 feet of grey and red quartzite, minor amounts of siltstone, slate, shale and conglomerate, and about 1,400 feet of gabbro in the form of sills. The upper unit, conformably overlying the quartzite succession, comprises 2,250 feet of light grey to yellow dolomite. Neither the upper nor the lower contact of this assemblage was seen but it appears to be younger than the granite and gneiss which outcrop along the west coast of Somerset Island, and older than the Allen Bay formation of OrdovicianSilurian age which outcrops north of Aston Bay. Although less varied lithologically, this assemblage of quartzite and dolomite is of comparable thickness to that on Baffin Island, about 200 miles to the east. The Somerset strata dip at an angle ranging from 10 to 30 degrees to the northeast. It is possible that at least part of their deformation dates from the Paleozoic. Here and there along the east coast of Prince of Wales Island there are outcrops of strata that are probably Proterozoic in age. Dolomite has been reported from Prescott Island (Tozer, 1956, manuscript report) and an examination of air photographs suggests that the band from which the dolomite was reported extends many miles to the south. CORONATION GULF AND VICTORIA ISLAND Rocks and structures apparently similar to those of Proterozoic formations of the Coppermine River and Bathurst Inlet regions of the Mackenzie dis-

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trict occur in the region of Wellington Bay and from Minto Inlet to Hadley Bay on Victoria Island, and on southern Banks Island. No comprehensive stratigraphie nor structural study has been made of those occurrences. PALEOGEOGRAPHY If the tentative correlations suggested in this paper are correct, then in some late Precambrian time, long after the formation of the granitic and gneissic complexes and after their deformation, volcanic rocks were deposited in the vicinity of Admiralty Inlet, Baffin Island, and whether contemporaneously or not, dolomite was deposited at Copes Bay, Ellesmere Island, and sandstone and limestone—the Etah formation—were deposited in northwestern Greenland. The southern margin of the basin in which the dolomite was deposited at Copes Bay may have been north of Bache Peninsula thus explaining the absence of this dolomite at Bache Peninsula. Subsequently sandstone was deposited over an extensive area. Whether there was a single basin of deposition extending northward from Fury and Hccla Strait at least to Copes Bay and from Somerset Island eastward to central Greenland, or whether there were several restricted basins, cannot lie determined with the information at present available. At Dundas Harbour, Devon Island, a gneissic complex is directly overlain by fossiliferous Lower Cambrian limestone (Kurtz, McNair, and Wales, 1952). This locality is in the centre of the area over which the Proterozoic rocks outcrop in the eastern Canadian Arctic, and their complete absence gives support to the hypothesis that there were several separate basins of deposition and that Devon Island was an elevated land mass during all of Late Precambrian time. Following the deposition of the sandstone, conditions again changed and shale or shaly dolomite was deposited at many localities; this was followed by the deposition of dolomite, which in some places exceeds 2,000 feet in thickness. Conditions of deposition once more changed and whereas at some localities a period of erosion followed the deposition of the dolomite, yet at others, such as Thule, Greenland, or northwestern Baffin Island, considerable thicknesses of siltstone, mudstone and sandstone were laid down above the dolomite. The presence of crossbedded and ripple-marked sandstone and conglomerate in many of the quartzite and sandstone deposits, and the presence in the stratigraphie column of great thicknesses of siltstone and mudstone, such as the Narssarssuk formation in Greenland or the Uluksan group in northwestern Baffin Island, suggests that the basins of sedimentation were relatively shallow during much of the depositional history of the area. Cryptozoon reefs have been noted in the Rensselaer Bay sandstone and what may be algae were collected from a calcareous layer in the sandstone unit on Somerset Island. In general, however, organic remains appear to be lacking in most of the Proterozoic sediments.

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Too little is known concerning the bed-rock geology between eastern Prince of Wales Island and central Victoria Island to permit even suggestive correlation between the Thule type rocks of the eastern Arctic and the sedimentary and trap succession found in the Victoria Island trough. According to O'Neill this basin is oval in shape and extends eastward from Cape Lyon to Boothia Peninsula, a distance of about 600 miles, and has a width of 300 miles. In this trough the great thicknesses of the Coppermine River series were deposited. The basin is assumed to have formed after the deposition of the Goulbourn quartzite. The Coppermine River series has always been correlated, on a lithological basis, with the succession on the south shore of Lake Superior, termed Middle and Upper Keweenawan by Leith. To date no more definite dating has been proposed for the Coppermine River series. Apparently a period of uplift followed the deposition of the Coppermine River series because, as far as is known, the oldest Paleozoic rorks overlying the scries arc Ordovician in age. Minor folding and subsidence probably occurred during the uplift and this caused the gentle dips which characterize the entire Coppermine River series. REFERENCES BKNTHAM. R. (1941). Structure and glaciers of southern Ellesmere Island; Geog. Jour., vol. 97, no. 1, pp. 36-45. BLACKADAR, R. G. (1955). Geological reconnaissance, Admiralty Inlet, Northwestern Baffin Island; Gc-ol. Surv., Can., Paper 55-6. (1956). Geology of Somerset Island: manuscript report. KOCH, L. (1929). Stratigraphy of Greenland; Meddelelscr om Gr0nland, Bd. 73, pp. 218-225. ( 1933). The geology of Ingleficld Land; Meddclelser om Gr0nland, Bd. 73, Nr. 2. KRUEGER, H. K. E. (1928). Zur Géologie von Wcstgronland; Mcddelclser om Gr0nland, Bd. 74, p. 118. KURTZ, V. E. and WALES, D. B. (1951). Geology of the Thule area, Greenland; Proc. Oklahoma Acad. Sci., vol. 31 (1950), pp. 83-92. KURTZ, V. E., MC\AIR, A. H., and WALKS, D. B. (1952). Stratigraphy of Dundas Harbour area. Devon Island, Arctic Archipelago; Am. Jour. Sci., vol. 250. pp. 636-655. O'N'F.ii.L. J. J. (1924). Report of the Canadian Arctic Expedition 1913-18, vol. 11, pt. A, pp. 21-24. TEICHF.RT, C. (1937). Report of the Fifth Thule Expedition. 1921-24, vol. 1, no. 5. THORSTEINSSON, R. (1956). Geology of eastern Ellesmere Island; manuscript report. TOZER, E. T. (1956). Geology of Prince of Wales; manuscript report. TROELSEN, J. C. (1950). Contributions to the geology of northwest Greenland, Ellesmere Island and Axel Heiberg Island; Meddelelscr om Gr0nland, Bd. 149, Nr. 7. WORDIE, J. M. (1938). An expedition to northwest Greenland and the Canadian Arctic in 1937; Geol. Jour., vol. 92, p. 399.

PROTEROZOIC ROCKS OF THE NORTHERN PART OF THE LABRADOR GEOSYNCLINE, THE CAPE SMITH BELT, AND THE RICHMOND GULF AREA* Robert Bergeron THIS PAPER outlines the major features of the geology of three areas of Proterozoic rocks in Ungava, namely, the northern part of the Labrador geosyncline, the Cape Smith belt, and the Richmond Gulf area. THE NORTHERN PART OF THE LABRADOR GEOSYNCLINE Location and Limits The part of the Labrador geosyncline here reviewed extends northward from latitude 58 degrees to about 40 miles north of Payne River. At the southern limit of this area the geosyncline is approximately 35 miles wide. It decreases northward to a width of a few miles in the Hopes Advance Bay area and from there it increases to about 20 miles at Payne Bay and tapers to nothing north of Roberts Lake (see Fig. 1). General Character and Succession The stratigraphie succession in the area between the Koksoak and Leaf rivers is given in the following table of formations. North of Leaf Bay some of the formations disappear and others lose their identity as a result of metamorphic alterations. Massive white, pink, grey or brown quartzite is present almost everywhere at the contact between the rocks of the Kaniapiskau group and the underlying granitic rocks. Near or at the base of the quartzite, local conglomeratic lenses composed of quartz fragments with some granite or gneiss pebbles are found. In a few other places the basal rock may be argillaceous and slightly metamorphosed, as south of Leaf Bay, or it may be mica schist, as north of the bay. South of Leaf Bay, the Sokoman iron-formation rests on the quartzite but north of the bay mica schist, garnetiferous mica schist, or phyllite imme*Published with the permission of the Deputy Minister of Mines, Quebec. 101

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FIGURE 1. Late Precambrian rocks of Quebec.

diately overlie the quartzite. The schistose rocks are usually composed of sericite, chlorite, and biotite with some quartz. They contain in places an appreciable amount of garnet or magnetite. The contact of these rocks with the overlying iron-formation is gradational. Throughout the northern part of the Labrador geosyncline the Sokoman (iron) formation can be divided into three members: a lower silicate member, magnetite-hematite metallic iron-formation, and an upper carbonate member which is capped by carbonates. The silicate member is almost everywhere present although in many places it is only a few feet thick. It

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TABLE OF FORMATIONS KANIAPISKAU?

KANIAPISKAU GROUP

Granite gneisses and paragneisses Metagabbros Intrusive contact Larch River series: sedimentary and volcanic rocks Abner dolomite Chioak formation : sandstones, conglomerates, shale Disconformity Sokoman formation (iron-formation) Quartzite locally overlain and/or underlain by siltstoncs, shale, phyllite, slate or schists Unconformity

BASAL GRANITE-GNEISS COMPLEX

consists of iron silicates, chert, carbonate, and magnetite. North of Leaf Bay this member is metamorphosed, and actinolite, cummingtonite, and garnet are abundant. The magnetite-hematite metallic iron-formation consists of massive or thin-bedded magnetite and hematite with intercalated chert and jasper layers or lenses. In the northern part of the geosyncline this member is mildly to strongly metamorphosed, and consists of a lower hematitic and an upper magnetitic sub-member. The hematitic beds are bluish-grey and composed mainly of specular hematite with dark amphiboles, ferriferous chlorites, quartz and some magnetite. The beds of the magnetitic sub-member are thick and massive and composed of recrystallized chert with disseminated magnetite. In some places beds two to three inches thick of greenish or grey recrystallized chert sprinkled with magnetite alternate with black metallic beds of the same thicknesses composed of fine magnetite and quartz. The top part of the metallic member is commonly a magnetite-carbonate rock. It is a tough rock composed of fine magnetite and quartz with spherical nodules one quarter to one inch in diameter of carbonates and amphiboles. The uppermost or carbonate member of the Sokoman consists of fine-grained siderite or ferrodolomite with lenses, beds, or nodules of chert. South of Leaf Bay the Sokoman is overlain by conglomerates, arkoses, sandstones, grits, and shales of the Chioak formation. These rock types do not form definite horizons but a series of interbedded lenses. The Chioak is overlain by a light grey or buff weathering dolomite, the Abner dolomite. Secondary quartz distributed through the rock makes the weathered surface rough. North of Leaf Bay neither the Chioak formation nor the Abner dolomite were definitely recognized. Here, phyllites and mica schists overlie the Sokoman. However, calcareous schists seen in a few localities may be stratigraphically equivalent to the Abner dolomites. The youngest sequence of the northern part of the geosyncline south of Leaf Bay is the Larch River series, a series of argillaceous and silty rocks

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with minor sandstones, dolomite and iron-formation. The upper part of the series has intercalated, thick, volcanic flows. Iron-formation usually occurs as thin lenses of magnetic shales and schists. However, a band two to three hundred feet thick is found east of Gerido Lake. It consists of a lithologically variable assemblage of ferruginous sedimentary rocks, the most common member being a fine-grained, thin-bedded, dark or black magnetic rock with intercalated lenses of jasper. The volcanic rocks are of intermediate to basic composition and include pillow lavas, massive lavas, tuffs and breccias. The massive flows are the most plentiful. Their colour is variable, ranging from greenish-grey to dark grey. They are of fine to coarse granularity. The pillow lavas are fine or very fine grained, grey-green or dark green in colour. The average diameter of the pillows is approximately three feet. All the volcanic rocks are essentially composed of altered feldspar, tremolite, and chlorite. The tops of individual flows are often characterized by breccias consisting of angular fragments up to four or five inches long set in a finer matrix of the same material and accompanied, in some places, by scoria. The fragments and matrix are of similar composition to that of the lava flows. Before the Labrador geosyncline was folded, extensive sills of amphibolitized gabbro were intruded into the upper part of the sedimentary sequence of the Kaniapiskau group, and some into the lavas. The gabbro possesses variable granularity, ranging from nearly aphanitic to pegmatitic. The sills range from 20 to 3,000 feet in thickness. However, the thicker sills almost certainly are the result of multiple intrusions, as is indicated by the presence of fine-grained zones and of sedimentary rocks within the sills. The most common rocks are actinolite gabbros, the principal minerals being actinolite, altered feldspars, a pyroxene, chlorites and, occasionally, bluish quartz. One kind of gabbro, the "blotchy gabbro," is important because it is related to the sulfide mineralization that has aroused considerable interest in the area. It is a dark coarse gabbro characterized by crystal aggregates of altered feldspar measuring from one-half inch up to ten inches in diameter. In several localities these aggregates make up more than 50 per cent of the rock and thus it may be referred to as a glomeroporphyritic gabbro. The rocks definitely recognized as belonging to the Kaniapiskau group are in contact to the east with gneisses. South of Leaf Bay these rocks include hornblende-biotite and granite gneisses as well as paragneisses. A small area along the contact was mapped by Henry S. de Romer under the supervision of the writer, in the summer of 1955, east of Thèvenet Lake. There is, in that area, a sharp lithological, structural and topographical break between the gneisses and the rocks to the west, and it is apparent that a fault of considerable magnitude is present. Nevertheless, we believe that some of the gneisses are essentially of the same material and age as the Kaniapiskau rocks. East of the fault the rocks are predominantly paragneisses that grade into migmatites and granitic gneisses. Numerous aplitic

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and pegmatitic dykes cut this complex. Amphibolite inclusions were also observed. As described later in this paper, the degree of metamorphism increases gradually from the west boundary of the Labrador geosyncline eastward until the gneissic rocks are encountered. The metamorphism is therefore younger than the géosynclinal strata. The Labrador geosyncline, commonly called the "Labrador Trough," is not the structurally simple basin that it was formerly thought to be. It is a broad belt of highly folded and faulted Proterozoic rocks. This belt trends northwest, and is bounded to the east by a fault line which separates lowgrade sedimentary, volcanic and intrusive rocks and higher-grade schists and gneiss. It has been suggested by Gill (de Rômer, 1956, p. 15) that the Labrador geosyncline represents a foothill zone of a truncated mountainbuilt belt typified by the gneiss complex to the east. The line of demarcation is usually straight but becomes quite sinuous in places. A resume of the hypothetical history of the area (de Rômer, 1956, pp. 85-88) is as follows. The complex gneissic zone is probably a part of a former basin filled with great thicknesses of sediments and flanked on its west side by a shallow miogeosynclinal basin in which shelf-type sediments and iron-formation were laid down. Thick scries of argillaceous and clastic material accumulated in an adjacent cugeosyncline. Extrusion of great masses of pillowed and massive lavas as well as pyroclastics occurred. Gabbro sills of the same composition as the lavas intruded the volcanic and sedimentary rocks. Eventually, the bottom of the basin reached a plastic zone where the rocks were impregnated with magmatic fluids and folded and faulted. The more mobile eugeosynclinal basin was thrust westward over the relatively stable miogeosyncline (Labrador geosyncline). Anticlines and synclines with axial planes overturned to the west and to the southwest formed in the activated part of the miogeosyncline and throughout the eugeosyncline. A series of reverse faults and low-angle thrust faults developed between the relatively stable miogeosyncline and the gneiss complex. The eastern contact of the Labrador geosyncline represents one of these faults. The invasion of igneous material in the mountain-built belt as well as pressure and heat due to depth of burial were the main factors responsible for the progressive regional metamorphism displayed in the area. The present writer agrees that this hypothetical geological history fits well the information we now have on the area west and southwest of Ungava Bay. He also would recommend that the term "Labrador Trough" be dropped in favour of Labrador geosyncline. Structure and Grade of Metamorphism The sedimentary rocks overlying the granitic rocks at the western edge of the geosyncline dip 10 to 20 degrees eastward, the sedimentary series becomes gradually thicker, and the thickness of the whole is further increased by the addition of volcanic rocks and of numerous gabbro sills. The whole group becomes folded into a succession of synclines and anticlines whose

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axes are parallel to the long axis of the Labrador geosyncline. The east limbs of many of these folds are overturned to the west and some are displaced by high-angle reverse faults. In the Roberts Lake area "the central zone has not been affected by folding to any great extent. The eastern border of the basin was, however, submitted to strong erogenic action." (Auger, 1954, p. 531.) It is evident that the deformation was caused by stress from the northeast. Along the western margin of the geosyncline and, north of Payne River, all around the nose of the geosyncline, the Kaniapiskau rocks rest unconformably over an Archean complex. Locally, or for short distances only, the contact is faulted. A disconformity marks the end of the period of deposition of the Sokoman (iron) formation. A conglomerate consisting of rounded to angular fragments of iron-formation in a matrix of finer fragments occurs at many localities at the top of the Sokoman. A minor angular unconformity was found a few miles south of Leaf Bay between the Sokoman and the overlying Chioak formation. Certain parts of the geosyncline are strongly metamorphosed. A gradual increase in metamorphism is observed from Larch River northward to Payne River along the geosyncline and from west to east across the geosyncline. The longitudinal change in metamorphism is particularly noticeable along the Sokoman formation. A few miles south of Leaf Bay iron amphiboles and garnet begin to appear in the iron-formation. The proportion of magnetite progressively increases to a point where, a few miles south of Payne Bay, coarse crystals of magnetite are enclosed in a matrix of secondary silicates. Hematite also shows the progressive metamorphism in being replaced little by little by specular hematite. Eastward across the geosyncline, the metamorphic changes are particularly noticeable in the argillaceous rocks which change from siltstone and argillite to slate, to phyllite, and to mica schist. There is some evidence of dynamic metamorphism, but the general metamorphism is best described as a regional thermal metamorphism which was not synchronous with deformation. The relationships between regional metamorphism and intrusive igneous rocks are not clear. The intrusives are uralitized and saussuritized gabbros, probably mostly altered during the late phases of the crystallization of the cooling magma. The sulphide mineralization present throughout the area is genetically related to these gabbro sills. However, contact metamorphism is not pronounced in the vicinity of the sills and is usually limited to a silicified zone a few inches wide in the adjacent sedimentary rocks.

THE CAPE SMITH BELT Location and Limits Cape Smith, on the east coast of Hudson Bay, is about 660 miles north of Moose Factory, at the south end of James Bay, and 125 miles south of Cape Wolstenholme, at the west end of Hudson Strait. A belt of altered volcanic

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and sedimentary rocks intruded by gabbro sills and dykes extends eastward from Cape Smith across the peninsula to Wakeham Bay, on Hudson Strait. The width of the belt is up to 40 miles in the western half and approximately 10 miles at Wakeham Bay. These rocks are bordered to the north and to the south by granite and granitic gneisses. General Character and Succession Gunning (1933, p. 144D-146D) has described the sequence of the Cape Smith belt as sedimentary rocks interbedded with and overlain by lavas, both being cut by gabbro sills and dykes. Sedimentary rocks. The sedimentaries are water-laid black to dark grey slate, impure limestone, and tuffaceous dark grey to almost white rocks. The last are siliceous and feldspathic tuffs consisting of quartz, feldspar, chlorite, pale green mica and minor amounts of epidote, actinolite and pyrite. Toward the north and south, where the sedimentary rocks are in contact with granite, they grade into phyllite and schists. Lavas. The volcanic rocks consist of both pillowed and massive lavas which are light to dark green or rusty brown on weathered surfaces and pale greyish-green on fresh surfaces. The pillows are generally two to four feet long. Many of the interstices between the pillows are filled with quartz, calcite, epidote, and other minerals. Volcanic breccias were noted at the base or at the top of some flows. The original minerals of the lavas, as seen under the microscope, have been altered to chlorite, calcite, epidote, zoisite, and actinolite. The feldspars have been too much altered to give a clue to the original composition of the lavas, but many remnants of sub-ophitic texture would suggest that they originally were of intermediate to basic composition. Intrusiues. The volcanic and sedimentary rocks are intruded by sills and dykes of augite diorite (Gunning). The intrusions are largely concordant. In places, however, they cut across the bedding of the sedimentary rocks. Where relatively fresh and coarse grained, augite crystals give the gabbros a greenish-grey to dark green mottled appearance. On weathered surfaces the rocks are generally brown. Structure and Grade of Metamorphism The rocks of the Cape Smith belt at the cape itself are folded into a series of tightly compressed anticlines and synclines, in places overturned, and plunging at angles of about 40 degrees toward the northeast. Apparently the same tightly folded structure extends across the peninsula. The direction of schistosity is parallel to the direction of the belt; the dips are vertical or nearly so in the northern part of the belt, but less steep in the southern part. The vertical or nearly vertical attitude of the schistosity suggests greater deformation, at least locally, in the northern part of the Cape Smith belt than in the southern part. The published descriptions of the rocks and the writer's examination of

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a few samples from the Wakeham Bay area lead to the following conclusions: that most of the rocks of the Cape Smith belt belong to the green schist faciès of metamorphism ; and that across widths up to approximately ten miles from the bordering granite and granitic gneisses, the rocks grade from the green schist faciès to the epidote-albite amphibolite faciès. Relation to Older Precambrian The granitic rocks bordering the Cape Smith belt are gneissic or massive, grey to pink, and medium to coarse grained. The contact between granite and greenstones was not observed by Gunning. He suggested (1933, p. 146D) that "the granites are later than and intrude the greenstones." He supported his conclusion on the facts that Low considered the granites to be later, and that parties who prospected the area in 1931 and 1932 reported "porphyries" cutting the greenstone. The similarity of the rocks of the Cape Smith belt with those of the Labrador geosyncline, and recent work done west of Wakeham Bay, indicate that the Cape Smith rocks are of a late Precambrian age and probably, at least at the southern edge of the belt, rest unconformably on a granitic basement complex. Low's opinion concerning the relationships between the rocks now called Proterozoic and the older granites is not supported by recent work. Apparently Low considered the basic volcanic rocks (now included in the late Precambrian ) to be equivalent in age to the old greenstones. Regarding the "porphyries" cutting the greenstones, the writer suggests the possibility that the "porphyries" may be the glomeroporphyritic gabbro sills (blotchy gabbro) of the Labrador geosyncline. RICHMOND GULF AREA Location and Limits Between Nastapoka River and the head of Manitounouk Sound, a band about seven miles wide (it attains a maximum development of 20 miles at Richmond Gulf) of sedimentary rocks capped by basalt flanks the east coast of Hudson Bay for a distance of 90 miles. South of the head of Manitounouk Sound, the strip passes from the coast to form the Manitounouk Islands. From the head of the sound southward, sedimentary rocks outcrop at intervals along the mainland. The last exposures to the south are on Long Island, just off Cape Jones at the northeast corner of James Bay, where sedimentary and associated volcanic rocks are seen. The Nastapoka Islands parallel the coast for 125 miles. These and the Hopewell Islands, which parallel the coast as far as Portland Promontory, are also underlain by Richmond Gulf rocks. There is a gap of 26 miles between the two chains of islands. Also included in this sequence are the rocks of Belcher Islands. General Character and Succession The Richmond Gulf rocks can be broken down into a lower and an upper

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group. Leith (1910) has called the lower the Richmond group. It consists of 2,450 feet (Low's estimate) of coarse ferruginous arkoses grading upward into sandstones and argillites. Basic lavas are interbedded with these rocks. The Richmond group also contains lean siliceous iron ores in beds overlying and underlying the lavas. The younger rocks of the Richmond Gulf area are referred to as the Nastapoka group. This consists of cherty limestones and dolomites overlain by quartzite which grades laterally into iron-formation. The iron-formation is not represented on the mainland but outcrops on Hopewell, Nastapoka and Manitounouk islands. These sedimentary rocks, whose maximum thickness is probably of the order of 1,400 feet, are capped by basaltic rocks with minor pyroclastics. The lower part of this Nastapoka group—the cherty limestones and dolomites—is of particular interest because it contains an ore-bearing horizon. The host rock is a cherty concretionary limestone. The concretions are of algal origin. They are circular or elliptical in plan and composed of concentric, sometimes silicified layers. Similar algal structures were described on the Belcher Islands (Moore, 1918) and in the Labrador geosyncline (Bergeron, 1954). Pyrite, galena and sphalerite together with calcite and quartz occur as fracture fillings or as disseminations in the limestone, or as a selective replacement of the algal concretions. These mineralized zones have been explored by Gulf Lead Mines Limited in recent years, but no large quantity of ore has been found. Structure and Grade of Metamorphism Low, in his explorations of the Richmond Gulf area in 1898, 1899 and 1901, observed an unconformity between the Richmond and Nastapoka groups which he believed was caused by thrusting of the upper beds over the lower. Leith postulated an erosional unconformity, stating that the Richmond group is more complexly folded than the upper group ; the latter dips gently seaward at an angle of 5 to 10 degrees. Recent geological work in the area (Parkes, 1949; Harwood, 1949) seems to indicate the existence of an erosional unconformity between the two groups, along which plane movement has taken place locally. In many places the upper Nastapoka group rests unconformably on granite. Structurally, Richmond Gulf, according to Kranck (1951, p. 10) "evidently represents a fault graben connected by fault lines." Particularly north of the gulf, there "is a very clear structural boundary between the uplifted blocks on the north side and the low terrane around the bay" (Kranck, 1951, p. 11). The Nastapoka Islands occupy their present position by tectonic movement. A westerly dipping fault line probably exists between the islands and the mainland, the islands being on the upthrusted side. A fault rather than a fold is suggested by the deep water in Nastapoka Sound. The sedimentary rocks of the mainland have been subjected to only slight folding, and are essentially unmetamorphosed.

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Relation to Older Precambrian Low's opinion concerning the relationships between the Proterozoic rocks and the underlying granite and granitic gneiss is not very clear. He wrote : "The granite outbursts were not universal throughout this vast region, and the areas of so-called Cambrian sedimentary rocks represent areas where the earlier crust remained unbroken by any such intrusion; and it is along the edges of these areas that the evidence of the later age of the granites is found, as there the granites are seen in a few places to penetrate the sedimentary strata and their associated diabases" (Low, 1903, p. 48D). Low did not indicate the localities where he saw granite cutting the sedimentary rocks. He was probably referring to areas where granite cuts Archean greenstones, since he apparently considered such greenstones to be equivalent in age to what we now refer to as Proterozoic. All geologists who have visited the east coast of Hudson Bay in recent years agree that the Richmond and Nastapoka groups rest unconformably on the granite and granitic gneiss. They have found no evidence of granite cutting the sedimentary and volcanic rocks which belong to these two groups. CORRELATION FROM SUB-AREA TO SUB-AREA Lithological similarities, the outpouring of lava flows, the intrusion of intermediate to basic sills, and essential continuity of the Labrador geosyncline, the Cape Smith belt and the Richmond Gulf sequence suggest that the sedimentary rocks outcropping within these areas were deposited in a continuous gcosyncline. This geosyncline would have extended at least from the general headwater area of the Hamilton River to Wakeham Bay on the south border of Hudson Strait, from there westward to Cape Smith and southward along the eastern part of Hudson Bay to Cape Jones. The Proterozoic rocks of the Labrador geosyncline are interrupted about 15 miles north of Roberts Lake, but similar sedimentary rocks (including ironformation) were discovered in the summer of 1955 to the northwest of the granitic area bordering the northern end of the geosyncline. Thus, most of the northwestern part of the province of Quebec was a land-mass probably free of tcctonism for a long period before the deposition, around most of it, of the Proterozoic sediments. It would appear that most of the clastic sediments deposited in this Proterozoic trough were not derived from this land-mass; this, at least, is the case in the Labrador geosyncline for the formations overlying the iron-formation. Shales, siltstones, and sandstones thicken to the northeast, thus suggesting that their source was from that direction. Volcanic rocks occur in force on the northeast side of the Labrador geosyncline. The iron-formations of the northeast side of the geosyncline are thin and lenticular, whereas those on the southwest side are thicker and more continuous. It seems, therefore, that an active block on the northeast side of the Labrador geosyncline shed clastic sediments to the southwest. The land on the southwest side was much less subject to

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movement and supported an extensively peneplained area. Such areas of mature weathering would provide material of the type found in the ironformations. The load carried by the rivers would be precipitated as it met marine waters along the southwest edge of the geosyncline. REFERENCES AUGER, P. E. (1954). The stratigraphy and structure of the northern Labrador Trough, Ungava, New Quebec; Can. Inst. Min. Met. Bull., vol. 47, no. 508, pp. 529-532. BERGERON, ROBERT (1954). A study of the Quebec-Labrador iron belt between Dcrry Lake and Larch River; Ph.D. thesis, Laval Univ. DE RÔMER, HENRY S. (1956). The geology of the eastern border of the "Labrador Trough," east of Thèvenct Lake, New Québec; M.Se. thesis, McGill Univ. GUNNING, H. C. (1933). Sulphuric deposits at Cape Smith, east coast of Hudson Bay; Gcol. Surv., Can., Sum. Rept., pp. 139D-145D. HARWOOD, T. A. (1949). The lead-zinc deposits of Richmond Gulf; B.A.Sc. thesis, Univ. of Toronto. KRANCK, E. H. (1951). On the geology of the east coast of Hudson Bay and James Bay; Acta Geographica, vol. 2, no. 2. LKITH. C. K. ( 1 9 1 0 ) . An Algonkian basin in Hudson Bay—a comparison with the Lake Superior basin; EC. Gcol., vol. 5, no. 6, pp. 433—458. Low. A. P. (1898). Report on a traverse of the northern part of the Labrador Peninsula from Richmond Gulf to Ungavn Bay; C.G.S.. Ann. Rcpt. 9, L43 pp. (1899). Report on explorations of the east coast of Hudson Bay; C.G.S., Sum. Rept., 1898 (Ann. Rcpt. 11), pp. 124A-133A. (1902). Report on an exploration of the cast coast of Hudson Bay from Cape Wolstenholme to the south end of James Bay; C.G.S.. Ann. Rept, 13, D84 pp. (1903). Report on the geology and physical characters of the Nastapoka Islands, Hudson Bay; C.G.S., Ann. Rcpt. 13, DD31 pp. MOORE, E. S. (1918). The iron formation of Belcher Islands. Hudson Bay, with special reference to its origin and its associated algal limestone: Jour. Gcol., vol. 26, pp. 412-438. PARKS, THURNF. (1949). A report on the geology of the Nastapoka group of sediments (Hudson Bay) with its contained lead and /inc bearing strata; B.A.Sc. thesis, Univ. of Toronto.

GEOLOGY OF CERTAIN PROTEROZOIC ROCKS IN QUEBEC AND LABRADOR W. F. Fahrig TWO BELTS of Proterozoic rock will be discussed in this paper. These are, first, that section of the Labrador "Trough" between Koksoak River in the north and Menihek Lake in the south, and second, the Labrador belt of Protero'/.oic rocks known here as the Scal-Crotcau groups. THE LABRADOR TROUGH The geology of the Labrador Trough has been described in a large number of scientific and semi-scientific papers beginning about the turn of the century with the remarkable reports of A. P. Low. In recent years the published material has dealt primarily with the Knob Lake area and particularly with the great iron ore deposits. This discussion will attempt to describe the Labrador Trough and its geological setting in somewhat more general terms. In simplest terms the part of the Labrador Trough here described consists of a western belt about 25 miles wide, of predominantly sedimentary rocks, and an eastern belt of similar width, of predominantly basic igneous rocks (see Fig. 1, and Harrison, 1952). In order to prepare a generalized geological map of that part of the Labrador Trough shown on Figure 1 it was found necessary to group the sedimentary strata into four units. These units are : ( 1 ) all rocks except dolomite, lying below iron-formation, (2) dolomite, (3) iron-formation, and (4) all strata lying above the iron-formation. Iron-formation forms a natural unit because of its economic importance and because it is probably a reliable time-stratigraphic unit. The iron-formation therefore divides the sequence into three units. The fourth unit consists of dolomite and is mapped separately because of its continuity over large areas and because, like ironformation, it represents in part chemical sedimentation. The grouping of sedimentary units on the map is thus based mainly on stratigraphy rather than lithology. Stratigraphy of the Sedimentary Rocks The stratigraphie sequence in the Knob Lake area has been described by J. M. Harrison (1952). The section consists from the base upward of 112

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quartzite, slate, dolomite, chert-breccia, quartzite, iron-formation, and slate. The origin of this sequence may be explained most simply in terms of three tectonic stages. (1) Rapid downwarp, marine transgression; quartzite and deep-water slate deposition fairly close to the present western boundary of the Labrador Trough. (2) Gradual filling of the basin of sedimentation; development of reef-structures; calcarenite formation from inshore reef spreading seaward over slates; possible deposition of bedded chert. Probable minor uplift, marine regression; partial erosion of dolomite and chert beds; spreading of orthoquartzite seaward. (3) Gradual downwarp, marine transgression, orthoquartzite, iron-formation, and finally slate spreading to the west. All of the formations, with the exceptions of quartzite, iron-formation and upper slate, wedge out toward the western boundary of the Trough. On the west boundary of the Trough, the orthoquartzite in this section may form a single formation deposited during various stages of marine transgression and regression (Harrison, 1952, diag. l e ) . The sharp westward wedging-out of lower formations is not present along some of the northern parts of this section of the Trough, particularly in the Cambrian Lake area. In this area, a remarkable axis of sedimentation extended from west of Cambrian Lake to at least the present region of Chakonipau Lake.1 The axis of this depression therefore crosses the present trend of Trough strata. It is unlikely that this depression had a tectonic origin. Most probably it was a wide erosional valley which formed an embaymcnt of the sea at an early stage of regional downwarping. During marine transgression this valley filled with coarse clastic sedimentary material, and with continued subsidence this was overlain by a thick sequence of shales. It may be possible to confirm that sediment transport was from west to east in this valley by a study of sedimentary structures in the coarser clastic portions. The thicknesses of sedimentary rocks along this depositional axis is of particular interest if the rocks do represent the filling of an erosional valley. West of Cambrian Lake, the medium- and coarse-grained clastic rocks have a thickness of several thousand feet, indicating that prior to subsidence local relief west of the present Trough must have been of considerable magnitude. It should be emphasized that the above interpretation would be considerably modified if it can be shown that this transverse depression had a tectonic origin. The sedimentary stratigraphie sequence from the Knob Lake area to the Larch River shows some remarkable similarities. One of the major changes is the absence of dolomite between Cambrian Lake and Forbes Lake. It seems likely that the presence of the previously described esturian basin through Cambrian Lake could, in part, account for the sharp lateral change in stratigraphy. 'S. M. Roscoc first pointed out the existence of a very thick series of basal elastics around Chakonipau Lake; Geol. Surv., Can., report on the East Cambrian Lake maparea, in preparation.

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The writer suggests that the above described major tectonic movements which affected sedimentation in the Knob Lake area were effective throughout the length of the Trough. It is likely that faciès variations along the Trough were primarily a function of the pre-Proterozoic topography. Igneous Rocks of the Labrador Trough The igneous rocks of the Labrador Trough consist of basalt, gabbro, basic p\ roclastics, ultramafics, and minor intermediate intrusive and extrusive rocks. The basalts are mainly pillowed and massive submarine flows but the belts of volcanic rocks northwest of Birch Lake have characteristics suggestive of sub-aerial extrusion. Stratigraphie evidence suggests that volcanism began early in the history of Trough sedimentation. The major part of the basic pyroclastic rocks appears to form a restricted layer in the volcanic sequence of any one section of the Trough. In the \\ illbob Lake area (Frarcy, 1952) they form a group lying below the major part of the submarine basaltic flows. It seems likely that the pyroclastic group forms a timc-stratigraphic unit and as such will be particularly useful in the correlation of the histories of sedimentation along the Labrador Trough. Most of the gabbroic intrusions of the Trough are sills and undoubtedly are intrusive equivalents of the submarine flows. A common, oddly textured variety of the gabbro, a glomeroporphyry variously called leopard rock, blotchy gabbro, etc., is probably a consanguineous variant of the gabbro. The serpcntinized periodotite (Fahrig, 1951; Frarey, 1952) is probably a differentiate of the gabbro. Metamorphism of the Labrador Trough and Eastern Trough Boundary With the exception of the more easterly parts of the Trough, the metamorphic grade of Labrador Trough strata is low (Harrison, 1952). Near the eastern boundary it increases progressively, until beyond this boundary the character of the rocks is determined largely by the metamorphism. East of Knob Lake in the Griffis Lake area, metabasalts end abruptly against a northwest-trending shear zone. To the east the rocks consist of schists derived from sedimentary and basic igneous rocks. Still farther east the rocks give way gradually to granitoid gneisses. Two hundred miles northwest along the Trough a strikingly similar situation exists. In this area, however, strata of the Trough may be traced eastward through zones of gradually increasing metamorphism. For example, some miles west of Erlandson Lake, the rocks, though genetically identical with Trough strata, are intensely metamorphosed, reaching at least the epidote-amphibolite faciès of metamorphism (see Fig. 1). East of Erlandson Lake, the grade increases progressively to False River where mineral assemblages typical of the staurolite-kyanite subfacies are reached. Still further east the rocks are again essentially granitic gneisses. This increase in metamorphism northeast across the Trough and into

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the schists and gneisses beyond indicates two things : ( 1 ) The metamorphism responsible for the present character of the schists and gneisses on the eastern boundary of the Trough is younger than the strata of the Labrador Trough. (2) The rock material which was metamorphosed to form the schists and gneisses east of the Trough is probably the same age as the Trough strata. Thus, although we may define the Labrador Trough geographically, the rocks to the east of the Trough are also Proterozoic in age, and since their present character is a function of later metamorphism, their major features must be considered younger than the Trough strata. To test these conclusions it is expected that the approximate ages of the granitic rocks to the east and west of the Trough will be determined in the near future. These age determinations will be carried out in laboratories of the Geological Survey of Canada using the potassium-argon method. For many years it has been assumed that the eastern "boundary"' of the Labrador Trough consists at least in part of a great thrust fault (sec Tectonic Map of Canada, 1950). Within the section of the Trough here discussed, this supposed fault separates commonly low-grade metabasalts and sediments on the west from higher-grade schists and gneisses on the east. This boundary is further emphasized in some areas by a zone of intense shear. It is likely that a fault zone does exist along this line of demarcation. If it is desirable to retain the term "Labrador Trough" as a geographic term, its eastern boundary should be defined by this fault. The Western Boundary of the Labrador Trough The presence of rather similar unmetamorphosed (Proterozoic?) rocks in areas reasonably close to the Labrador Trough raises the question of whether all or part of these rocks were laid down contemporaneously in isolated basins or even in a single basin. The rocks of the Belcher Islands, Seal Lake. South Baffin Island, Cape Smith, and Mistassini are some of those which might be thought to have such a relationship to the Labrador Trough. The solution to this problem lies in attempting to date the sedimentary and igneous rocks of the belts by geochemical methods; by tracing the belts through metamorphic terrain; and by lithologie and stratigraphie correlation of non-contiguous strata. Since this paper deals with only a section of the Labrador Trough, only one aspect of the problem is discussed, namely the original lateral extent of this section of the "Trough strata." The problem of the eastern boundary of the Trough has already been considered under the heading "metamorphism," so there is left to be discussed here only the western border. As described above, lower strata in the Knob Lake section lens out rapidly towards the west limit of the Trough. This rapid thinning and disappearance of at least four of the lower formatons (Harrison, 1952) suggests that this was a tectonically neutral zone at an early stage in the development of the sedimentary basin and subsequent folded belt. Three formations from the base upward, quartzite, iron-formation, and slate apparently extended west of the present Trough boundary.

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Of these, the lower two formations, orthoquartzite and iron-formation, are thin and may not have extended far to the west. However, the upper slate formation is indicative of deep-water sedimentation. Graded bedding in West Lac Herodier area (Fahrig, 1955) and its presence along this whole section of the Trough indicate a considerable marine transgression with a shifting of the axis of sedimentation some distance to the west. Probably the deep-water upper slates extended with their shallow-water facies a considerable distance west of the present Trough boundary, and the present boundary is in fact an older tcctonically neutral zone. [EDITOR'S NOTE: Dr. I. W. Jones has contributed the following information about the Mount Wright Area: "The rocks of the southern part of the Labrador 'Trough' (Geosyncline) form two branches, one swinging southwestward toward Wabush Lake and the other southeasterly toward the Ossokmanuan Lake area. Mapping has not proved yet a geographical continuity between the Wabush rocks and those outcropping in the Mount Wright area, but the lithological similarities of the rocks in the two areas and the relatively short distance between them point to a continuation of the Labrador geosyncline from Wabush Lake to the Mount Wright area. The rocks of this latter area are highly folded and highly metamorphosed quartzites, quartz-specular hematite-magnetite iron-formation, quartz-gruneritemagnetite iron-formation, biotite-feldspar gneiss, amphibolites, and gabbro. The iron-formations are of sedimentary origin."] SEAL-CROTEAU GROUPS Introduction The Seal and Croteau groups (Fig. 2) form a belt of Proterozoic rocks about 80 miles northwest of Goose Bay, Labrador. The general outline of the belt and distribution of rock types is shown on Geological Survey of Canada Preliminary Map 53-14 (Christie et al., 1953). The Seal group comprises the strata west of a fault trending south from Pockctknife Lake, and the Croteau group comprises the Proterozoic strata to the east. Croteau Lake, from which this latter group is named, lies just east of Pocketknife Lake. The Seal group extends a few miles to the west of 63 degrees longitude. An excellent study of an area which includes the Seal and Croteau groups was made by R. A. Halet ( 1946). However, his report is not generally available, so a few details, particularly referring to the rocks of the area bounded by 54 to 55 degrees north latitude, 61 to 62 degrees longitude, are presented below.2 In addition, the readers are referred to a report by E. L. Evans (1952) for a brief description of copper deposits in the Seal Lake area. The Seal and Croteau groups lie on "Early Precambrian" gneisses, massive granitic rocks, anorthosite, and related gabbroic rocks. It is assumed that the anorthositic and related rocks postdate the gneisses, probably forming contemporaneously with the metamorphism and granitization of the latter. 2

W. F. Fahrig, Geol. Surv., Can., report on East Sncgamook Lake Area, in press.

FIGURE 2. The Seal and Crotcau groups.

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It is possible that some of the granitoid gneisses are metamorphosed younger rocks, particularly near the south boundary of the Seal and Croteau rocks. It is also possible that some of the granitic intrusives north of the Proterozoic rocks are younger than the Seal-Croteau groups. The major reason for assuming that the granitic gneisses to the south of the Proterozoic rocks are of the same general Early Precambrian age as those to the north is the presence in both areas of large anorthosite masses. The variations in the character of the Early Precambrian granitoid rocks are too many to describe within these notes. The rock type is best described as a migmatite ranging from white-weathering muscovite granite to amphibolite gneiss. The banding of the gneisses east of Mistinippi is generally straight and fairly regular. It appears to dip steeply towards the anorthosite body along its western contact with the anorthosite. The belt of igneous rocks southwest of Otter Lake consists mainly of coarse-grained light-weathering porphyritic granitic to dioritic rock containing potash feldspar, plagioclase, hornblende, biotite, quartz, epidote and chlorite. It is not strongly gneissic and may be younger than the pre-Croteau granitic-anorthositic rocks. Part of the belt consists of a later fine-grained pink-weathering granitic rock containing blebs of epidote. Croteau Group The Croteau group is a northeast-trending sequence of sedimentary and igneous rocks. Members of the Croteau group have been mapped by exploration companies for a distance of at least 30 miles northeast from Croteau Lake, and a patch of volcanic rocks still further east, north of Micmac Lake, may be a continuation of this group. It is assumed that the strata north of Croteau Lake form the basal part of the group, and that these are separated from the gneisses further north by an erosional unconformity. The lower strata are composed predominantly of pyritic black shales with minor quartzite, greywacke and dolomite, and green metabasalt. These rocks are overlain along Croteau Lake by boulder conglomerate, and on Pocketknife Lake by arkose. The conglomerates on Croteau Lake and just to the north consist of boulders of red quartzite and cobbles of jasper in a sandstone matrix. The associated sandstones north and south of Croteau Lake, and the sandstone along the east arm of Pocketknife are feldspathic and banded pink-maroon-cream in colour. This sandstone locally contains cherty layers, is crossbedded in its coarser phases, and locally has a carbonate cement. These sandstones have the following microscopic characteristics: they are composed of moderately to poorly sorted and rounded grains of feldspar, quartz, chert and other rock fragments; feldspar is more abundant than quartz and the matrix is now sericite and fine-grained quartz. Southeast of Pocketknife Lake there are a number of thin light-weathering sandstone layers between the overlying strata. These resemble the sandstones around Pocketknife, but at least locally contain abundant microscopically visible

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devitrified volcanic glass shards. These now are composed mainly of a fine quartz mosaic. The major part of the rocks of the Croteau group between Croteau and Otter lakes are volcanic rocks of rhyolitic to andesitic composition. Their colour is generally purple, pink, dark green or light grey-green. They are almost universally porphyritic; most commonly the phenocrysts are feldspar but locally small quartz eyes are present. A common variety consists of quarter-inch euhedral feldspar crystals and irregular clots of ferromagnesians and epidote in an aphanitic purplish groundmass. Amygdaloidal and flow breccia layers are common in the volcanic rocks southeast of Pocketknife Lake. Fold axes in the Croteau group generally trend in a northeasterly direction. The felsitic volcanics along the south limit of the group are schistose in many places. A large number of mineral prospects have been reported from rocks of the Croteau group, the more common being chalcopyrite, chalcocite, bornite, galena and sphalerite. Seal Group To the west of the Croteau group lie the unmetamorphosed rocks of the Seal group, consisting of continental and marine type sedimentary rocks and volcanic rocks intruded by diabasic and gabbroic sills. The stratigraphie sequence consists from the base upward of a unit composed mainly of coarsegrained clastic rocks and red shales with minor volcanics; a thick volcanic sequence with interbedded and overlying grit and arkose layers; a zone of interbeddcd red slates and volcanics; and an upper zone composed almost entirely of quartzite and arkose with minor slate. The diabase and gabbro >ills arc unevenly distributed through the section, the vast majority having been intruded above the lower volcanic zone, about half way up in the section. It is believed that the Seal group along the Canairiktok River lies unconformably on a surface of anorthosite and granitic gneisses. The conglomerates of the group generally are composed of pink granite boulders and cobbles in a red sandy matrix. The grit and sandstone phases are generally pink or red in colour. The grit layers are composed of either coarse white quartz fragments or white quartz with pink feldspar. Shear zones in the grits are yellowish or greenish owing to formation of sericite in the matrix. Some of the quartzite, particularly between Seal and Wuchusk lakes, is white in colour and composed of well sorted and rounded quartz grains. The uppermost quartzite south of Seal Lake is composed of moderately sorted and rounded quartz grains, cemented by quartz and containing a small proportion of feldspar, chert, amphibole and fine-grained rock fragments. Hematite is present as a thin film around the clastic grains and selectively replaces certain grains, particularly rock fragments. Its presence to an extent of less than 3 per cent accounts for the red colour of the quartzite.

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The fine-grained rocks of the Seal group are mainly red and maroon argillites and slates with minor grey and black slate and metamorphosed equivalents. Narrow bands of varicoloured, hard, bedded, fine-grained sediments were observed within the zone of diabase intrusions, particularly southeast of the Thomas River. One such exposure forms a cliff along the south shore of Nascaupi Lake. It is composed of alternate cream and light purple layers, one-half to four inches in thickness, of about equal hardness. Other darker colours have been observed in this rock type. In general this rock exhibits no cleavage and has a cherty texture. It was observed to pass into banded red shales away from the diabase contact, and in thin-section appears to be a hornf els. The volcanic rocks are mainly light or dark green, hematite-rich, amygdaloidal flows which are dark maroon or green on fresh surfaces. Where oxidation is pronounced the weathered surface has a similar reddish colour. Variation in colour is believed due to degree of oxidation and hematite enrichment rather than to primary difference in lava type. The lavas are generally redder near the boundary of flow units and near columnar joints. The green parts of flows contain unaltered pyroxene and feldspar, while epidote is more common in the reddish parts. The amygdules of the flows are composed of calcite, chlorite, quartz, epidote and zeolite. Small tearshaped or more irregular very fine-grained dark green inclusions are also commonly present. The amygdules are noticeably concentrated near the top and base of the flow units. The latter generally are a few tens of feet thick. Poorly developed pillows were observed, but these arc rare. The volcanic rocks of the Seal group are basic, probably basaltic in composition. Diabasic and gabbroic sills form a considerable part of the Seal group. These are concentrated in a zone above the lower thick volcanic sequence and below the interbedded slate and volcanics. Individual sills range up to several hundred feet in thickness, and in the central parts of these sills, plagioclase and pyroxene crystals attain major diameters of at least an inch. Olivine is commonly observed in thin-sections of these rocks. Small bodies of probably intrusive gabbro were mapped in areas of Croteau rocks. In addition, some small outcrops of serpentinized peridotite are present just east of Croteau Lake, and numerous boulders of this rock are present in the drift north of the east arm of Pocketknife Lake. The Seal group forms an arcuate east-west trending synclinorium which is overturned towards the north and truncated on the south by a zone of moderately dipping thrust faults. The general appearance of the structure is that of a monoclinal sequence dipping towards the south at a low angle. This monoclinal appearance is the result of the overturning of beds towards the north, reverse strike faulting, and a consistent south-dipping cleavage. The features most readily available for top determinations are crossbedding and ripple-marks. These structures are restricted mainly to arenaceous and coarser-grained horizons but fortunately are common at these horizons. The type of crossbedding most commonly observed was concave

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crossbedding, a less commonly observed type being torrential crossbedding. Slaty cleavage (flow cleavage) in the Seal group is not consistently parallel to bedding planes, and since much of the red slate lacks a pronounced primary banding there is a tendency to assume that bedding and cleavage planes are parallel. Flow cleavage, fracture cleavage, and schistosity all dip consistently to the south. A number of east-west strike faults have been suggested within and on the south border of the Seal group. Evidences for these are prominent straight lineaments, which in places truncate structure, and zones of schistose and cataclastic rock. The fault truncating the late Proterozoic rocks about 8 miles south of Seal Lake is bordered on the north by a schistose band of quartzite and greenschist of variable width, and on the south by a narrow band of cataclastic Early Precambrian granitic rock. This is probably a reverse fault of considerable magnitude. Stratigraphie Relation of the Seal and Croteau Groups The stratigraphy of the Seal and Croteau groups shows many contrasting features. The lower part of the Croteau group has features suggesting deposition in a marine environment, while the upper sedimentary parts of the group have characteristics suggesting deposition in a shallow-water, possibly continental, environment. On the other hand, all of the sedimentary strata of the Seal group indicate continental or epicontinental marine deposition. The major part of the volcanic rocks of the Croteau group is intermediate or acidic in composition, while that of the Seal group is predominantly basic. The stratigraphie differences between the two groups are great enough, and change abruptly enough to suggest a difference in age between the groups. The structural relations of the two groups suggest that the Croteau group is in part contemporaneous but generally older than the Seal group. If such is the case, the Croteau group has been raised relative to the Seal group along the fault which runs south from Pocketknife Lake and generally separates the two groups. THE LABRADOR TROUGH AND THE SEAL-CROTEAU GROUP Although there are similar strata, some of which require rather special conditions of sedimentation, in the Seal-Croteau and Trough areas, there is no reason to believe that these strata formed contemporaneously. Tectonically, the two areas are strikingly different. In the Seal-Croteau area, continental sedimentary and volcanic rocks are bounded by Early Precambrian granitoid gneiss which includes great masses of anorthosite. In the Labrador Trough area epicontinental and géosynclinal sedimentary and igneous rocks are bounded on the west by older gneisses and on the east by gneisses formed during a later period of metamorphism. Apparently the Seal-Croteau groups formed on a Continental Crustal Area and the Labrador Trough developed on a Marginal or Oceanic

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Crustal Area. It will be of interest to determine by geochemical methods the relative ages of the gneiss just northeast of the Trough and of that around the Seal-Croteau group. If the gneiss northeast of the Trough is older than the basement gneiss in the Seal-Croteau Area, then the strata in the Trough must predate the Seal-Croteau groups. If the two gneissic areas are of the same age, the Trough strata still must predate the Seal-Croteau groups. Finally, if the gneiss in the Seal-Croteau area is older than that northeast of the Trough, then the relative ages of the Seal-Croteau and Trough strata are not defined. The writer believes that the basement gneiss of the Seal-Croteau area will be found to postdate the gneiss northeast of the Trough and hence that the Seal-Croteau groups are younger than the Labrador Trough. The gneiss underlying the Seal-Croteau rocks is generally thought of as Early Precambrian and the Trough strata may predate these gneisses. This illustrates that the terms Early and Late Precambrian (Archean and Proterozoic) as commonly used have only local and relative meaning. REFERENCES CHRISTIE. A. M., ROSCOE, S. M., nnd FAHRIG. W. F. (1953). Central Labrador Coast, Newfoundland; Geo!. Surv., Can., Paper 53—14. EVAXS, E. L. (1946). Native copper discoveries in the Seal Lake area, Labrador; Proc. Geol. Assoc.. Cnn., vol. 5, pp. 111-llf). FAHRIG, W. F. (1951). Griffis Lake, Quebec; Geol. Surv.. Can.. Paper 51-23. (1955). Lac Herodicr, New Quebec: Geol. Surv.. Can., Paper 55-1. (in press). East Snegamook Lake: Gcol. Surv., Can. FRAREY, M. J. (1952). Willbob Lake, Quebec and Newfoundland: Gcol. Surv., Can.. Paper 52-16. HALET, R. A. (1946). Geological reconnaissance of the Nascaupi Mountains and adjoining coastal region; private rept., Dome Exploration Limited. HARRISON, J. M. (1952). The Quebec-Labrador iron belt, Quebec and Newfoundland; Geol. Surv.. Can., Paper 52-20. ROSCOE, S. M. (in préparation). Cambrian Lake, New Quebec; Geol. Surv.. Can.

LATE PRECAMBRIAN ROCKS OF THE NORTH SHORE OF THE ST. LAWRENCE RIVER AND OF THE MISTASSINI AND OTISH MOUNTAINS AREAS, QUEBEC* Robert Bergeron THE ROCKS described in this paper are assumed to be of late Precambrian age. They include the Wakeham, Mistassini and Otish Mountains groups. THE WAKEHAM GROUP Location and Limits The Wakeham rocks outcrop east of Havre-St. Pierre from Romaine River to a few miles east of Natashquan River and extend inland from the shore of the Gulf of St. Lawrence 60 to 80 miles. They occupy an area of over 1,000 square miles (see Fig. 1). General Character and Succession The group consists of a thick sequence of sedimentary rocks intruded by gabbro sills. Quartzite is the predominant sedimentary rock and includes white vitreous, grey micaceous, feldspathic, and hematite-rutile varieties. The purer varieties are usually thick bedded; the impure varieties are thin bedded and, in places, crossbedded. A few beds of slate and conglomerate are interbedded with quartzite, and here and there bands of metamorphosed argillaceous rocks several hundred feet thick outcrop in place of the quartzite. Intrusive into the Wakeham sedimentary rocks are numerous sills of hornblende gabbro ranging in thickness from 50 to 1,500 feet. The gabbro is greenish black, massive, and generally coarse grained and ophitic. It consists of about equal amounts of hornblende and plagioclase with minor ephidote, biotite, sphene, and magnetite. Late granites also cut the Wakeham rocks. "Published with the permission of the Deputy Minister of Mines, Quebec.

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Structure and Grade of Metamorphism The sedimentary rocks have been folded into anticlines and synclines whose axes are generally north-south. Dips are variable in direction and amount. The gabbroic magma invaded the sedimentary rocks before and during deformation as sills and as discordant masses at the noses of many folds. The maximum apparent thickness of the Wakeham sedimentary rocks, measured across the limbs of some of the folds, is between 15,000 and 20,000 feet. It is very likely that the true thickness is much less than this

FIGURE 1. Late Precambrian rocks of Quebec.

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figure and that certain parts of the sequence have been repeated by faulting. Although evidences of faulting are very few, it is quite possible that unrecognized faults, particularly longitudinal breaks, exist. The relationships between rocks of the Wakeham group and the granitic gneisses to the west are obscure owing to the complex structure and the presence of masses of late granites with local gneissic faciès. But the relationships with the granitic rocks to the east are known. Biais (1955, p. 7) has found evidence in the Pashashibou area of a major fault bordering the Wakeham quartzites and gabbros. This is indicated by hydrothermal alteration, intrusion of pegmatite and porphyry bodies, innumerable quartz stringers, drag-folds, a few zones of shearing and brecciation, and abrupt termination of granitic terranes. The mineral association of the sedimentary and igneous rocks of the Wakeham group is characteristic of the albite-epidote amphibolite faciès. Staurolite schists are found along some faults, but these rocks were formed as a result of a dynarno-metamorphism. Correlation Following the recent work done by Biais in the Pashashibou area, there is little doubt that the sedimentary rocks of the Wakeham group are younger than the Grenville paragneiss. Bcland (1950), Grenier (1952), and Biais (1955) have suggested that these rocks were deposited under similar conditions and at approximately the same time as the rocks at the southern end of the Labrador geosyncline. Such long-range correlation is always open to doubt. However, it is supported in the present case by similarities of composition, texture, degree of metamorphism (weak in both cases ), and of association with gabbro sills. THE MISTASSINI GROUP Location and Limits Sedimentary rocks outcrop in the Lake Mistassini area 160 miles north of Lake St. Jean and 20 miles northwest of the divide between the Hudson Bay and St. Lawrence River drainage systems. These rocks were recognized by early visitors in the area and referred to as "Mistassini Lake sediments" or as "Mistassini series" (Barlow, 1911). According to recent stratigraphie usage it would be preferable to use the word "group" instead of "series." The group crops out as a segment of a circle, the arc of which, measuring roughly 100 miles, coincides with the northwest shore of Lake Mistassini. General Character and Succession The Mistassini group consists of five formations, from bottom to top : the Papaskwasati, the Cheno River, the Lower Albanel, the Upper Albanel, and the Temiscamie.

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The Papaskwasati consists of well-bedded, quartzose sandstone which crops out about ten miles north of the northeastern extremity of Lake Mistassini. It overlies Archean rocks with great unconformity, and appears to be conformable with the overlying Cheno River formation. The lowermost beds contain quartz pebble beds and layers of pebble conglomerate up to four feet thick. The average diameter of the rounded quartz pebbles is three-quarters of an inch. The Cheno River formation consists largely of arkosic conglomerate, greywacke, pebble conglomerate, sandstone, and fine-grained dolomite. In some places it overlies the Papaskwasati sandstone and in others rests with great unconformity over Archean rocks. The main character of the formation is an interbedding of rock types which represents conditions of deposition transitional from coarse clastic rocks to the dolomitic formations that occupy the greater part of the Mistassini basin. The Lower Albanel formation is made up of massive bluish-grey, grey or dark grey dolomite grading, in places, into shaly, slaty, or ferruginous dolomite. Nodules of chert up to eight inches in diameter occur in the shaly beds overlying the massive basal beds. Traces of intraformational conglomerates are found in some exposures of the massive type. The ferruginous dolomite shows alternating layers of fine-grained, grey dolomite and reddishbrown ferruginous dolomite. The latter rock contains lenses of jasper and vugs lined with calcite and ferrodolomite. The Upper Albanel formation consists of a thick sequence, from bottom to top, of sandy dolomite, dolomitic sandstone, interbedded massive dolomite and sandy dolomite, and a sandy dolomite characterized by algal structures at two or more horizons. These algal structures are similar in all points to other cryptozo'on-like structures found in the Labrador geosyncline (Fahrig, 1955, p. 5) and on the Belcher Islands (Moore, 1918). In the Lake Mistassini basin, these structures are elliptical or circular in plan and range from six inches to fifteen feet in diameter. Seen in vertical cross-section they consist of concentric bands of convex-upward laminae. The Temiscamie formation is made up of four distinct members which are, from bottom to top, quartzite and conglomerate, a lower slate, an iron-formation, and an upper slate. The lower part of the iron-formation is composed of ferruginous chert with siderite aggregates. This lower horizon shows in places intercalations of lenses of jasper and of magnetite-rich zones. The upper part of the iron-formation consists of alternating layers of chert and siderite. Individual layers of siderite are up to six inches thick; those of chert are usually two inches thick, but may be as much as three feet thick. Structure and Grade of Metamorphism Detailed measurements of the thickness of the Mistassini group are lacking, but Neilson (1950) has estimated this to be at least 6,500 feet and possibly more than 8,400 feet. Unconformities observed within the Mistassini group represent surfaces

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of disconformity rather than major breaks. The sandy dolomite of the Upper Albanel formation truncates the crests of folds in the underlying rocks. The disconformity at the base of the Temiscamie formation appears to be a break resulting from scour and non-deposition. The basal beds of this formation truncate minor folds and algal structures. The Mistassini rocks have a regional strike varying between 10 and 45 degrees northeast and dips ranging from 5 to 75 degrees southeast. In the northwest part of the Mistassini basin the strata are moderately inclined but southeastward the dips increase and there is some overturning. Two directions of folding are observed. An earlier period of deformation produced undulations with eastward- and southeastward-trending axes. The later produced a northeast-trending fold and fault system, and major regional deformation. Most of the recognizable faults southeast of Lake Albanel strike northeast or north-northeast; the movements appear to have been of the normal type, with small displacements. As mentioned above, to the northwest of the Mistassini basin the sedimentary rocks rest unconformably over granitic rocks. To the southeast, the rocks of the Mistassini group are in contact with igneous and metamorphic rocks. Mapping done by the Quebec Department of Mines has established the existence along the southeastern margin of the Mistassini sedimentary basin of a high angle reverse fault dipping 45 to 55 degrees toward the southeast. Adjacent to the southeastern contact of the sedimentary basin, the sedimentary rocks are steeply dipping and the igneous and metamorphic rocks are commonly crushed and sheared. The shear surfaces are parallel to the contact and dip 40 to 60 degrees southeast. The intensity of the shearing decreases eastward. The magnitude of the movement along the. fault is difficult to evaluate but its vertical component does not seem to be of very great extent. The sedimentary rocks are but little metamorphosed. Neilson (1953, p. 22-23) gives the composition of one member of the Temiscamie formation, an iron-silicate slate, as being 75 per cent siderite, 5 per cent chert, and 20 per cent minnesotaite and stilpnomelane. Although there is no unanimity concerning the sedimentary or metamorphic origin of these two iron silicates, James (1955, p. 1474) states that minnesotaite and stilpnomelane are the characteristic silicates of the chlorite and biotite zones of metamorphism. Thus, the mineral assemblage of the iron-formation would indicate that the rocks of the Mistassini group in general are at the low level of metamorphism. Correlation The low grade of metamorphism and the unconformable relationships to more ancient rocks indicate that the Mistassini group is late Precambrian in age. These rocks are lithologically similar to Proterozoic rocks of the Labrador geosyncline and along the east coast of Hudson bay. One cannot disagree with Wahl's conclusion regarding the problem of correlating the

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rocks of these three areas when he wrote: "The stratigraphy, lithology, and geological history of these three regions are so similar as to indicate that the sediments to form their underlying rocks were deposited at approximately the same time and perhaps in the same epicontinental sea" (Wahl, 1953, p. 26). The lowest formation of the Mistassini group, the Papaskwasati sandstone, could be a correlative of the Chibougamau series. In composition, undeformed nature, attitude of beds, and stratigraphie relation to the basement rocks, this formation resembles the Chibougamau series as described by Mawdsley and Norman (1935, p. 43). THE OTISH MOUNTAINS GROUP Location and Limits Sedimentary and associated igneous rocks outcrop in the Otish Mountains area. The sedimentary rocks were deposited in a basin which likely was the northeastern extension of the Lake Mistassini sedimentary basin. The Otish Mountains group extends northeastward to a point roughly 120 miles northeast of Lake Mistassini and is approximately 20 miles wide. It consists mainly of sandstones with sills and discordant intrusions of gabbro. The Otish Mountains are located slightly north of 52 degrees north latitude and between longitudes of 70 and 71 degrees, or some 400 miles north of Quebec City. They are an easterly-trending series of hills about 40 miles long and 10 miles wide. They consist of gently dipping sedimentary rocks forming irregularly dissected cliffs and steep north-facing slopes. The highest elevation is 3,700 feet, and the average is 3,200 feet above sea level. Local relief varies from 300 to 800 feet. The region south of the mountains is covered with a thick mantle of morainic material and, in the vicinity of major drainage, with fluvio-glacial deposits. General Character and Succession* The Otish Mountains group consists of a thick series of conglomerate and sandstone overlain by red sandstone and red shale. Sills and discordant bodies of gabbro intrude the red beds. The group rests unconformably over Archean rocks. The sandstone and conglomerate sequence is approximately 3,000 feet thick. The most abundant rock of this sequence is a medium- to coarsegrained, thick-bedded sandstone containing, near the base, numerous lenses of grit and conglomerate. The conglomerate lenses are made up mainly of white quartz pebbles up to six inches in diameter. In places this medium- to coarse-grained sandstone grades into a well-bedded, fine-grained, mediumgrey sandstone composed ma/inly of sub-rounded quartz and feldspar grains. In many places the feldspar content exceeds 10 per cent and the rock grades •During the summers of 1953 and 1954, Kennco Explorations, Ltd., conducted a broad survey of the Otish Mountains area. Permission to publish geological information gathered by them has been kindly granted to the writer.

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into feldspathic sandstone and arkose. A few gabbro sills have invaded this lower sequence. The sandstones and conglomerates are overlain by red sandstone and red shale. The red sandstone is similar to the well-bedded faciès of the lower sequence in granularity, composition and bedding. The reddish colour results from a dissemination of fine-grained hematite. The red shales are very fissile and, in many places, quite silty. The matrix consists of a micaceous and argillaceous material including fine dusty hematite and patches of carbonate. This upper sequence is the host rock of most of the gabbro intrusions of the area. The weathered surface of the gabbro is dark grey, whereas the fresh surface is slightly vitreous and greenish black. The margins of the gabbroic masses are fine grained, but inward the grain size becomes coarse and an ophitic texture is quite visible. The sills usually display a well-exposed columnar jointing. The gabbro is composed of 50 to 70 per cent of almost completely saussuritized plagioclase (Anoo), and of 30 to 35 per cent of a partly uralitized pyroxene, probably augite. Structure and Grade of Metamorphism The structure of the Otish Mountains rocks is that of a shallow basin consisting almost entirely of unmetamorphosed sedimentary rocks invaded by general concordant gabbroic masses. In the Otish Mountains, the dips are about 20 degrees southward. Southwest of the mountains, the dips are flat or 10 to 15 degrees eastward or southeastward. North of the Otish Mountains the sedimentary rocks lie on a flat erosion surface developed on the Archean, and it would seem that the same relationships persist southward and westward. Local irregularities of the floor of the basin may account for some small variations of the attitude of the overlying sedimentary rocks. Little information is available on the contact at the southeast side of the basin. Rocks near this contact arc more metamorphosed than elsewhere and consist of metamorphosed sandstones, metadolomites and iron-formation. It would appear, therefore, that the over-all structure of the Otish Mountains basin is similar to that of the Mistassini basin and that the rocks are more disturbed in the vicinity of the southeast contact. Correlation There is little doubt that extension of the mapping already done in the Mistassini area will prove that the Otish Mountains basin is the northeastward extension of the Lake Mistassini basin. The lower sandstones and conglomerates of the Otish Mountains group appear to be equivalent to the Papaskwasati sandstone of the Mistassini, and possibly to the Chibougamau series. Also, some of the rocks of the Otish Mountains upper sequence are similar to some members of the Cheno River formation. However, definite correlation is impossible at the moment.

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REFERENCES BARLOW, A. E., FARIBAULT, E. R., and GWILLIM, J. C. (1911). Geology and mineral resources of the Chibougamau region, Quebec; Que. Dept. Mines and Fisheries, Mines Branch, pp. 131-133. BÉLAND, RENE (1950). Le synclinal du lac Wakeham et la Fosse du Labrador; Naturaliste Canadien, vol. 77, pp. 291-304. BLAIS, RocER-A. (1955). Preliminary report on Pashashibou area, Drucourt and Costebelle townships, Saguenay county; Que. Dept. Mines, P.R. no. 316. (1955a). La discordance tectonique entre le groupe de Wakeham et les gneiss granitiques de la région de Natashquan; XXIIIe Congrès de l'ACFAS, Ottawa. FAHRIC, W. F. (1955). Lac Herodier, New Quebec; Gcol. Surv., Can., Paper 55-1. GRENIER, PAUL-E. (1952). Géologie et petrologie de la région du lac Beetz, compté de Saguenay; unpublished Ph.D. thesis, Laval Univ. JAMES, HAROLD L. (1956). Zones of regional metamorphism in the Precambrian of northern Michigan; Bull. Gcol. Soc. Am., vol. 66. pp. 1455—1488. NEII.SON, JAMES M. (1950). Geology of the Lake Mistassini region, Northern Quebec; unpublished Ph.D. thesis, Univ. of Minnesota. (1953). Albancl area, Mistnssini territory; Que. Dept. Mines, Geol. Rept. 53. MOORE, E. S. (1918). The iron formation on Belcher Islands, Hudson Bay, with special reference to its origin and its associated algal limestone; Jour. Geol., vol. 26, pp. 412-438. WAHL, W. G. (1953). Temiseamie River orea, Mistassini territory: Que. Dept. Mines. Geol. Rept. 54.

THE GRENVILLE PROVINCE D. F. Hewitt, F.R.S.C. THE QUESTION of whether the sedimentary and volcanic rocks of the Grenville province of the Canadian Precambrian belong wholly or in part to the Proterozoic is a matter of controversy. The recent symposium on The Grenville Problem published by the Royal Society of Canada1 deals with this and other problems of the Grenville, and readers are referred to it for a more complete discussion of the subject. Unfortunately there is no general agreement on the age and correlation of the Grenville series, nor even on what constitutes the Grenville scries. However, this paper gives a summary of some of the ideas on the Grenville province or subprovince, the Grenville erogenic belt, the Grenville front and the Grenville series. THE GRENVILLE PROVINCE OR SUBPROVINCE The Grenville and Timiskaming subprovinces formerly made up the St. Lawrence province of the Canadian Precambrian Shield, but the latter term has fallen into disuse. Modern age determinations and study of regional tectonics of the Shield have indicated that the Canadian Shield can be divided into several provinces which formed orogenic belts during different times in Precambrian history (Gill, 1949). Since the Grenville region is not now a subdivision of any larger Precambrian province from a structural viewpoint, and since it forms a large tectonic unit within the Shield, it seems better to use the name "province" rather than "subprovince," although the latter has the merit of long usage. In 1925 M. E. Wilson (1925, p. 389), in summarizing the results of twelve years of field work in the Grenville of Ontario and Quebec, states that "Southeastern Ontario, the Southern Laurentian highlands of Quebec, and the Adirondack region together form the Grenville subprovince." He defines the Timiskaming subprovince as the region extending northeast from Lake Superior and Lake Huron to Lake Timiskaming and Lake Mistassini. He further notes that between the Grenville and Timiskaming subprovinces there intervenes a belt of banded gneisses, mainly granitic, twenty-five to forty miles wide. As originally defined then, the Grenville and Timiskaming subprovinces were separated by a belt of banded gneisses (amphibolites, paragneisses, l The Grenville Problem, ed. James E. Thomson, Royal Society of Canada, Special Publications, no. 1 (University of Toronto Press, 1956, 128 pp.).

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migmatites and granitic gneisses) which made correlation difficult. However, although the difference in grade of metamorphism, lithology and tectonic style between the Timiskaming subprovince and this belt of banded gneisses is quite marked and now forms the line called the "Grenville front," there is little or no difference in lithology, grade of metamorphism and tectonic style between this belt of banded gneisses and the gneisses of the Grenville province to the south. Gradually the term "Grenville subprovince" came to refer to the whole area south of the Grenville front. The "Grenville province" is defined geographically as the area lying southeast of the "Grenville front" ; it is a structural and tectonic unit and is coincident with the "Grenville orogenic belt." The Grenville province, as constituted at present, is that area of the Canadian Precambrian Shield which was directly involved in the folding and mountain building of the Grenville orogenic cycle which apparently culminated with the intrusion of granites and pegmatites, and a period of deep-zone regional metamorphism, about one thousand million years ago. The Grenville province cannot be defined as that area in which the Grenville series occurs, because it has become apparent that the term "Grenville" really has no defined timestratigraphic value: the Grenville is not a time-stratigraphic unit, but rather a poorly defined lithologie assemblage. Workers in the Grenville region are agreed, as indicated by papers and discussion at the Grenville Symposium, that rocks of diverse age occur in the Grenville province, and that the whole area has been the site of Late Precambrian mountain building. Work in Ontario and particularly in Quebec has indicated that in several places the Grenville front is not a fault, but a metamorphic front, and that rock units can be traced across this front. This is true in the Sudbury area of Ontario, and in the Chibougamau area of Quebec. It therefore becomes apparent that the term "Grenville series" cannot be applied indiscriminately to any high-grade metamorphic paragneisses merely because of their geographical position within the Grenville province. As F. F. Osborne (1956) points out, in the Grenville region of Quebec a convention grew up that any obviously metamorphic rocks, probably originally sedimentary or volcanic, are referred to the Grenville series. This resulted in referring almost all the metasedimentary and metavolcanic rocks in some areas, regardless of age or stratigraphie relationships, to the Grenville series. The term "Grenville" is being used in different senses by different workers for time, lithology and tectonics: this paper is intended to clarify the usage of "Grenville province," "Grenville fault," and "Grenville orogenic belt" as valid and useful concepts in a geographic, structural and tectonic sense, separate and distinct from the usage of "Grenville series" which may be properly applied to a restricted lithology or rock sequence in the original Grenville area. In Ontario it seems likely that rocks which have been referred to the

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Huronian in the Sudbury area are traceable across the Grenville front. In the Grenville Symposium volume (1956) W. G. Robinson's paper on the Grenville of New Quebec deals with rocks that are geographically and tectonically within the Grenville province, but which are certainly of Proterozoic type. GRENVILLE OROGENIC BELT Publication of the tectonic map of Canada (1950) and papers by J. T. Wilson (1949) and J. E. Gill (1948, 1949) have clarified the broader structural features of the Canadian Shield. The shield is divided into separate erogenic belts which each went through a complete cycle of sedimentation, volcanism, igneous invasion (intrusion and granitization) and mountain building, and then became stable areas. It is suggested by both J. T. Wilson and J. E. Gill that the Precambrian Shield was built up by progressive orogenies. The Grenville province is that part of the Precambrian Shield which was involved in the Grenville orogeny, and the rocks now exposed are the roots of the Grenville mountains. Structural evidence and age determinations both substantiate the hypothesis that the Grenville province marks the site of an erogenic belt. These data also indicate that the Grenville orogenic cycle was Late Precambrian (Proterozoic) in age and that this orogenic belt was formed later than the orogeny which affected the Superior (Keewatin, Timiskaming) province. The structural and tectonic trends within the Grenville province are northeast, and the Grenville front which bounds the province on the northwest truncates structures in the Timiskaming subprovince. Quirke and Collins'(1930) describe the change in metamorphic grade and "disappearance of the Huronian" south of Sudbury. Farther east the NorandaBell River belt of Archean rocks which trend east-west are truncated at the Grenville front. Similarly three bands of Archean rocks in the WaswanipiChibougamau belt, which also trend east-west, are cut off by the northeasttrending Grenville orogenic belt. In discussing the Chibougamau area and comparing it with the area to the southwest along the Grenville front, G. W. H. Norman (1936, p. 128) states that "the axes of the Huronian folds of the Penokean disturbance in the Panache map area (Ontario) strike almost due east to be cut off by northeasterly striking gneisses, a set of conditions almost identical with that found in the Chibougamau district. It seems necessary to conclude, therefore, that the movements under consideration were definitely later than the Penokean orogeny. . . . " At the southern end of the Labrador trough the Proterozoic rocks become highly metamorphosed, intruded by granite and granitic gneisses, and disappear to the south. As W. G. Robinson (1956) points out, the field evidence suggests that the rocks of the Labrador trough at the southern end were involved in the Grenville mountain building. Age determinations on pegmatites and granites in the Grenville province made over a period of many years indicate an age of 900 to 1,200 million

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years for the majority of the samples. It is significant that no older ages than about 1,300 million years have turned up, although a diverse group of granites and pegmatites have been examined. Thus the coincidence of ages in the 900 to 1,200 million year bracket within the Grenville province appears definitely to date a major orogeny which affected nearly all the Precambrian rocks of the province. When we date the age of metamorphic gneisses of the Grenville region we are dating this period of orogeny and recrystallization, rather than the time of original formation of the rocks. Significantly the age determinations in the Superior province (Timiskaming subprovince) average about 2,200 million years indicating that a much older orogenic belt lies to the north of the Grenville province, and this lends support to the theory that these two provinces represent successive periods of growth of the continental mass during geologic time (Shillibeer and Gumming, 1956). Thus the structural evidence of truncation of Archean and Proterozoic rocks by the Grenville orogenic belt, and the evidence from age determinations, indicate a Proterozoic age for the Grenville orogeny. THE GRENVILLE FRONT The Grenville front can be defined as the line or zone marking the boundary between the Grenville orogenic belt ( Grenville province ) and the Timiskaming subprovince ( Superior province ). This line is a metamorphic front which divides the Archean and Proterozoic rocks of low-grade metamorphism belonging to the Timiskaming subprovince from the highly metamorphosed gneissic complex of the Grenville province which has been involved in the late Proterozoic Grenville orogeny. For some time it was suggested that the Grenville front was a major fault zone. The tectonic map of Canada (1950) shows the Grenville front as a fault from Killarney on Georgian Bay to the Chibougamau area: it is now certain that through parts of this length the front is not a fault, but an unfaulted line or zone where there is either a very rapid or a gradual metamorphic change. In the Sudbury area, work by T. C. Phemister in 1956 has shown that the metamorphic change from Timiskaming subprovince to Grenville province can be measured in places in feet. In the Chibougamau area the zone of transition measures 2 to 3 miles (Norman, 1936). Map areas straddling the Grenville front in Ontario were mapped by A. E. Barlow (1907), E. W. Todd (1925), Quirke and Collins (1930), and W. G. Q. Johnston (1954). Barlow (1907) recognized that the gneiss complex to the southeast of the Grenville front was lithologically and structurally similar to the Grenville region of Ontario and Quebec, and he believed that the gneiss complex and its granitic components were younger than the Cobalt rocks to the north and northwest, and that the granite intruded the Cobalt series. Todd (1925) recognized the important northeasterly faulting along the zone of the Grenville front in the Montreal River (Matabitchuan) area but concluded that the granites on the Grenville side

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were older than the Cobalt series on the Timiskaming side of the front. The work of Quirke and Collins in the area along the Grenville front northeast from Killarney on Georgian Bay drew wide attention to the problem after publication of their Geological Survey of Canada Memoir 160, "The Disappearance of the Huronian." They introduced the idea of a major zone of thrust faulting bounding the south margin of the Huronian in Ontario and this zone of faulting became the Grenville fault or front. Quirke and Collins drew attention to the fact that the great thickness of over 20,000 feet of Huronian sediments, which were folded in the east-west Penokean disturbance, were cut off at the line now known as the Grenville front. To the southeast they recognized the Killarnean granite and granitic gneisses which were intrusive into the Huronian. Large inclusions of metasedimentary rocks, paragneisses, limestones and quartzites were found on the Grenville side of the front and Quirke concluded that these sedimentary relicts in the granitic gneiss terrane were remnants of the Huronian series. He suggested that the Grenville series of southeastern Ontario and Quebec is the marine equivalent of the Bruce series, their lithological differences being due to different conditions of deposition. In 1936 G. W. H. Norman (1936) described the Grenville-Timiskaming boundary in the Chibougamau area and suggested that the GrenvilleTimiskaming boundary from Georgian Bay to Lake Chibougamau might mark a major zone of thrust faulting of late Precambrian age. Subsequently J. T. Wilson (1949) called it the Huron-Mistassini fault. Later the Tectonic map of Canada (1950) shows it as the "Grenville Front or Fault Zone." In 1954 W. G. Q. Johnston published the result of his field work in the Temagami area in an article entitled "Geology of the TemiskamingGrenville Contact Southeast of Lake Temagami, Northern Ontario, Canada." He concluded that "east of Lake Temagami the Temiskaming and Grenville subprovinces are in fault contact along a northeast-trending, eastward-dipping, complex zone of faulting." He also states that the emplacement of the granite, metamorphism and deformation in the Grenville subprovince all postdate the Archean rocks in the Timiskaming subprovince, a conclusion which is generally accepted through the length of the Grenville front. Although this idea that the Grenville front was marked by a fault or fault zone has become widely accepted, the earlier writings of Quirke and Collins (1930) and Norman (1936) make it clear that they did not intend to imply that the boundary was faulted throughout. Norman (1936, p. 123) states: ". . . near Surprise Lake pre-Huronian lava and sediments apparently grade eastward into garnetiferous gneisses and schists." He further points out: . . . the contact between the pre-Huronian rocks and the gneisses is a transition zone rather than a linear feature, though, in comparison to the extent of the pre-Huronian rocks westward, and the gneisses eastward from it, the transition zone, with its maximum width of two or three miles, is remarkably narrow. There does not appear to be any one continuous line of demarcation across

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which the structure and lithology is so different that such a line can be considered one continuous fault. It would seem that any faulting between the typical pre-Huronian rocks and the gneisses must be distributed in a zone several miles wide.

Quirke and Collins ( 1930, pp. 38-9 ) in describing the character of the Timiskaming-Grenville contact northeast of Killarney state that although the contact is notably straight from Killarney to Timber Berth 68 and is probably a major fault, northeast of Timber Berth 68 the conditions are very different and the contact is dentate and irregular. Immediately ~ast of Panache Lake faulting does not appear to mark the contact between the subprovinces. Work by T. C. Phemister in 1956 and W. J. Pearson in 1953 in Dryden township, Sudbury district, indicates that the high grade kyanite and sillimanite gneisses, and garnetiferous amphibolites of the Grenville province are in fault contact with the quartzitcs of the Timiskaming subprovince along the Wanapitci River. However, south of Coniston the fault swings off to the west into the Timiskaming subprovince and the contact between the Grenville and Timiskaming subprovinces is an unfaulted metamorphic front marked by an abrupt change in metamorphic grade from grits and quartzites to banded gneisses, sometimes along a zone only a few feet in width. This unfaulted metamorphic front has been mapped from Coniston to highway No. 69, a distance of about 7 miles. In discussing the Grenville front northeast of Lake Mistassini, W. G. Robinson (1956) describes major thrust faulting in the Seal Lake area which presumably marks the Grenville front. He states that the Indicator Lake series northeast of Lake Mistassini dips steeply as the Grenville front is approached and appears to be truncated on the southeast by faulting. The Proterozoic rocks of the Labrador trough also have been involved in the Grenville orogeny at the extreme south end of the trough, with marked changes in structure, tectonic style and grade of metamorphism. Robinson (1956) states: . . . from the Labrador trough to the Seal Lake area there is little evidence of faulting, but the general trend of the formations is northeasterly. This same trend continues past the Seal Lake area to near Cape Makkovic on the Labrador coast. Thus between Lake Mistassini and Cape Makkovic, the general trend of the formations is northeasterly, thrust faulting occurs in places parallel to this trend, and Proterozoic-type formations occur in scattered areas. To the south the formations resemble those of the Grenville province, and so it is probable that the Lake Mistassini-Cape Makkovic line marks the northern edge of the Grenville province. THE GRENVILLE SERIES The previous discussion has attempted to answer the questions, what are the Grenville province, the Grenville erogenic belt, and the Grenville front? It becomes apparent that the concept of the Grenville province being the site of the Grenville erogenic belt, divided from the Superior province

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(Timiskaming subprovince) by the Grenville front, must be divorced from the term "Grenville series." Much confusion has arisen from the correlation of all metasedimentary rocks within the geographic confines of the Grenville province with the Grenville series. The Grenville series of Precambrian sediments was first named, but not clearly defined, by Sir William Logan near the town of Grenville, Argenteuil county, Quebec. In this area the Grenville consists of marble, quartzite, amphibolite and garnetiferous paragneiss intruded by anorthosite, gabbro, shonkinitic and charnockitic syenite, quartz syenite and granite. Early work in Ontario and Quebec indicated that a similar sequence of rocks extended \vcstward into Ontario, and north and west for about 100 miles in Quebec, and these rocks were correlated with the Grenville series. Gradually a convention grew up that all highly metamorphosed sedimentary gneisses and limestones in the Grenville province were referred to the Grenville series. Various criteria were used at one time and another to correlate rocks with the Grenville series and these are reviewed by Osborne (1956). The presence of limestone in the sequence was regarded as diagnostic of Grenvilletype sedimentation, and as Osborne points out, if we define the Grenville on the basis of the presence of 2 per cent or more of limestone in the sedimentary assemblage, this would restrict the usage of the term Grenville series to the rocks of the original Grenville area and the adjacent area 100 miles to the north and west, together with the part of southeastern Ontario extending as far west as the Haliburton-Bancroft area. However, this hardly seems a valid criterion for correlation. The presence of quartzite and amphibolite were also regarded as "characteristic" of the Grenville and the presence of garnet in the paragneisses as a useful criterion. High grade of metamorphism—usually of the amphibolite-granulite faciès—characterizes the Grenville rocks. These criteria seem to be based on two fundamental concepts. First, that there is a close relation between the carbonate-rich, clastic sedimentary sequence of the Grenville, the associated igneous intrusions and the erogenic cycle. The presence of abundant limestone and sandstone in the Grenville scries suggests that there has been rather deep-weathering and perhaps more than one cycle of weathering, in contrast, for example, to the type of sedimentation characteristic of the Timiskaming series of Northern Ontario. The characteristic assemblage of the Grenville series indicates a rather stable site of sedimentation. Second, the concept that the whole Grenville province was involved in the Grenville orogeny accounts for the high grade of metamorphism, the uniformity in type and sequence of intrusion, and the characteristic tectonic style of the Grenville region. Analysis of these criteria generally used for correlation of rocks as "Grenville series" shows that although there is some basis for their limited use, none of them are really valid criteria for stratigraphie use. Correlation on lithology and grade of metamorphism is dangerous. It has been demonstrated in several areas along the Grenville front that Archean and Protero-

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zoic rocks of the Timiskaming subprovince cross the front into the Grenville province with marked change in metamorphic grade and mineral assemblage. In eastern Ontario a faciès change from a predominantly clastic arkose-sandstone-limestone sequence, to one in which tuffs and volcanics are common, occurs along strike within a distance of about 15 miles. In face of the lack of valid criteria for correlating rocks outside the limited original Grenville area with the Grenville series, it would seem that the term should be restricted to that area and not extended throughout the Grenville province. In Hastings county, Eastern Ontario, there is an extensive area of sediments consisting of conglomerate, arkose, quartzite, argillite, schist and well-bedded blue-grey limestones called the Hastings series. Apart from the presence of conglomerate in the section, and their lower grade of metamorphism, there is little to distinguish these rocks from the Grenville-type rocks outside the Hastings basin. AGE OF THE GRENVILLE SERIES The age of the Grenville series has long been in dispute. Early workers correlated the Grenville with the Huronian. Miller and Knight (1914) correlated the Hastings series with the Timiskaming, largely on the basis of the conglomerate, and placed the Grenville as post-Keewatin, preTimiskaming. Quirke and Collins (1930) correlated the Grenville rocks with the Huronian by tracing relicts of the supposed Huronian south into the Grenville province across the Grenville front. H. C. Cooke (1946) questioned the correlation of the rocks on the north side of the Grenville front with the Huronian. These rocks, which he called the Coniston group, he correlated with the Sudbury series of probable Archean age. He agreed that relicts of what appeared to be this same series occurred south of the Grenville front but placed them as Archean in age. F. F. Osborne (1956) in discussing the age of the Grcnville series points out that an Archean age for the Grenville is not disproved, and that the Grenville may possibly be pre-Timiskaming since pebbles of anorthosite occur in conglomerate of Mattagami (Timiskaming) age in Quebec; the source of the anorthosite pebbles presumably being in the Grenville region of Quebec. As previously discussed, age determinations in the Grenville province indicate that the Grenville orogeny, and the emplacement of granitic intrusives and pegmatites, occurred in late Precambrian times about 900 to 1,100 million years ago. J. T. Wilson (1949) has suggested that sedimentation, volcanism, igneous invasion and mountain building occur in cycles and that the deposition of the Grenville series could not have long preceded the period of mountain building. He places the Grenville series as well as the orogeny as late Precambrian in age and regards the Grenville as a "primary mountain belt" from which the Huronian sediments of the Huronian subprovince (Timiskaming subprovince) were derived. J. E. Gill in discussion at the Grenville Symposium agreed with the view of the probable late Precambrian age of the Grenville and pointed out that

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the presence of quartz-rich sandstone and arkose in quantity may be of use as an indicator of age since these rocks are rare in the preserved remnants of older Precambrian terranes. This could be because there was little, if any, granite in the original crust of the earth. He also considered that the rarity of limestones in rocks classed as "Archean" and their relative abundance in rocks classed as "Proterozoic" could be due to secular changes. In a later paper (Gill, 1955, p. 118) he emphasizes the possibility that this could also be due to the practice of placing all shelf-type sediments in the "Proterozoic." Limestones characteristically form in the continental shelf or epicontinental sea environment. Since such seas almost certainly existed early in Precambrian time, some of "Proterozoic" sediments may prove to be much older than heretofore thought. Both Proterozoic and Archean rocks are traceable from the Timiskaming subprovince across the Grenville front into the Grenville province as previously described. Determination of the age of the so-called Grenville series and the Hastings series which appears to lie conformably above the Grenville series awaits further evidence. The writer, however, concludes that the present evidence favours a late Precambrian age for the Grenville series. REFERENCES BARLOW, A. E. ( Ï 9 0 7 ) . Report on the geology and natural resources of the area included by the Nipissing and Temiskaming map sheets; Geol. Surv., Can., Pub. no. 962, 303 pp. COOKE, H. C. (1946). Some problems of Sudbury geology: Geol. Surv.. Can.. Bull. 3. 77 pp. DELANO, A. N. (1956). The boundary between the Timiskaming and Grenvillc subprovinces in the Surpise Lake area, Quebec; paper given at the Can. Inst. Min. Met. meeting, Quebec City, April 1956. GILL. J. E. (1948). The Canadian Precambrian Shield; Structural Geology of Canadian Ore Deposits, Can. Inst. Min. Met., Symposium vol., 1948, pp. 20—48. (1949). Natural divisions of the Canadian Shield; Trans. Roy. Soc. Can., Ser. Ill, vol. 43, Sec. IV, pp. 61-69. (1955). Precambrian history of the Canadian Shield with notes on correlation and nomenclature; Proc. Geol. Assoc. Can., vol. 7, pt. 2, pp. 117-124. JOHNSTON, W. G. Q. (1954). Geology of the Timiskaming-Grenville contact southeast of Lake Temagami, Northern Ontario, Canada; Bull. Geol. Soc. Am., vol. 65, pp. 1047-1074. MILLER, W. G., and KNIGHT, C. W. (1914). The Precambrian geology of southeastern Ontario; Ont. Bur. Mines, vol. 22 pt. 2, 151 pp. NORMAN, G. W. H. (1936). The northeast trend of late Precambrian tectonic features in the Chibougamau district, Quebec; Trans. Roy. Soc. Can., Series III, vol. 30, Sec. IV, pp. 119-128. OSBORNE, F. F. (1956). The Grenville region of Quebec; The Grenville Problem, Roy. Soc. Can., Special Publications, no. 1, 128 pp. QUIRKE, T. T., and COLLINS, W. H. (1930). The disappearance of the Huronian: Geol. Surv., Can., Mem. 160, 129 pp. ROBINSON, W. G. (1956). The Grenville of New Quebec; The Grenville Problem, Roy. Soc. Can., Special Publications, no. 1, 128 pp. SHILLIBEER, H. A., and GUMMING, G. L. (1956). The bearing of age determinations on the relation between the Keewatin and Grenville provinces; The Grenville Problem, Roy. Soc. Can., Special Publications, no. 1, 128 pp. TODD, E. W. (1925). The Matabitchuan area: Ont. Dept. Mines, vol. 36, pt. 3, 38 pp. WILSON, J. T. (1949). Some major structures of the Canadian Shield: Trans. Can. Inst. Min. Met., vol. 52, pp. 231-242. WILSON, M. E. (1925). The Grenville Precambrian subprovince; Jour. Geol., vol. 33, pp. 389-407.

THE PROTEROZOIC OF EASTERN CANADIAN APPALACHIA* L. J. Weeks, F.R.S.C. PRECAMBRIAN ROCKS have been known in eastern Canadian Appalachia (here assumed to comprise the four Atlantic Provinces—New Brunswick, Nova Scotia, Prince Edward Island, and Newfoundland), since the region was first studied geologically. The areas believed to be underlain by such rocks have, however, varied considerably from time to time as ideas changed regarding their age. In the past ten years, for instance, four groups of rocks underlying large areas in Nova Scotia and Newfoundland that were previously considered to be Precambrian are now referred, in part at least, and probably wholly, to the Paleozoic. The purpose of this paper is to gather together information on the known Precambrian rocks of Appalachia; to discuss their probable, or improbable Proterozoic age; and to present a tentative correlation of such of these rocks as will permit it. DISTRIBUTION Figure 1 shows the distribution of probable Precambrian rocks on which information is readily available. Other areas of such rocks may exist in the more or less unstudied parts of Newfoundland. Structurally these rocks occur in two belts, both of which are exposed on the island of Newfoundland. That the eastern of the two Newfoundland Precambrian belts is related structurally with such rocks in Cape Breton Island and in New Brunswick, is indicated by the similarity of faunal remains in Cambrian rocks associated throughout with these rocks; and the dissimilarity of such faunas with those of the Cambrian rocks associated with Precambrian rocks of the western belt. DESCRIPTIVE GEOLOGY Gneiss, Schist, Carbonate What are believed to be the oldest Precambrian rocks in the region comprise a series or system composed mainly of gneiss and schist, but characterized in all localities except one by varying amounts of interbedded 'Published with the permission of the Deputy Minister, Department of Mines and Technical Surveys, Ottawa. 141

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FIGURE 1. Sketch map of southeastern Canada showing extent of Prccambrian rocks in the eastern Appalachian region.

crystalline limestone or other metamorphosed carbonate rock. The rocks are cut extensively by granite and pegmatite and little is known of the absolute age of these intrusions. In New Brunswick the gneiss-schist-carbonate rocks are called the Green Head group (Alcock, 1938) and are overlain with considerable angular unconformity by volcanic rocks of the Coldbrook group for which a late Protcrozoic age will be proposed later. The Green Head, unlike other successions of similar eastern rocks, has yielded concentric concretionary markings which may be of organic origin and have been described under the name of Archaeozoon acadiense. In Cape Breton Island these rocks are referred to as the George River group (Weeks, 1954) which is lithologically similar to the Green Head except that some volcanic rocks, greywacke and hornfelsic shale are included (Guernsey, 1928). Attempts have been made to subdivide the George River but such subdivisions cannot be carried beyond the area where these were first described. The presence of crystalline limestone beds is characteristic, however, and is indeed the basis of correlation from area to area. The George River is nowhere known to be in contact with later Precambrian rocks, but at one locality is overlain with a large angular unconformity by Middle Cambrian beds. Gneiss-schist-carbonate rocks are shown in three areas of Newfoundland. The two northern of these are believed without much doubt to be Precambrian and are characterized by crystalline limestones that are associated with paragneisses, schists and intrusive rocks. The larger of these areas, that

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of the Long Range Mountains in the Northern Peninsula, is flanked by Lower Cambrian beds which rest with great unconformity on the older rocks (Foley, 1937). To the east of White Bay, rocks of the smaller of these two northern areas are overlain by Middle or Late Lower Ordovirian beds (Baird, 1951), which are considered to prove the Precambrian age of the gneisses and schists, inasmuch as this region was believed to have been land during Cambrian time. Rocks of the most southern Newfoundland area, those underlying the Long Range, exist only in faulted contact with Paleozoic rocks (Barnes and Riley, in prep. ), and their age cannot be inferred on structural grounds. The rocks are paragneiss and schist cut by numerous intrusions. Crystalline limestone has not been reported from these rocks but may occur. It should be mentioned here that the Lower Devonian Bay du Nord group, which lies to the southeast of these rocks, would, a few years ago, have been classed with them as probably Precambrian. The Bay du Nord has been intimately injected by granitic intrusions and, in places, has been metamorphosed to a kyanite schist ( Cooper, 1954 ). The discovery of fossils in these considerably metamorphosed beds has cast doubt on the age of all such rocks in Newfoundland that are not known on structural grounds to be Precambrian. The southern of the three western Newfoundland areas may therefore be post-Precambrian. The absolute age of the gneiss-schist-carbonate rocks is unknown. Previous writers have, without significant exception, referred to them as probably Archean. In no place are they in contact with rocks of what is believed to be the next youngest subdivision, the Harbour Main group and its correlatives. Harbour Main, Love Cove The second subdivision shown on Figure 1 comprises rocks of the Harbour Main and Love Cove groups, and is found only in eastern Newfoundland, where three areas are known. The two smaller, eastern of these are underlain by rocks of the Harbour Main group (Rose, 1952; McCartney, 1954). Rocks of the western area are known in the north as the Love Cove group (Jenness, in prep.), and in the south are, as yet, unnamed. They all are believed to be equivalent in age and to represent the oldest Precambrian rocks in eastern Newfoundland. The rocks are primarily volcanic in origin with both acid and basic types represented and associated with interbedded conglomerate, greywacke, sandstone and shale. The bottom of these successions is nowhere known. The Harbour Main group is overlain unconformably to possibly disconformably by the Conception group of probable Proterozoic age (see Fig. 2). The Love Cove group is in faulted contact only, with the probable equivalent of the Conception group, and the unnamed groups similar to the Harbour Main are in faulted contact with postConception rocks at the southern end of the Burin Peninsula. No equivalents of the Harbour Main group have been found in western Newfoundland or

FIGURE 2. Tentative correlation of Precambrian rocks in eastern Newfoundland, Cape Breton Island and New Brunswick. Sections located as follows: 1—St. John's to Topsail, Nfld.; 2—Manuels River, Nfld.; 3—Harbour Grace map area, Nfld.; 4—east side Dildo map area, Nfld.; 5—west side Dildo map area, Nfld.; 6—west side Trinity Bay, Nfld.; 7—Louisburg to Mira River, N.S.; 8—St. Esprit, N.S.; 9—Saint John area, N.B.

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on the mainland. The rocks are considered to be younger than those of the gneiss-schist-carbonate rocks because in parts of the Harbour Grace area (section 3, Fig. 2) there appears to be very little interruption in sedimentation from the Harbour Main to the Cambrian, even though elsewhere as many as two unconformities exist. The absence of any rocks similar to those of the earliest groups in this sedimentary column is construed to indicate that they existed previous to the accumulation of Harbour Main rocks. Their relative degrees of metamorphism substantiates this assumption. Van Ingen (1914) ascribed a Huronian age to the Harbour Main (then called Avondale) and a Keweenawan age to the Random. A Huronian or Lower Huronian age has been ascribed in general to these rocks since then, until very lately. Rose (1952) suggested that possibly they were Archean, but believed they were Proterozoic, and later Weeks ( 1954 ) classed them as Archean(?). LATE PROTEROZOIC ROCKS Conception Group and Its Correlatives In the Avalon Peninsula of Newfoundland the Harbour Main is succeeded with an angular unconformity or, in one place, a disconformity, by the Conception group of conglomerate, sandstone, siltstone and shale. These rocks are believed to be the correlatives of the Connecting Point group (Hayes, 1948; Christie, 1950) west of Trinity Bay, which is in faulted contact only with equivalents of the Harbour Main group. The Connecting Point rocks are similar but have much more greywacke reported from them, a fact that is not considered to be of great importance inasmuch as this difference is believed to be mainly due to different lithological classifications used by the different authors. The writer, personally, would apply the term "greywacke" to many of the Conception rocks of the St. John's and Harbour Grace areas. The conglomerates reported from the Conception and Connecting Point are mainly of the type commonly associated with greywacke, that in which the cobbles are set in a matrix of greywacke. The shales, which, in general, are altered to slates, apparently are in gradational contact with coarser beds. The whole assemblage then represents a succession that could have been deposited in a deep, rapidly subsiding basin, conditions that have been referred to as eugeosynclinal. Volcanic rocks, which are commonly present under such conditions of sedimentation, are absent. Thick coarse conglomerates which are commonly associated with greywackes near the borders of such basins of deposition are also absent. As volcanic rocks in an eugeosynclinal sequence are more plentiful near the active border of the basin than near its centre, it would appear that the lack of volcanic rocks and of coarse conglomerates may be related, and that the sections of Conception rocks at our present-day disposal represent sediment laid down at some distance from the active border of the basin. The Conception and Connecting Point are without doubt Proterozoic. In two of the four sections in which these rocks occur, they are succeeded

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conformably by rocks that continue upward to the Cambrian with little or no break. Cabot Group and Its Correlatives The Conception and Connecting Point were succeeded by, first, a group of mainly acid volcanic rocks which were poured out or ejected from a centre in the far west of the region under discussion, and which resulted in no stratigraphie representatives in the extreme east. These rocks were followed, secondly, by a group of shallow-water, supposedly marine, beds which are much more developed in the east than in the west, and, in fact, are lacking in the most westerly section (section 8, Fig. 2) and are only 130 feet thick some 280 miles to the east (section 7, Fig. 2). It is assumed that ejected volcanic material in the extreme west built up the land surface in latest Proterozoic time to such an extent that it was under erosion until Lower Cambrian time. Conception rocks are in contact only with the sedimentary members of this later succession which are termed the Cabot group in the St. John's area, and the Hodgcwatcr group in the Harbour Grace area. The contact near St. John's is regarded as a disconformity, and that near Harbour Grace, as conformable. The Connecting Point is only in contact with the volcanic members, which are termed the Bull Arm group by McCartney ( 1956), and the Bull Arm member of the Musgravctown group by Hayes (1948) and Christie (1950). The first of these contacts is reported to be a conformity, whereas Hayes reports an angular unconformity between the Connecting Point and the Bull Arm. The Fourchu group of Cape Breton is correlated with the Bull Arm because both conformably underlie shallowwater beds similar in many respects, termed the Morrison River in Cape Breton (Weeks, 1954), the upper part of the Musgravetown west of Trinity Bay (Hayes, 1948), and Musgravetown in the western part of the Dildo area (McCartney, 1956). The Coldbrook group of New Brunswick is correlated with the Fourchu and the Bull Arm because, although Coldbrook pebbles occur in the basal Cambrian conglomerates, there is no evidence of extensive folding between the times of deposition of the two groups. The correlation of the Morrison River with the upper part of the Musgravetown and of it, in turn, with the Hodgewater is strengthened bv the presence of a characteristic white quartzite, recognizable at all localities, that forms the uppermost member of the Morrison River formation in Cape Breton, and is termed the Random where it overlies the Musgravetown and Hodgewater. The white quartzite lies with conformable to disconformable contacts below the lowermost Cambrian. Before proceeding to a discussion of absolute age, one section should be mentioned which is apparently at variance with some of these conclusions (section 2, Fig. 2). At Manuels, a few miles west of St. John's, and just across Conception Bay from Harbour Grace, Lower Cambrian conglomerates rest on Harbour Main volcanic rocks, thus indicating a degree of unconformity unsuspected in adjacent sections. No adequate solution of this

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problem has been proposed, the most plausible being that proposed by Mr. W. D. McCartney, who has done much work in the Avalon Peninsula— that block faulting elevated a section of Harbour Main and Conception rocks in Cabot time, and from this block, Conception beds were eroded.1 FORMATIONS FORMERLY REFERRED TO THE PRECAMBRIAN Certain formations of this region have at one time or another been classed as Precambrian, but at present are believed to be younger. These units will be briefly mentioned, together with the reasons for ascribing them to the Paleozoic. The Meguma group of Nova Scotia, comprising a lower, greywackequartzite, Goldenville formation, and an upper, slate, Halifax formation was originally ascribed to the Lower Paleozoic, then later to the Protcrozoic. Crosby ( 1951 ) found Ordovician fossils in Halifax beds described by E. R. Faribault and considered by him to be the type section of thai formation. The Baie d'Espoir group of south-central Newfoundland, although not known as long as has been the Meguma group of Nova Scotia, had been generally ascribed to the Precambrian. Like the Meguma, it comprises a lower, quartzite-greywacke division, and an upper, slate division. Fossils were discovered in 1951 by H. S. Scott in what is probably the northeast extension of this group. These fossils were identified by A. E. Wilson2 of the Geological Survey and ascribed to the early Middle Ordovician. In the La Poile-Cinq Cerf area of southwestern Newfoundland, J. R. Cooper (1954) describes the fossiliferous Lower Devonian Bay du Nord group, and the associated La Poile group and an unnamed group of schists and paragneisses which he believes, on structural grounds, to be younger than the Bay du Nord. Many of these rocks are highly metamorphosed, and all were included in the Precambrian by earlier workers. DISCUSSION AND CONCLUSIONS These Precambrian rocks as described fall naturally into four subdivisions based on mode of deposition and prominence of the unconformities or other type of contact that separates them ; four only, because there is good reason to suspect that the Coldbrook—Fourchu—Bull Arm volcanic successions are related to and part of the Morrison River-Musgravetown-HodgewaterCabot successions together with the overlying Random formation. The oldest of these four subdivisions is composed of gneiss-schist-carbonate rocks that are nowhere in contact with those of the second or third group but which would probably be separated from them by an unconformity of considerable magnitude, should such a contact be found. The second comprises Harbour Main and Love Cove volcanic rocks exposed only in eastern Newfoundland where they represent the oldest strata. The third subdivision, rocks of the Conception and Connecting Point groups, were laid down with what is commonly an angular unconformity, but may be a disconformity J W. 2

D. McCartney, Geol. Surv., Can., personal communication. A. E. Wilson, Geol. Surv., Can., office report, Nov. 14, 1951.

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in places, on the Harbour Main group. The fourth and last subdivision includes in the east a succession of shallow marine deposits that are overlain by the Cambrian, and in the west a succession of volcanic rocks that are overlain by what is believed to be the same succession of shallow marine beds. These two lithological entities are believed to be equivalent in time because, first, no sign of an unconformity has been found between them, and, second, because on the west side of Trinity Bay, the Bull Arm felsite member of the Musgravetown group has a basal conglomerate of typical Musgravetown sediment. Although this fourth subdivision rests with complete conformity on the third in two localities, and with reported unconformity in only one place, the former is given the status of a major subdivision because of the great change in the environment of deposition that took place between the times of sedimentation of the third and fourth subdivisions. In Appalachia there are no known Archean rocks. Therefore, to class any Precambrian rock as either Proterozoic or Archean it is necessary to compare either the succession as a whole, or individual units thereof, with successions or units in areas where such are known to be either Archean or Proterozoic. Although parts of the Shield lie a few hundred miles from these studied sections, those parts that are well known and particularly those that have both Archean and Proterozoic present are nearly two thousand miles distant. To classify these Appalachian subdivisions as to time, a worker is then forced either to follow his hunches, as have many earlier authors, or to use the most meagre of evidence—evidence that could by no means be termed conclusive. Let us examine what evidence there is regarding the age of the oldest of these subdivisions. The similarity of the gneiss-schist-carbonate rocks of this region to those of the Grenville series of Quebec and Ontario has not been mentioned before but will be apparent to most, as it has been to many authors in the past. However, to make a correlation on lithology over a distance of 2,000 miles is not sound practice, even were there unanimity of opinion regarding the age of the Grenville. Whether or not the earth was capable chemically of producing carbonate rocks in Archean time is, however, another matter, and should the consensus be that such was not the case, a very clear argument for a Proterozoic age of these earliest rocks would be established. Markings that resemble algal growths have been found in the Green Head of New Brunswick. They have not been found in the George River or the calcareous members of the oldest rocks in Newfoundland. And they may not be organic in origin. But if they are, a strong argument is put forward for a post-Archean age of these rocks. These two items comprise all the evidence presently available. One would hesitate to state on the basis of these points whether the oldest Precambrian subdivision should be considered Archean or Proterozoic. Inasmuch as no direct correlation is possible with rocks in areas where both Archean and Proterozoic are present, the matter for the present may be considered purely academic. We know the succession with reasonable certainty ; we know that

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the most pronounced visible break is between the second and third subdivision; and we are reasonably sure that the third and fourth subdivisions are Proterozoic. Until the oldest rocks are dated geophysically, why not leave it at that? REFERENCES ALCOCK, F. J. (1938). Geology of the Saint John region, N.B.; Geol. Surv., Can., Mem. 216. BAIRD, D. M. (1951). Burlington Peninsula, Nfld.; Gcol. Surv., Can., Paper 51-21. BARNES, F. Q., and RILEY, G. C. (in preparation). St. Georges Bay area. Nfld.; Geol. Surv., Can., map. CHRISTIE, A. M. (1950). Bonavista map area; Geol. Surv., Can.. Paper 50-7. COOPER, J. R. (1954). La Poile-Cinq Cerf area. Nfld.: Geol. Surv., Can., Mem. 276. CROSBY, D. G. (1951). The Wolfville area. N.S. ; Ph'D. thesis, Leland Stanford Jr. Univ.: also Geol. Surv., Can.. Mem. in preparation. FOLEY, F. C. (1937). Hawke Bay-Great Harbour Deep, Newfoundland: Geol. Surv., Nfld., Bull. 10. GUERNSEY, T. D. (1928). North Mountain. Cape Breton; Gcol. Surv.. Can.. Sum. Rept. 1927C. HAYES, A. O. (1948). Area between Bonavista and Trinity bays. Nfld.: Gcol. Surv., Nfld., Bull. 32. HUTCHINSON. R. D. (1953). Harbour Grace area, Nfld.; Geol. Surv., Can., Mem. 275. JENNESS, S. E. (in preparation). Terra Nova map area, Nfld.: Geol. Surv., Can., map. MCCARTNEY. W. D. (1954). Holy-rood, Nfld.: Geol. Surv., Can., Preliminary map 54-3. (1956). Dildo, Nfld.; Geol. Surv.. Can.. Map 13-1956. ROSE, E. R. (1952). Torbay map area. Nfld.; Geol. Surv.. Can., Mem. 265. WEEKS. L. J. (1954). Southeastern Cape Breton Island; Geol. Surv., Can., Mem. 277. VAN INGEN, G. (1914). Table of Geological Formations about Conception and Trinity Bays, Nfld.; Princeton Univ., 1914.

THE PROTEROZOIC OF THE CORDILLERA IN SOUTHEASTERN BRITISH COLUMBIA AND SOUTHWESTERN ALBERTA* J. E. Reesor AREAL EXTENT THE PRECAMBRIAN ROCKS of this region form a limited northward extension into Canada of the Belt Terrain of northwestern Montana and northern Idaho. They consist of the dominantly clastic rocks of the Purcell and Windermere systems. At the International Boundary, rocks of the Purcell system are found in all ranges of the Rocky Mountains but disappear rapidly northward beneath younger rocks in a few tens of miles. Purcell rocks are present as far north as latitude 50 degrees in the Western Ranges of the Rocky Mountains, but Windermere rocks are found northward a few miles into the Stanford Range (Henderson, 1954). In the main ranges of the Rocky Mountains isolated areas of Precambrian rocks, which may be correlated with the Windermere, are known, as at Mt. Assiniboine (Deiss, 1940, p. 756), along the Bow River anticline (Walcott, 1910, pp. 423^131), and northward to Jasper and beyond (Hughes, 1955, pp. 69-116; Walcott, 1813, p. 340). The Purcell system occurs in a north plunging geanticline in the Purcell Range. At the 49th Parallel, rocks of the Lower Purcell underlie the entire Purcell Range. Northward these rocks are flanked on either side by the Upper Purcell and by the Windermere system. With considerable repetition by major northeast- to eastward-curving faults and by local changes in direction of plunge, the older rocks in the south are followed by successively younger rocks northward along the axis of the geanticline, until, near latitude 51 degrees, rocks of the Purcell system plunge beneath the base of the Windermere system. In turn, these rocks plunge northward beneath the lower Paleozoic, although rocks that may be correlated with the Horsethief Creek series of the Windermere system again come to the surface in the northern Selkirks within the Big Bend of the Columbia River (Gunning, 1929, pp. 136A-153A). In the Cariboo Mountains, about 120 miles to the •Published with the permission of the Acting Deputy Minister, Department of Mines and Technical Surveys, Ottawa. 150

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northwest of Big Bend area (Brown and Holland, 1956), rocks which are again correlated with the Windermere system underlie conformably Lower Cambrian strata. The Shuswap complex, including doubtful correlatives of the above systems, outcrops over a large area west of Revelstoke and the Arrow Lakes and east of Okanagan Lake (see Index Map, Fig. 1 ). PURCELL SYSTEM Divisions and Lithology The Purcell system is a conformable, gradational succession of predominantly very fine-grained clastic rocks up to 45,000 feet thick (Rice, 1937, 1941; Walker, 1926; Reesor, 1954). This thickness includes the Fort Steele formation at the base of the system (6,000+ feet thick) found nowhere west of the Rocky Mountain Trench (Rice, 1937, pp. 4-6). The exposed part of the Fort Steele formation of Cranbrook area (Rice, 1937, p. 5) consists of a lower member of more than 1,000 feet of quartzite and argillaceous quartzite with striking crossbedding. This grades upward to 2,000 to 3,000 feet of laminated dark grey argillite and white to grey quartzite, which is in turn followed by 2,000 to 3,000 feet of massive, black, calcareous or dolomitic argillite, followed by grey-green dolomitic argillite. This is followed, east of the Trench, by the Aldridge formation, 16,000 feet of massive and laminated black argillite. Westward, in the Purcell Range, these rocks are not found, and the corresponding Aldridge formation consists of at least 15,000 feet of rusty-weathering, very fine-grained quartzites, with minor interbeds of black argillite, grading to 1,000 to 2,000 feet of black, laminated, silty, argillite at the top. The overlying Crestón formation consists of at least 6,300 feet of greenish, or grey-weathering, impure, finegrained quartzites and argillites. No calcareous or dolomitic rocks have been found below the top of the Crestón formation in the Purcell Range. Conformably and gradationally overlying the Crestón are the Kitchener (5,000 feet), Siyeh (2,800 feet), Dutch Creek (3,500 feet) and Mt. Nelson (3,400 feet) formations. They consist essentially of a conformable, gradational succession of calcareous quartzites and siltstones, varicoloured argillites, calcareous argillites, limestones and dolomites (Rice, 1937, 1941; Walker, 1926; Leech, 1953; Reesor, 1954). Massive dolomitic limestones are most plentiful in the Mt. Nelson formation of Windermere area. Individual formations in the above series are divided solely on the basis of varying proportions of the component rock-types, distinctive, widespread marker horizons being rare. A white quartzite of variable thickness at the base of the Mt. Nelson formation is perhaps the best marker horizon in the succession (Walker, 1926, p. 10; Rice, 1941, p. 12). Leech, mapping in Canal Flats area (personal communication) finds the Kitchener, Siyeh, and Dutch Creek to be an indistinguishable succession of predominantly slates, argillites, limestone, and dolomite with some quartzite, which in a structurally complex area south of Findlay Creek can be divided

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into separate formations only with great difficulty. Similarly the writer, mapping to the west along the Purcell divide, found the separation of this succession into formations exactly equivalent to Kitchener, Siyeh, Dutch Creek, and Mt. Nelson almost impossible. Walker, in Windermere area (1926, p. 7), placed an arbitrary division between the Upper and Lower Purcell, based on the occurrence of Purcell lavas at the top of the Siyeh to the south and southeast to latitude 49 degrees and beyond. The lavas do not occur in the region north of about latitude 50 degrees or west of longitude 116 degrees and in this region the precise division into Upper and Lower Purcell is impossible. Nevertheless, the Purcell lavas are found everywhere to the south and east of this dividing line, and the separation of the Purcell system into Upper and Lower Purcell at the top of the Siyeh formation appears to be valid southward to the International Boundary and beyond, and eastward to the Waterton area (Douglas, 1952). In the succession of Purcell sediments in the Purcell Mountains, primary structures such as ripple-marks, crossbedding, and mud-cracks, are common to plentiful at some locality in every formation with the exception of the upper part of the Aldridge. In Cranbrook area, the Gateway, equivalent to the lower strata of the Dutch Creek of Windermere area, contains casts of salt crystals. Correlation and Variation of Lithology Correlation of an unfossiliferous succession of gradational, conformable strata as represented by the Purcell system may be based on three methods (Clapp and Deiss, 1931, p. 689): (1) the continuity of the deposits; ( 2 ) detailed lithological character of each formation ; ( 3 ) comparison of the sequence of formations in the different stratigraphie sections. The first is the only method that may be used with certainty if rapid faciès changes occur. The attached correlation chart (Fig. 2) summarizes the sections mapped in this region and shows the correlation of formations based on knowledge at present available. Along the western limit of known outcrop of Purcell-Belt rocks from Cœur d'Alêne (Ransome and Calkins, 1908) district approximately 100 miles south of the 49th Parallel, to Findlay Creek area about the same distance north of the 49th Parallel, both the succession of formations and the lithology are remarkably similar throughout (Kirkham and Ellis, 1926; Rice, 1941 ; Reesor, 1954) and no problem of regional correlation is encountered. Correlation across the strike of the formations in the region to the east, however, is not as easy. Many uncertainties exist, owing in part to lack of continuous mapping, in part to rapid variation in lithology from west to east. Known equivalents of the Fort Steele formation have not been discovered and this formation has been mapped only in the vicinity of the Rocky Mountain Trench in Cranbrook area (Rice, 1937). The Aldridge formation varies rapidly from predominantly quartzitic rocks west of the Trench to

FIGURE 2. Correlation table of Lower Cambrian and Precambrian formations, southeastern Canadian Cordillera.

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predominantly argillaceous rocks just east of the Trench (Rice, 1937, pp. 6-8 ). Similarly, to the south, the predominantly quartzitic Pritchard formation (correlative of the Aldridge) of Boundary county, Idaho (Kirkham and Ellis, 1926) changes rapidly eastward to the predominantly argillaceous equivalent of Libby quadrangle (Gibson, 1948, p. 10). Clapp and Deiss (1931, p. 694) show that, much farther to the southeast in Montana, the Pritchard-Aldridge argillites pinch out completely to the east and are not found in the Belt Mountains where the underlying Neihart quartzite, the basal Belt formation, lies directly on the Archean. In the succession in the Belt Mountains the Chamberlain shale, considered the equivalent of the Creston-Ravalli by Clapp and Deiss, lies on the Neihart quartzite (Walcott, 1906). In the Purcell Range more detailed work in the formations of the Lower Purcell shows interesting lateral variation in lithology and grain size (Leech, 1953; Reesor 1954 a, b). In the general area both south and north of latitude 50 degrees the upper 2,000 feet of the Aldridge formation becomes increasingly quartzitic to the west and north. On the other hand the upper part of the overlying Crestón is found to contain many beds of medium to coarse quartzites and intraformational conglomerates near the Rocky Mountain Trench. In this region these are the coarsest rocks in the entire Purcell system, with the exception of local intraformational conglomerate in the Aldridge formation. The upper Crestón becomes more calcareous and argillaceous toward the west. Similarly the overlying Kitchener appears to contain more limestone to the west. Correlations north of the 49th Parallel from the Purcell Mountains eastward to Waterton area (see Figs. 1 and 2) must be based largely on comparison of the stratigraphie sections at various localities since there has been no continuous mapping throughout the region. These correlations also depend largely on the supposition that the Purcell basaltic lavas occur everywhere at, or near, the same horizon. Schofield (1915, pp. 50-52) in Cranbrook area (see Fig. 2) correlates Kitchener of Purcell Range with Daly's (1912, pp. 97-110) Altyn, Hefty, MacDonald and Wigwam formations of the Galton Range, immediately east of the Rocky Mountain Trench. Thus, no equivalent of either Crestón or Aldridge is exposed, if this correlation is correct. It is of interest to note that no further information contributing to a more exact interpretation has since become available. Between Galton Range and Waterton area, Daly, working along the 49th Parallel ( 1912, p. 163), considered the Hefty formation the equivalent of lowermost Appekuny, and Grinnel formation equivalent to the Wigwam formation plus the upper part of the McDonald. Daly concluded that rocks older than the middle and lower Altyn and Waterton formations of Waterton area are not exposed in Galton Range. These sections appear to be rather similar throughout, though varying proportions of argillite and sandstone and calcareous equivalents occur from locality to locality. In Waterton

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area, in the Front Ranges of the Rocky Mountains, the basal quartzite of the Appekuny formation, consisting of coarse sands with sun-cracks and with some intraformational conglomerate, becomes coarser in the more northeasterly fault slices and thins in the southwesterly fault slices ( Douglas, personal communication). Thus a nearby eastern or northeastern source for the sediments is postulated. The Siyeh, the uppermost formation of the Lower Purcell, consists essentially of argillite in the Purcell Range (Schofield, 1915), but becomes increasingly calcareous and dolomitic eastward. In Waterton area (Douglas, personal communication) the lower Siyeh consists of grit and sandstone with salt crystal impressions, though the upper Siyeh is predominantly carbonate and shale. Rocks of the Upper Purcell are found throughout, from the Purcell Range to Waterton area. They are roughly equivalent, as shown in Table I, though they are everywhere truncated by erosion surfaces or overlain with an erosional unconformity by younger rocks. Within the eastern part of the Rocky Mountains the Upper Purcell consists essentially of shallow-water deposits with predominantly argillaceous and quartzitic beds (Douglas, 1952; Hage, 1943). Westward, calcareous and dolomitic rocks become more plentiful. In Windermere area a thick sequence of dolomitic limestones dominate the Mt. Nelson formation, and both Dutch Creek and Mt. Nelson formations are here uniformly fine grained, but again at some horizons show evidence of deposition in shallow water (Walker, 1926, pp. 7-12). On the other hand, in the Gateway formation, in part equivalent to the Dutch Creek, Schofield (1915, pp. 36—38) notes the presence in Cranbrook area, 100 miles southeast, of a fine-grained grit containing pebbles of Purcell lava as well as pebbles of quartzite, all overlain by thin-bedded sandy argillites with abundant casts of salt crystals. Once the Purcell sediments have been traced and correlated southward from Cranbrook area and eastward beyond Elko, much additional information regarding precise correlation and variation in lithology may be obtained. Deposition and Origin A hypothesis outlining the origin and conditions of deposition of these sediments must account for the following features : ( 1 ) dominant fine-grain size of the clastic sediments; rare localities, within the basin, of intraformational conglomerate and coarser-grained sediments (e.g., Crestón of Dewar Creek and Gateway of Cranbrook areas) ; (2) shallow-water features from place to place in the region and intermittently throughout the stratigraphie section; (3) continuity of the deposits, at least along the western limits of the basin, along a general northerly trend ; conversely, a rapid faciès change across the strike in an easterly direction; (4) greater thicknesses exposed in the west than in the east, with the lower part of the western section composed entirely of clastic sediments; (5) the only unequivocal indication of source area in these sediments, found along the eastern boundary of the basin, and indicating an eastern source.

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Deposition has occurred in a slowly subsiding basin or trough of relatively great tectonic stability. Throughout, rate of subsidence has kept close pace with quantity of supply, apparently with some localities exposed to the air while others nearby were submerged well below wave-base, as shown by the presence of shallow-water features in some localities and in some formations, and their absence in others. Such conditions would exist, as already pointed out by Walker (1926, p. 11-12), on the flood-plain of a large subsiding delta. Perhaps long-shore currents distributed laterally, for great distances, the sediments supplied to the delta. The sediments are everywhere fine grained and could be easily transported long distances without much separation of clay from quartz particles. It has been noted that the lower part of the succession in the west, along the region of the present Purcell Range, consists of medium-grained (Ft. Steele formation) and fine-grained (Aldridge and Crestón formations) clastic sediments. Equivalents to the east are not known, but may be buried beneath younger sedimentary rocks. Nevertheless east of Phillipsburg area, in Montana, as already noted (Clapp and Deiss, 1931), the Pritchard, equivalent of the Aldridge, thins eastward and disappears. It is possible that the lower clastic rocks of the Purcell Range similarly pinch to the east. If this is the case, deposition of the Purcell sediments may well have begun in the west, and, with further subsidence of the basin of deposition, extended eastward. Further, local ridges or higher areas within the basin may have supplied sediments to nearby lower areas. This would explain local coarse-grained sediments as found in the upper Crestón of Dewar Creek area (Reesor, 1954) or in the Gateway formation of Cranbrook area (Schofield, 1915). In all, the character of the sediments and conditions of deposition are not unlike those of the Gulf Coast geosyncline of the present day. Any consideration of source area for these sediments must take into account that no specific direction of source is indicated within the basin. Only locally and in certain formations is an eastern source indicated along the eastern limit of the Purcell-Belt terrain. A conclusion as to source area must be founded on plausibility and conformity with current geological theory rather than on any specific data available at the present time. Granitic and gneissic rocks are known from deep wells to underly rocks of Paleozoic age in the plains of central Alberta in the Kdmonton area and southward, as well as the Peace River area to the northwest and Athabasca area to the northeast ( Burwash, 1951 ). Siirnlarlv, the Belt rocks lie unconformably on Archean crystalline rocks in the Belt and Little Belt mountains in western Montana (Clapp and Deiss. 1931). Furthermore, much of the region of northern and central North America (the Canadian Shield) provided an extensive source of sediment at this time. It thus appears that a known, distant, relatively tectonically stable source area existed throughout the deposition of the Purcell-Belt sediments to the north and east of the known present outcrop

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area of these sediments. No known source exists to the west and no evidence is found within the stratigraphie column which indicates a western source. WlNDERMERE SYSTEM

Walker (1926) first recognized and defined the Windermere system in Windermere area. He divided it into a basal conglomerate, the Toby formation; and the Horsethief formation. Later (1929, p. 9) Walker extended the Windermere to include the younger Hamill series, Badshot formation, and Lardeau series of Lardeau area. Evans (1933), in Dogtooth Range of the northern Purcells, renamed the Horsethief formation, the Horsethief Creek formation, and finally Walker (1934) in Salmo area near the 49th Parallel named it the Horsethief Creek series. Park and Cannon (1943) and Campbell (1947) in northeastern Washington and Little ( 1950) in Salmo area discovered fossils in rocks equivalent to the upper part of the Hamill series, the Badshot formation and the succeeding Lardeau series that indicated these rocks to be of Lower Cambrian and later age. Therefore, Walker's original definition of the Windemere is retained to include only the Toby formation and the Horsethief Creek series and, locally, the Irene volcanic formation. Divisions and Lithology The Toby formation, or its equivalents, occur west of the Purcell divide and south into northeastern Washington. It is found north of latitude 50 degrees in the Purcell Range and in a few adjacent localities of the western Rocky Mountains. No Toby conglomerate is known to exist southeast of the boundary marked by the 50th parallel and the Purcell divide. This formation is extremely variable in thickness as well as in lithology. In Windermere area (Walker, 1926, p. 14) it varies from 50 to 2,000 feet thick, but at the International Boundary at longitude 117 degrees it is several thousand feet thick. Not far southward, in northeastern Washington (Park and Connor, 1943), the equivalent Shedroof conglomerate thins to zero in a few miles. This variation in thickness is noted in detail by Leech in Canal Flats area (1954, p. 6). He states: "The Toby is not more than 45 feet thick 3/4 mile south of the area but is 1,000 feet thick l/ 2 miles farther north along its trace, on a spur l/2 mile within the area [south and north of latitude 50 degrees respectively]. This change in thickness may be in part a feature of original deposition." The Toby conglomerate varies from a breccia to a conglomerate, and component particles may be of pebble, cobble or boulder size predominantly composed of dolomite, quartzite, or argillaceous quartzite from locality to locality. The matrix may be argillite, siliceous argillite, or limestone, and the ratio of particles to matrix may, in different localities, range from zero to 90 per cent of the rock (Walker, 1926; Rice, 1941; Reesor, field work, 1954 and 1955).

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In Canal Flats area (Leech, personal communication) in the vicinity of the Rocky Mountain Trench the top of the Toby formation consists, in places, almost entirely of gneissic granite boulders up to 2 feet in diameter. Along strike Leech finds that component boulders in the Toby conglomerate may alternately consist of quartzite, dolomite, and, in localities, greenstone that may be derived from the Purcell lavas. The Toby conglomerate lies with angular unconformity on Dutch Creek and Mt. Nelson formations of the Upper Purcell. Walker (1926, p. 14-15) reports an angular unconformity, though Rice (1941, p. 14) notes the unconformity only when the Toby is seen to lie on different horizons of the Upper Purcell from locality to locality. The Irene volcanic formation occurs in the vicinity of the International Boundary and conformably overlies the Toby formation. It consists essentially of greenstone of andesitic composition; but interbeds of conglomerate and limestone are found well within the greenstone (Rice, 1941, p. 16). The greenstones thin rapidly northward and disappear within a few miles of the 49th Parallel. The Irene volcanics are correlated with the Leola volcanics of Metaline Quadrangle (Park and Cannon, 1943, p. 9) in northeastern Washington. The Leola volcanics vary in thickness from 4,500 feet to possibly 9,000 feet (pp. 9-10), and are similarly interbedded with, and conformably overlie the Shedroof conglomerate (equivalent of Toby). The Horsethief Creek series conformably overlies the Toby conglomerate, although it may in places be interbedded with one or more horizons of Toby-like conglomerates. In Windermere area the Horsethief is about 5,000 feet thick (Walker, 1926, p. 15) though it may be somewhat thicker to the northwest (Reesor, field work, 1955). Essentially the Horsethief Creek consists of argillites, phyllites, and slates as well as prominent successions of quartzites, pebbly and feldspathic pebbly quartzites, and quartz pebble conglomerates. Beds of limestone occur, and northwestward in the Purcell Mountains and in Dogtooth Mountains (Evans, 1926, p. 117A) a member several hundred feet thick consisting of buff-weathering, bluish limestone as well as black, arenaceous limestone occurs not far below the top of the series. In this area, also, the Horsethief Creek may be divided into a number of separate members'alternately consisting predominantly of argillite or slate, and quartzite with some strata of pebbly or feldspathic pebbly quartzite. Individual beds or even successions of beds are not persistent along strike. Nevertheless the members noted above persist for some distance in the northern Purcells, though they are not recognizable southward due to greater structural complexity and a higher degree of metamorphism (Rice 1941, pp. 17, 18). The lateral changes in lithology within the Horsethief Creek are significant. Walker (1926, p. 15) in Windermere area notes that it is coarser in the east and appears to thicken westward. North and northwest of Windermere area (Reesor, field work, 1954 and 1955) the coarse members of

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the Horsethief Creek are more plentiful and coarser grained in the southeast but become progressively fewer in number and finer grained northwestward. Rice, in Nelson east-half map-area, southward from Windermere area (1941, p. 17) notes that conglomerates similar to the Toby occur throughout the Horsethief Creek series. Thus, here again the coarse members occur along the eastern margin of the present limit of outcrop. Rice (1941, p. 19) further notes that southward toward the 49th Parallel the upper slaty and argillaceous beds of the Horsethief Creek series grade into the Three Sisters formation of Salmo area, consisting of "5400 feet . . . of fine and coarse grits with some quartzite and conglomerate beds." The Horsethief is therefore a correlative of the Monk and Three Sisters formation of Salmo area (Little, 1950) and of the Monk and lower Gypsy quartzite of Metaline Quadrangle in northeastern Washington (Park and Cannon, 1943). The Horsethief Creek series may be correlated with the Hector and Corral Creek formations (Walcott, 1910, pp. 423-431) in the main ranges of the Rocky Mountains, and the Miette formation of Yellowhead PassRobson district (Walcott, 1928, p. 364). (See Table I.) This correlation is based solely on the similar relative stratigraphie position of all these formations as well as the fact that coarse pebbly quartzites and conglomerates found in Hector, Corral, and Miette formations are typical of the Horsethief Creek series and are nowhere found in the Purcell system to the south. Déposition and Origin The Toby conglomerate represents the only major break in deposition between the late Precambrian Purcell and Windermere systems. Significantly, as shown below, no break of similar magnitude occurs between the Windermere rocks and the succeeding lower Paleozoic rocks. Walker (1926, p. 15) considered the Toby to be a fanglomerate. Rice (1941, p. 23) considered it to have been deposited on a highland consisting of gently open-folded Purcell rocks, in the wake of an advancing sea. This explains the variability of the Toby from a breccia to a boulder conglomerate, to a slate or phyllite, as well as the wide distribution of the formation. Nevertheless, this hypothesis does not explain the accumulation of the great thicknesses of Toby conglomerate near the 49th Parallel, and perhaps in such regions it did accumulate as a fanglomerate. It is in any case evident from the unconformity represented by the presence of the Toby conglomerate at the base of the Windermere system that the Purcell basin was deformed at the end of Purcell time. No evidence exists that this deformation was intense, but it was, rather, open folding and gentle upwarping. Deiss (1935, p. 113) considers that, "following the cessation of Beltian deposition, central and southwestern Montana, was elevated to mountainous heights, at least 20,000 feet above the region to the west. . . . the strata were thrown into gentle folds, but no examples are known of Beltian rocks

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that were closely folded or faulted in pre-Camhrian time. The elevated mass formed in western Montana is designated the Helena Mountains." In British Columbia these mountains were limited approximately to the present outcrop area of the Purcell system in the southern Canadian Rockies and approximately to the region of the present Purcell Range as far north as latitude 50 degrees. Some of this deformed area remained high until at least the Lower Cambrian and some until Middle Cambrian (see Deiss, 1935). The relation of the Proterozoic with the Cambrian is discussed in some detail below. The rocks of the Windermere system (consisting of Toby and Horsethief Creek) were deposited in a subsiding trough to the northwest and west of the ancestral Belt-Purcell Mountains (Helena Mountains of Deiss). Subsidence was apparently greatest west of latitude 117 degrees in the vicinity of the International Boundary, for the Toby-Irene-Shedroof conglomerates are here the thickest, and the overlying Horsethief Creek contains more coarse conglomerate than farther north. For the same reason, elevation in the adjoining mountain mass to the east was apparently greater than farther north in the vicinity of latitude 50 degrees. The predominance of fine-grained elastics in the Horsethief Creek reflects merely a source area in the universally fine-grained Purcell system. No evidence for a western source exists within the Windermere. The source of feldspar, though it usually represents less than 10 per cent of any succession of beds within the Horsethief Creek, is puzzling. No such feldspar exists in the Purcell series. No granites of Precambrian age are known within the Purcell-Belt terrain. The feldspar, so far as examined by the writer, is predominantly microcline, orthoclase and albite-oligoclase, not andesinelabradorite as might be expected from weathering of Movie intrusions. The only evidence bearing on this problem is the occurrence of gneissic granite boulders in quantity at localities in Toby formation of Canal Flats area just northeast of the intersection of latitude 50 degrees and longitude 116 degrees (Leech, 1955, personal communication). This has a significant bearing on the source of the feldspar in the Horsethief Creek, for such boulders have not been moved great distances. They therefore indicate a nearby granitic source, for which there are two possible explanations : ( 1 ) a post-Purcell, pre-Windermere granitic intrusive occurs nearby; (2) an arch of Archean rocks, to be found not far east, buried now beneath the Paleozoic rocks of the Rocky Mountains. The first alternative is not considered likely, for no intrusive rock of granitic composition is at present known anywhere in the Belt-Purcell terrain to the southeast. Furthermore, there does not appear to have been sufficient deformation within the Purcell system at the end of Purcell time to produce a gneissic granite of the type found here. Therefore an Archean source must be considered. Again, a western Archean source is unlikely, for evidence at present available indicates great subsidence of the area immediately west, within the region of the present Purcell divide and westward

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into the Selkirk Mountains. Transportation of such boulders of gneissic granite across a subsiding trough from any distance is unlikely. It is therefore necessary to consider the plausibility of a nearby source, or one which lay not far to the east. In considering this it is interesting to note the information that has become available from deep drilling east of the Rocky Mountains in the search for oil during the past few years. Burwash (1951) summarized the results of an examination of twentyeight cores from wells in the central Alberta plains of the Edmonton, Peace River, and Athabasca areas as follows: ". . . all information . . . supports the concept that the Precambrian underlying the central plains of Alberta is an extension of the Canadian Shield. . . . Intrusive igneous rocks are the dominant group, quantitatively, comprising 24 of the 28 crystalline cores. . . . Within the intrusive igneous group the granites and granodiorites are dominant. . . . Most of the samples examined showed marked evidence of metamorphism." (Burwash, 1951, p. 39.) Similarly Webb (1954, pp. 6-7) notes the presence of the Precambrian Shield beneath the plains of northern and central Alberta. He considers it likely "that a low westward extension of the great pre-Cambrian Shield upland existed across the McMurray-Peace River belt, where sediments of positive Cambrian age are now lacking." (P. 7. See also isopach map of Cambrian system, p. 6.) Webb also points out that the "thinness of the southeast Alberta section and somewhat greater thickness on the east in Saskatchewan (of the Cambrian system) suggests the presence of ancestral features related to the Cenozoic Sweetgrass arch and Williston basin" (p. 7). It is evident from the foregoing that a source area for the granitic boulders in the Toby conglomerate and particularly for the coarse quartzitic and fcldspathic material of the Horsethief Creek series lies at no great distance to the east and north of the known region of deposition of the Windermere system. No western source for these sediments need be postulated. RELATION BETWEEN THE LOWER PALEOZOIC AND THE PROTEROZOIC Much uncertainty exists as to the precise location of the CambrianPrecambrian boundary, and as a result uncertainty in the actual distribution of the Lower Paleozoic rocks, particularly in the western part of this region. These difficulties are caused by a number of factors : scarcity of fossils in the Lower Cambrian strata in the entire region; reconnaissance nature of most work in the western Cordillera; metamorphism and intense deformation in the probable Lower Paleozoic of the Selkirk Mountains. The following is an attempt to review and to interpret the stratigraphy and the tectonics of the Proterozoic-Lower Paleozoic sequence of this region. These results are based on currently available, incomplete information and are therefore of temporary value and subject to constant revision as more information becomes available.

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The Cambrian System A remarkably uniform quartz arenite of considerable thickness occurs throughout the region of the northern and western Purcell Mountains as well as throughout much of the adjacent Selkirk Mountains to the west. A similar quartz arenite is found in the Rocky Mountains from Mt. Assiniboine near latitude 51 degrees northward to Mt. Robson and district and probably beyond. The correlation of these Lower Cambrian formations is summarized in Table I. Throughout, these rocks consist of quartz arenites or quartzite of varying grain size, with local, discontinuous interbeds of shale, slate or schist (e.g., Lake Louise shale). Local pebble bandings occur, as well as a small percentage of clastic potash and sodic feldspar in some of the coarser-grained interbeds. Significant variations in thickness, distribution, and grain size are noted from locality to locality throughout the region. In northeastern Washington the Gypsy quartzite (Park and Cannon, 1943, pp. 13, 14) is 8,500 feet thick cast of Pend Oreille River but a few miles to the west across the Pend Oreille River the thickness is only 5,300 feet. The reason is given as follows: "The 4,525 feet of grits and conglomerate present at the base of the formation east of Pend Oreille River are represented by only 1,260 feet near the Oriole mine [west of the Pend Oreille River]. The quartzites above the basal grits and conglomerates are surprisingly similar in the two sections." Furthermore, the Gypsy quartzite here grades downward into the Monk formation and upward through the Maiden phyllite to the Metaline limestone of the Middle Cambrian. Fragments of trilobites in the uppermost beds of the Gypsy quartzite indicate a Lower Cambrian age for this formation (Park and Cannon, 1943, p. 15). Little (1950) working in Salmo area just north of the International Boundary notes that the Quartzite Range formation (equivalent to upper Gypsy quartzite) and the Three Sisters formation (equivalent to lower Gypsy quartzite) show a lateral variation in lithology similar to that in the Gypsy quartzite to the south. Little ( 1950, pp. 12, 13) points out that these formations "in the eastern part of the map-area consist of thick, massive beds in which crossbeds are conspicuous. To the west, the beds are relatively thin and platy, and show few crossbeds, and overall thickness is much less. This indicates that the sediments composing the Three Sisters and Quartzite Range formations were derived from the region to the east or southeast of Salmo map-area." Pleosponges of Lower Cambrian age have been collected from the base of the overlying Laib group (Little, 1950, p. 18). Little, however, considers the Quartzite Range formation to be Lower Cambrian on the basis of the fossils found to the south by Park and Cannon. In any case the Precambrian-Cambrian boundary is arbitrary and the Precambrian and Lower Paleozoic formations of this region follow one another conformably. In the area to the north, east of Kootenay Lake, the quartzites of the lower part of the Hamill series are equivalent to, and lithologically identical with those of the Quartzite Range formation (Rice, 1941, pp. 19-20). In

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this area the upper beds of the Horsethief Creek series are equivalent to the Three Sisters of Salmo area. The entire sequence is conformable and gradational and no break has been found between rocks of known Windermere age (Horsethief Creek) and those correlated with rocks of Lower Cambrian age (the quartzites of Hamill series). It must be noted, however, that no fauna have been discovered in the area immediately east of Kootenay Lake indicating a Cambrian or later age for the Hamill and immediately succeeding rocks. Therefore the Hamill quartzites are considered to be of Cambrian age only on the basis of their correlation with identical, sparsely fossiliferous rocks to the south. Within the same area (Rice, 1941, map 603A) along latitude 49 degrees 30 minutes, approximately on the divide of the Purcell Range, no apparent equivalent of the great thickness of Hamill quartzite is found. In this locality a coarse Lower Cambrian conglomerate ( Cranbrook conglomerate ) rests unconformably on Lower Purcell strata (Rice, 1941, p. 29). The Cranbrook conglomerate is here overlain conformably by approximately 2,000 feet of argillite, impure quartzite, calcareous argillite and impure limestone of the Eager formation ( Leech, personal communication ). Fossils found near the base of the Eager are of upper Lower Cambrian age. This locality lies approximately 20 miles from the last outcrop of Hamill quartzite to the west. About an equal distance farther east, in Cranbrook area (Rice, 1937, p. 18-21), the Cranbrook formation consists of 600 feet of massive, coarse-grained, siliceous quartzite, white, rose-red, green, and grey in colour, overlain by about 150 feet of rock magnesite, with gradational sandy magnesite between. A series of green quartzites, about 200 feet thick, lie above the magnesite. According to Gunning (personal communication) there is evidence, in this area, of an angular discordance of ten degrees or more between the Cranbrook formation (basal Cambrian) and the bevelled surface of the underlying Siyeh formation (Upper Purcell). In addition the underlying Siyeh formation has undergone considerably more metamorphism than the overlying Cranbrook formation. Approximately another 50 miles southeast of Cranbrook area at Elko, in the western Rocky Mountains, Middle Cambrian with perhaps a few basal beds of Lower Cambrian (Schofield 1914) lies unconformably on Upper Purcell. It may be noted from the foregoing that if the rocks of the Lower Cambrian are correctly correlated, and there seems to be no alternative, then these sediments have been deposited under much different conditions on either side of the present Purcell divide. West of this divide a conformable sequence of great thickness, of both Windermere and Lower Paleozoic rocks, have been deposited. On the other hand, on and east of the Purcell divide no Windermere rocks exist and the Lower Cambrian consists, at most, of a few hundred feet of conglomerates or coarse quartzites. A few miles farther east no Lower Cambrian exists. It may be concluded, then, that no Windermere rocks were deposited in this area east of the present Purcell divide and

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that during this time the ancestral Purcell Mountains were undergoing active erosion. During the Lower Cambrian a great thickness of quartzite and other fine elastics as well as some limestone were being deposited in the region west of the present Purcell divide. At the same time, in the area immediately to the east, equivalent conglomerates and coarse quartzites as well as some rock magnesite were being deposited on the bevelled surface of the gently folded Purcell rocks. The present distribution of the Cranbrook indicates that many thousands of feet of sedimentary rocks (all of Upper Purcell and perhaps one-half of Lower Purcell) were eroded from the ancestral Purcell Mountains between the end of Purcell time and before the beginning of the Cambrian (i.e., during Windermere time). Further, the area of the present Rocky Mountains farther east remained a "high" during Lower Cambrian, for here Middle Cambrian rests on the Upper Purcell. Farther north, in the general vicinity of latitude 50 degrees 30 minutes, stratigraphie relations are noted that indicate conditions of deposition similar to those at latitude 49 degrees 30 minutes (Walker, 1926; Henderson, 1954; Leech, personal communication; and Reesor, field work 1954, 1955). In this area 5,000 feet of Hamill quartzite occurs northeast of Kootenay Lake. These quartzites thin eastward to about 3,000 feet at the Purcell divide (Reesor, field work, 1954, 1955). The contact with the underlying Horsethief Creek appears to be, at least in part, gradational. About 25 miles farther east, approximately along latitude 50 degrees 30 minutes in Windermere area (Walker, 1926, p. 21), rocks of Middle and/or Upper Cambrian age rest directly on Horsethief Creek rocks. No equivalent of the great thickness of Hamill quartzite of probable Lower Cambrian age, found everywhere to the west, exists here. It has been pointed out, in the discussion of the Windermere system above, that it also thins and disappears southward in a few miles in Canal Flats area. Therefore at this latitude the basin of deposition of the Windermere has extended much farther east than it did to the south, in the region immediately north of the 49th Parallel, and the Purcell "high" apparently terminated in the vicinity of the 50th parallel (see Fig. 1). The stratigraphy along a line from Canal Flats area northwestward, in the general vicinity of the Rocky Mountain Trench, further indicates that the basin deepened northward and northwestward at least during the Lower Cambrian. In Canal Flats area Leech (1954, pp. 7, 8) notes the occurrence of several hundred feet of quartzites and pebbly conglomerates overlain by impure quartzites and slates. Fauna in the upper part of these rocks show them to be of Lower Cambrian age and to be a correlative of the lower part of the Eager formation to the south in Cranbrook area. In considering the possibility of an unconformity beneath the Cambrian, Leech (1954, p. 6) notes that, "the Toby formation is overlain disconformably, and perhaps with angular unconformity, by quartzite of the Lower Cambrian Cranbrook formation. Individual outcrops have revealed no clear-cut angular un-

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conformity but the Toby formation appears to be bevelled by the Cranbrook formation." A few miles north of Canal Flats area for a distance of 30 to 40 miles northwest, no Lower Cambrian rocks exist, and Jubilee formation of Middle and/or Upper Cambrian age lies directly on Horsethief Creek "formation" (Walker, 1926; Henderson, 1954). This locality was therefore a "high" during the Lower Cambrian or immediately post Lower Cambrian (Henderson, 1954, p. 51). Another 25 miles northwest, on Jubilee Mountain (Evans, 1933, pp. 120A, 123A) a few hundred feet of Lower Cambrian strata occur. These strata thicken rapidly to the northwest and in 50 miles the Lower Cambrian formations are nearly 7,000 feet thick. These rocks consist of a lower quartzitic sequence of the Fort Mountain, Lake Louise, St. Piran formations, in all 5,400 feet thick, and the overlying Donald formation of limestone, slate and impure limestone, about 1,500 feet thick in this locality. The Donald formation contains sufficient fauna to indicate clearly a Lower Cambrian age, but Callavia and vertical worm burrows in the upper 20 feet of the St. Piran are the only fossils found in this great thickness of quartzitic rocks (Evans, 1933, p. 120A). Evans considers the base of the Fort Mountain—St. Piran sequence to rest "with an undulating unconformity" above the Horsethief Creek series, of Windermere age. The correlation of the sparsely fossiliferous Fort Mountain, Lake Louise and St. Piran with the quartzites of the Hamill series to the south and west, in which no fossils have yet been found, must be based on almost identical lithology and on the same relative stratigraphie position of the two sequences. There seems no reasonable doubt that this correlation is correct, particularly in view of the fact that fossils at the top of the equivalent of the Hamill quartzites in northeastern Washington indicate these rocks to be Lower Cambrian. It may be possible that further work in the northern Selkirk Mountains northwest of Evans' work in the Dogtooth Mountains may show that Hamill-Fort Mountain-St. Piran rocks are continuous. Similarly, on the basis of rare fossils, similar lithology, and similar stratigraphic position Evans correlated the Fort Mountain-Lake Louise-St. Piran formations of the Dogtooth Mountains (northern Purcells) with the same formations of Field area in the central ranges of the Rocky Mountains to the east (p. 121A, II). Again, the relations with the underlying rocks, presumably of Precambrian age, is of considerable significance. At Mt. Assiniboine Deiss called the equivalent of the Fort Mountain-Lake Louise—St. Piran quartzites the Gog formation. This formation here consists of 1,235 feet of quartzites and sandstones with a basal conglomerate shown to rest with slight unconformity on the underlying slates and shales of the Hector formation (Deiss, 1940, p. 769). Deiss considers that boulders of this conglomerate, up to 17 inches in long diameter, could not have been

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transported far by wave action and marine currents and that therefore the north shore of the "Montana Island" was not far to the south. This is further substantiated by the fact that Middle Cambrian in southern Alberta and northwestern Montana rests directly on equivalents of the Upper Purcell rocks. About 60 miles northwest of Mt. Assiniboine in upper Bow Valley, Walcott (1910, p. 427-430) found the Lower Cambrian quartzites in apparent local conformity with the underlying Hector, but considered the contact to be an unconformity on the basis of the following : ( 1 ) varying thickness of the basal Cambrian conglomerate; (2) different character of the Proterozoic in different places (p. 426). Deiss (1939, p. 1011) considering the same unconformity notes that similar quartz pebble conglomerates occur in the Fort Mountain sandstone as in the underlying Prccambrian Hector. In addition, the Hector, on Ptarmigan Mountain, lies apparently conformably under the Fort Mountain formation and contains intraformational conglomerate with angular and rounded pebbles of grey, maroon, and green fine-grained limestone greater than 18 inches in diameter. Also a shale similar to that of the Hector is found near the base of the Fort Mountain. "Thus the Hector is partly transitional into the overlying Fort Mountain" (p. 1011). It would appear, therefore, that in this locality any break between the Cambrian Ft. Mountain and the preceding Hector is based more on change of character of the rock from a predominantly argillaceous Hector to a predominantly quartz arenite of the Fort Mountain—St. Piran succession, not on any fundamental, important or extensive disconformity or angular unconformity. The break in this region between Cambrian and Precambrian is not profound. Furthermore, even in this region fossil evidence indicating a Lower Cambrian age for Fort Mountain—St. Piran rocks is not plentiful. Rasetti (1951, p. 56) notes: "Fossils occur sparsely through the silicious portion of the St. Piran sandstone. . . . The oldest faunule . . . [occurs] 2,000 feet below the top of the Lower Cambrian, near Vermilion Pass at the base of Storm Mountain." Hughes (1955), working about 60 miles north of the above locality, considers the entire sequence of Precambrian-Cambrian strata to be conformable. On the basis of lack of fossils Hughes considers the Jonas Creek formation (equivalent of Fort Mountain, Lake Louise and St. Piran formations to the south) to be in part Precambrian. The Jonas Creek formation here consists of approximately 5,000 feet of quartzites with local pebble bandings and interbedded shale lenses. "Cross-bedding in quartzite strata, apparently derived by currents flowing from east to west, occur at several places" (p. 76). A few tens of miles farther northwest along the Jasper-Banff highway, "the lower cliffs of a great many mountains are formed of thick quartzites.

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The lower Cambrian is here about 3,000 feet thick, and in most localities carries a good deal of limestone in the centre, with Olenellus" (North and Henderson, 1954, p. 51). In Mt. Robson area, a few miles farther northeast, Walcott ( 1913, p. 34) considers an unconformity at the base of the Lower Cambrian succession, between the McNaughton sandstone and the Precambrian Miette formation. However, Walcott (1928, p. 363) later says: "This [referring to the thickness of the McNaughton sandstone], however, is very uncertain, as it is difficult to determine the line of demarcation between the sandstone of Cambrian and pre-Cambrian (Beltian) age." It must therefore be concluded that no major unconformity exists between the Precambrian and Cambrian succession in this locality. As previously noted, there is little reasonable doubt that the Fort Mountain-Lake Louise-St. Piran sequence of Dogtooth Mountains in the northern Purcells (Evans, 1933) may be correlated with the quartzites of the Ham ¡11 series of the western Purcell and Selkirk Mountains. Okulitch (1949, pp. 9-10) correlated the Hamill, along the main line of Canadian Pacific Railway a few tens of miles due west of Dogtooth Mountains, with the Ft. Mountain-St. Piran sequence of the Dogtooth and Rocky Mountains as well as with a number of formations mapped earlier by Daly (1915) along the Canadian Pacific, and with the Hamill series mapped in Lardeau area to the south (Walker and Bancroft, 1929). In turn, part of the Hamill may be shown to be equivalent to rocks mapped by Gunning in Big Bend area a few miles farther west, which he called members IA and IB (Gunning, 1928, pp. 140-143A and p. 153A). Okulitch in this locality notes that the Hamill contains considerable argillaceous material and thins from 10,000 feet in the Sir Donald Range in the central Sclkirks to 4,000 feet a few miles west. Okulitch (1949, p. 7) considers a possible local disconformity between Horsethief Creek series and Hamill series. The apparent equivalents of the quartzitic members of the Hamill series in Big Bend area (Gunning, 1928, p. 143A) become increasingly fine grained to the west and north and consist, on the west slopes of Selkirk Mountains, essentially of mica schist, quartzite, chlorite schist, and discontinuous crystalline limestone rather than pure massive quartzites found farther east. It may be noted here that Walker and Bancroft (1929, p. 9) found the quartzites of the Hamill series a few miles to the south in Lardeau area to become increasingly argillaceous from east to west. Of considerable significance is the occurrence of greenstones interbedded with rocks, in part equivalent to the Hamill series in Big Bend area (Gunning, 1928, p. 143A). The greenstones occur as follows: "Over large areas they are massive to schistose, chloritic or serpentinous rocks, showing no trace of igneous texture or of flow structure. . . . In numerous places they are strongly amygdaloidal, the amygdules, generally less than one-half inch in diameter, being filled with quartz, epidote, or calcite, and occasionally with

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fresh, well-formed crystals of albite." (P. 143A.) The greenstones occur in bands 400 to 600 feet wide in these rocks. Gunning concludes (p. 144A) that "the greenstones found in members IA and IB are largely extrusive, but that some are definitely intrusive. [The intrusive phases] . . . so closely resemble intrusive material from the Triassic Kaslo scries of Lardeau and Slocan map-areas that the possibility of a Triassic age must not be overlooked." Nevertheless, he states that since greenstone flows are interbedded with members IA and IB, they must of course be of the same age. If the correlation proposed in this paper is correct these greenstones are of Lower Cambrian age.

Summary Okulitch (1949, pp. 16-20) has reviewed the problem of the age of the late Precambrian or early Paleozoic sequence in the northern Selkirk Mountains. In speaking of the Hamill series and its equivalents (p. 17) he says in part : . . . transgressive overlaps and deltaic deposits have resulted in vast sheets of sandy detritus continuous over large areas, but almost certainly of different ages at different localities. . . . Stratigraphie and structural evidence points to the fact that these rock-units transgress time and are older in the west than they are in the east. . . . Thus, whereas the Hamill series is apparently Precambrian in the Selkirk Mountains its rock-unit equivalent farther cast, the Fort Mountain, Lake Louise, and St. Piran formations, includes Lower Cambrian in the Dogtooth and Rocky Mountains. Okulitch arbitrarily places the Precambrian-Cambrian boundary at the top of the Horsethief Creek series, based partly on lack of fossils in the Horsethief Creek in any area, on disconformity between Horsethief Creek and Hamill in Dogtooth Mountains and Glacier area, and finally on the change in character of sediments deposited in Horsethief Creek and Hamill time. Okulitch (p. 20) further suggests that the sediments of the Cambrian and equivalent strata were derived from the east. In summary it may be noted that nowhere in the region under discussion do the Lower Cambrian quartzites contain abundant fauna indicating a precise age. Fauna so far discovered invariably indicate the upper part of the Lower Cambrian. In the region of the southern Purcells the Lower Cambrian quartzites and conglomerates unconformably overlie the Purcell rocks. Northward the Cambrian rocks overlie Windermere rocks with only little or perhaps no unconformity and westward the entire sequence of Windermere-Cambrian rocks appears to be conformable. The Lower Cambrian thickens greatly along the western flank of the Purcells and the adjacent Selkirk Mountains, and wherever evidence is available these sediments appear to have been derived from the east or southeast. Warren (1951, pp. 3, 4) has similarly suggested that these sediments may have been derived from the Archean granitic terrain now

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known to lie not far to the north (Peace River "high") and to the east beneath the central Alberta plains. In any case the evidence at present available indicates a source to the east for both the Windermere and the Lower Paleozoic rocks of this region. Both sequences have been deposited with little interruption within the same western trough, although the Lower Cambrian strata have overlapped the ancient Purcell-Belt Mountains within the region of the present southern Purcell Range, and the Middle Cambrian has overlapped nearly the entire eroded surface of this ancestral mountain belt. SUMMARY OF THE EARLY DEVELOPMENT OF THE GEOSYNCLINE IN THE SOUTHERN CANADIAN CORDILLERA Along the western limit of the central North American Shield, Purcell sediments were deposited over a long period in the late Precambrian in a basin of unknown westward and northwestward extent. The Shield has apparently been the source of sediments which were carried long distances perhaps by one or more major river systems from the north and east, and deposited in a slowly subsiding delta under contemporaneous subaerial or aerial conditions from point to point in the basin. To some extent, erosion of sedimentary and metamorphic rocks covering the granitic terrain of the Shield may explain the fineness of grain of the Purcell sediments. Local welts within the basin may explain local intraformational conglomerates and coarser quartzites. Subsidence and deposition perhaps began in the west, though the evidence is not unequivocal, and a great thickness of fine elastics as well as dolomitic and calcareous sediments are known in the central part of the basin. In any case supply of sediments and subsidence of the basin remained in remarkable balance throughout the deposition of the Purcell system. The conditions of deposition were perhaps similar to those in the Gulf Coast geosyncline at the present time, and the open sea may well have extended westward from the western limit of the Purcell sediments. Evidence at present available clearly indicates that initial deformation of this basin took place at the end of the Upper Purcell. Strata were deformed into gentle, open folds, and later rocks (Lower and Middle Cambrian) are in places found conformably overlying Purcell rocks, in places with an angular unconformity up to 30 degrees. Up to 20,000 feet of Purcell strata were eroded before the Cambrian. Purcell deformation was apparently confined approximately to the region of presently known outcrop of Purcell rocks. The western extent of deformation was in the region of the western Purcell Mountains, for a great thickness of Windermere sediments immediately succeeding the deformation were deposited in the Purcells west of 116 degrees 30 minutes, and in the adjacent Selkirk Mountains. The limit of Purcell deformation to the north appears to have been the vicinity of latitude 50 degrees. After the deformation in the Purcell-Belt terrain a great thickness of

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Windermere and Lower Paleozoic strata were deposited to the west, with lesser thicknesses to the north of the limit of Purcell deformation. Sediments now forming Windermere and Lower Paleozoic strata were derived from the ancestral Belt-Purcell Mountains and probably from the Archean granitic landmass farther east and north (Peace River "high" and central Alberta plains). Possibly an Archean granitic mass lay not far from the present site of granite boulders found in Toby conglomerate of the Canal Flats area. During the Lower Cambrian, the area of deposition transgressed over the site of the present southern Purcell Mountains and during Middle Cambrian over most of the region of the ancestral Purcell-Belt Mountains. From the evidence as outlined in the first three sections of this paper, two belts of deposition of contrasting character that formed after the deformation of the Purcell system may be delineated. The first, confined to the present site of the Rocky Mountains and the eastern and central part of the Purcell Mountains, contains fine-grained clastic sediments followed by or interbedded with much limestone and dolomite. These strata were deposited under relatively stable conditions, and the thickness of sediment is not great. Subsequent deformation is confined to simple folding and complex faulting with associated more complicated folding. Metamorphism is limited to recrystallization of sediments and dynamic metamorphism related to faulting. Rare interbedded volcanic material of andesitic composition occurs in this belt, and granitic intrusions are of minor importance only in the western part. Evidence of breaks in the sequence of strata between Precambrian and Lower Paleozoic is clear only in the region of the ancestral Purcell-Belt Mountains or within a few tens oï miles of this limit. This belt is clearly miogeosynclinal and may, in many of its salient lithological characteristics as well as in relative location with respect to the central North American Shield, be compared directly with the Champlain belt of the Appalachian Mountain system. The second belt lies west of the first, and the western limit of this belt is not at present known. Great thicknesses of fine clastic sediments of Windermere and Lower Paleozoic age are known. In contrast with the first belt the entire sequence of strata is here apparently conformable and gradational. Also in contrast with the first belt, subsequent deformation and metamorphism have been intense. Granitic intrusions are plentiful and scattered localities are known to contain considerable thicknesses of interbedded Windermere and possibly Lower Cambrian andesitic volcanic material. With further work in the region much more volcanic material of Lower Paleozoic age may be found. This belt is clearly eugeosynclinal and may be compared in character and position, relative to the Shield, with the Magog belt of the Appalachian Mountains. Throughout both belts fossil evidence indicating Lower Cambrian age is rare or non-existent. This is to some extent a result of the predominant Lower Cambrian rock: a uniform quartzite or quartz arenite throughout. Nevertheless rocks later than the Lower Cambrian in the miogeosynclinal

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belt contain abundant fauna of Middle Cambrian and later age. On the other hand rocks apparently in the same stratigraphie position in the eugeosynclinal belt are found to be sparsely fossiliferous only in the general vicinity of the International Boundary. Thus, to the north in the western Purcell and Selkirk Mountains correlations of the strata equivalent to those of the miogeosyncline to the east may only be made on a lithological basis. This lack of fossils may be explained in part by deposition of the eugeosynclinal sequence under rapid subsidence and perhaps deep marine conditions, and in part by subsequent intense deformation and metamorphism. The fact that these sediments may in fact be very late Precambrian has been noted above. The deformation and metamorphism of the early rocks with which we are here concerned in the western orthogeosyncline may be unravelled only by briefly considering, first, the correlation and deformation of the Shuswap terrain lying immediately west of the Selkirk Mountains, and second, the post-Cambrian sedimentation and deformation of this region. The time and place in deformation and metamorphism of the Proterozoic rocks of the southern Canadian Cordillera may then be briefly summarized. THE SHUSWAP TERRAIN The Shuswap terrain consists of highly deformed, metamorphic rocks underlying a wide region west of the Arrow Lakes. This terrain outcrops as far west as Lake Okanagan, and its extension beyond this is hidden by younger, overlying rocks. In addition exposures of Shuswap rocks are separated from known Proterozoic rocks to the east by the overlying younger rocks of the Selkirk Mountains. Therefore, correlation and age determination must be based on speculation and comparison of position in the stratigraphic column. The Shuswap terrain has been divided into three groups (Jones, 1953). The Monashee group outcrops between Arrow Lakes and Shuswap Lake and consists of not less than 50,000 feet of granitoid gneiss, mica-sillimanitegarnet schist, quartzite, hornblende gneiss, limestone, marble dolomite, slate and phyllite. Metamorphic rocks which may have been derived from volcanic flows or tuffs are scarce. No unconformities are known within the group. The Monashee group lies in fault contact with rocks of the Hamill and Lardeau series on the east. The Mt. Ida group is found in fault contact with the Monashee group along Shuswap Lake and southward. It consists of at least 60,000 feet of limestone, chlorite and biotite schist, argillite, and quartz-mica schist as well as a considerable thickness of altered andesitic lavas and volcanic fragmentais. This group is much less metamorphosed than the Monashee group and although structurally much deformed, may be divided into a number of separate formations (Rice and Jones, 1948). The Chapperon group is found south of Shuswap Lake (Jones, 1953), and consists of at least 5,000 feet of mixed sediments and volcanics.

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Although the stratigraphie relations of the various groups of the Shuswap terrain are uncertain, Jones does not consider the Monashee and Mt. Ida groups to be stratigraphically equivalent, for thick units of schist, limestone, and basic volcanic rocks are not present in the Monashee group. The latter is considered the older and has been more deeply buried and more intensely metamorphosed than the Mt. Ida group. The Chapperon group on the other hand may be equivalent to the upper part of Mt. Ida group (Eagle Bay formation). Intrusive rocks consist of pre-Older deformation gabbroic sills and dykes. Minor granite and pegmatites have been emplaced syntectonically with the pre-Cache Creek, Older deformation. There are in addition serpentinized ultramafic dykes found only south of Shuswap Lake within the outcrop area of the Chapperon group (Jones, 1953). Jones in an unpublished thesis states: . . . although superficially appearing unfolded, the rocks of the Shuswap terrain are nevertheless folded on a small scale to a degree that is most extraordinary. These folds average a few feet in amplitude and are recumbent and isoclinal with horizontal or shallow plunging axes. A remarkable feature of these folds is their prevalence throughout the Shuswap terrain and the evidence they provide of widespread, horizontal thrusting movements throughout the thick assemblage of nearly horizontal beds. Other folds in some places are clearly superimposed on the recumbent type and therefore belong to the Younger period of deformation. The important structural difference between the Monashee and Mt. Ida groups is the direction in which the minor recumbent folds are oriented. The tectonic axes in the Monashee group are oriented to the northeast and east, whereas in the Mt. Ida group they are oriented predominantly northwest or north, almost at right angles. The sense of the drag-folding in the Mt. Ida group, with minor, local exceptions, is as if upper layers sheared to the east or northeast over lower layers. Jones considers that no large sequences of strata are repeated to suggest the presence of large folds. The overturned middle limbs of these recumbent drag-folds range from a few millimeters to several tens of feet, with the average about 2 feet. Jones considers the deformation of the Monashee group to have taken place at much greater depth in the earth's crust than that of the Mt. Ida group. He states: ". . . it seems certain that the Monashee and Mt. Ida groups must have assumed the drag folds and related structure of the Older deformation while they lay at different levels in the earth's crust because no tectonic process could have developed in them such contrasting directions of intense rock movement while they lay in their present relative positions." Age and Correlation No reasonable doubt exists that rocks of the Cache Creek (Permian, and possibly in part Carboniferous age) lie with profound unconformity over rocks of the Shuswap terrain. Basal conglomerates of the Cache Creek con-

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tain boulders of metamorphosed Shuswap rocks. Thus metamorphism and deformation of the Shuswap rocks took place before the Permian. Jones notes, however, that the actual age of the Shuswap terrain remains largely a matter of conjecture. This is owing in part to the intensity of metamorphism and deformation, in part to the fact that it lies in fault contact with bordering rocks. Jones considers three possibilities for the age of the Shuswap : ( 1 ) it is of early Paleozoic and Windermere age; (2) it is equivalent to the Purcell system; there is considerable similarity between rocks of the Monashee group and those of the Purcell system that have been locally intensely metamorphosed by granitic intrusions; (3) it is Archean in age, for similar structures were not found in a cursory examination of Hamill or Lardeau rocks east of Upper Arrow Lake. Jones considers the last possibility the most likely and says: ". . . this would introduce no impossible features, but would account for all known facts." Nevertheless the possibility that the Mt. Ida group is equivalent to the Lower Paleozoic and Windermere must be considered for the following reasons: ( 1 ) If the Shuswap terrain was metamorphosed and deformed prior to the Windermere then presumably it must have been in part a source area for Windermere and Lower Paleozoic rocks to the east. No such evidence exists, and the Hamill series at least is finer grained along the western limits of its outcrop area (Gunning, 1928; Walker and Bancroft, 1929). (2) Volcanic rocks are found in the Windermere of Salmo area as well as in late Prccambrian or early Paleozoic of Big Bend area. It is not impossible that these could be correlatives of the volcanics of the Mt. Ida group to the west. (3) It must be noted that although an unconformity exists between the Lardeau series and the Mississippi an (and later) Milford group of Lardeau area (Walker and Bancroft, 1929) this is not apparently an unconformity of the same order as that noted in the Shuswap to the west. Further, Little (personal communication) found no evidence, to the south of Lardeau area, of a profound unconformity between Milford and Lower Paleozoic rocks. Thus deformation within the Shuswap area may have taken place in post-Lower Paleozoic and pre-Carboniferous and was limited to the region of the present outcrop of the Shuswap terrain. Therefore lack of structures characteristic of the Shuswap in the Hamill and Lardeau series is explained without the necessity to consider Shuswap deformation as preWindermcre. In connection with the time of deformation and metamorphism in this belt it is interesting to note that in the Cariboo 200 miles farther northwest Brown and Holland (1956) find an unconformity between Carboniferous and Lower Paleozoic, but none between the Lower Paleozoic and underlying Precambrian. Similarly Roots (1954, p. 107) finds that in Aiken

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Lake area, 300 miles farther northwest, metamorphism and deformation of the Wolverine complex have occurred after the Lower Cambrian and before the Mississippian. The Shuswap terrain lies on a southward continuation of this same belt and may thus be presumed to have been metamorphosed and deformed at the same time. On the other hand the Monashee group is found only in fault contact with either the Hamill and Lardeau series to the east and with the Mt. Ida group on the west. It may therefore be of either Archean or Purcell age as suggested by Jones, and has been faulted into its present position relative to younger rocks during the pre-Permian, post-Lower Paleozoic deformation. SUMMARY OF TIME AND PLACE IN SEDIMENTATION AND TECTONISM IN THE SOUTHERN CANADIAN CORDILLERA The correlation of the Shuswap terrain with the Precambrian systems to the east is based on pure supposition and must remain so based until further work and suitable independent methods of dating these rocks have been applied. Nevertheless, an outline of the processes which have resulted in the formation of the present character and configuration of the Precambrian rocks in this region may be attempted. ( 1 ) Sedimentation during Purcell time in which a great thickness of predominantly very fine-grained clastic rocks were deposited in a geosyncline which in this region extended from the foothills of the Rocky Mountains westward to the Purcell Mountains and perhaps to the Monashee Mountains and beyond. Possibly the open sea extended to the west. This geosyncline extended northward for a distance not at present known. Sediments were derived from the cover of the crystalline basement lying to the east and northeast. (2) Deformation of this geosyncline into open gentle folds, apparently confined to the area of the southern Purcell Mountains and the entire region of the Purcell-Belt terrain to the east and southeast. (3) Deposition of the Windermere system and overlying, conformable Paleozoic to a great thickness to the west, and a lesser thickness to the north of the ancestral Purcell-Belt Mountains; and overlap eastward of Cambrian formations along a sinuous shoreline over Windermere and Purcell rocks, but concurrent deposition without apparent break to the west in the eugeosyncline. Sediments of the Windermere and Lower Paleozoic were derived from the uplifted areas of Purcell rocks and from uncovered Archean granitic terrain to the east and north (Peace River "high"). Interbedded volcanics consist of: Irene volcanics of Windermere age; volcanics of Big Bend area of late Windermere or early Paleozoic age; and Mt. Ida volcanics of possible Lower Paleozoic and/or late Precambrian age. All volcanics occur within the limits of the eugeosyncline in Selkirk and Monashee Mountains. (4) After deep burial, intense deformation and metamorphism in the eugeosynclinal belt of the Monashee Mountains during post-Lower Paleo-

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zoic and pre-Permian (Cache Creek). The western limit of the deformation is not known, but the eastern limit was along the west slope of Selkirk Mountains, since no equivalent profound unconformity occurs between Mississippian (?) and earlier rocks in this area. The deformation in the west is, however, represented by a break between the Upper Paleozoic Milford group and the Lower Paleozoic (?) rocks of Selkirk Mountains and probably by the pre-Middle Devonian break of the Rocky Mountains. (5) After this pre-Cache Creek deformation for the first time a major proportion of the sediments deposited in the western orthogeosyncline were derived from the west. A great thickness of sediments and volcanics accumulated in the eastern part of the eugeosyncline, in the belt now occupied by the Selkirk Mountains. (6) Following deep burial, deformation during the late Jurassic resulted in further disruption of the Shuswap terrain as well as intense deformation, metamorphism, and emplacement of plutonic rocks in the region of the Selkirk Mountains. The Precambrian rocks of the Purcell Mountains were deformed to a lesser degree apparently at the same time. This final deformation of the eugeosyncline and the western part of the miogeosyncline in the Purcell Mountains culminated in the disruption of the Windermere, Paleozoic and Mesozoic strata, as well as in further deformation of the Purcell rocks of the Rocky Mountain miogeosyncline during what is termed the Laramide Revolution, in the late Cretaceous and early Tertiary. ACKNOWLEDGMENTS The writer wishes to acknowledge the help of many geologists in the preparation of this manuscript. Free use has been made of unpublished material by A. G. Jones on the Shuswap, and by R. Mulligan in the descriptions of aerial extent of the Precambrian of the Purcell, Selkirk, and Cariboo Mountains. Drs. G. B. Leech, and H. M. A. Rice have given suggestions throughout and have read and checked the manuscript. Their help is gratefully acknowledged. In particular, the writer wishes to express appreciation for the help of Dr. H. C. Gunning during preparation, and for presenting the paper to the Royal Society. REFERENCES BROWN, A. S., and HOLLAND, S. (1956, in press). Structure of the northeastern part of the Cariboo district. BURWASH, R. A. (1951). The Precambrian under the central plains of Alberta; M.Sc. thesis, Univ. of Alberta, unpublished. CAMPBELL, C. D. (1947). Cambrian rocks of northeastern Stevens county, Washington; Bui!. Geol. Soc. Am., vol. 58, pp. 597-612. CLAPP, C. H., and DEISS, C. F. (1931). Correlation of Montana Algonkian formations; Bull. Geol. Soc. Am., vol. 42, pp. 673-69R. DALY, R. A. (1912). North American Cordillera, Forty-ninth Parallel; Geol. Surv., Can., Mem. 38. (1915). A geological reconnaissance between Golden and Kamloops, B.C., along the Canadian Pacific Railway; Geol. Surv., Can., Mem. 58. DEISS, CHARLES (1935). Cambrian-Algonkian unconformity in western Montana; Bull. Geol. Soc. Am., vol. 46, pp. 95-124.

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(1939). Cambrian formations of southwestern Alberta and southeastern British Columbia; Bull. Geol. Soc. Am., vol. 50, pp. 951-1026. • (1940). Lower and Middle Cambrian stratigraphy of southwestern Alberta and southeastern British Columbia; Bull. Geol. Soc. Am., vol. 51, pp. 731-794. (1941). Cambrian geography and sedimentation in the central Cordilleran region; Bull. Geol. Soc. Am., vol. 52, pp. 1085-1116. DOUGLAS, R. J. W. (1952). Waterton Alberta; Geol. Surv., Can., Paper 52-10. EMMONS, W. H., and CALKINS, F. C. (1915). Geology and ore deposits of Phillipsburg Quandrangle; U.S. Geol. Surv., Folio 196. EVANS, C. S. (1933). Brisco-Dogtooth map-area, B.C.; Geol. Surv., Can., Sum. Rept. 1932, pt. All, pp. 106-176. GIBSON, RUSSELL (1948). Geology and ore deposits of the Libby Quadrangle, Montana; U.S. Geol. Surv., Bull. 956. GUNNING, H. C. (1929). Geology and mineral deposits of Big Bend map-area, B.C.; Geol. Surv., Can., Sum. Rept. 1928, pt. A, pp. 136-193. HAGE, C. O. (1943). Beaver Mines; Geol. Surv., Can., Map 739A. HENDERSON, G. G. L. (1954). Geology of the Stanford Range of the Rocky Mountains; British Columbia Dcpt. of Mines, Bull. 35. HUGHES, R. D. (1955). Geology of portions of Sunwapta and Southesk map-areas, Jasper National Park, Alberta, Canada; Guidebook, Alberta Soc. of Petroleum Geologists, Annual Field Conference, pp. 69—116. JONES, A. G. (1953; in manuscript). Geology of the Vcrnon map-area, British Columbia; Geol. Surv., Can., Mem. KIRKHAM, V. R., and ELLIS, W. E. ( 1 9 2 6 ) . Boundary County, Idaho; Idaho Bur. Mines, Bui!. 10. LEECH, G. B. (1952). St. Mary Lake, British Columbia: Geol. Surv., Can., Paper 52-15. (1954). Canal Flats, British Columbia; Geol. Surv., Can., Paper 54-7. LITTLE, H. W. (1950). Salmo map-area, British Columbia; Geol. Surv., Can., Paper 50-19. MCALLISTER, A. L. (1951). Ymir map-area, British Columbia; Geol. Surv., Can., Paper 51-4. NORTH, F. K., and HENDERSON, G. G. L. (1954). Summary of the geology of the southern Rocky Mountains of Canada; Guidebook, Alberta Soc. of Petroleum Geologists, Annual Field Conference, pp. 15—81. OKULITCH, V. J. (1949). Geology of part of the Selkirk Mountains in the vicinity of the main line of the Canadian Pacific Railway, British Columbia; Geol. Surv., Can., Bull. 14. PARK, C. F., and CANNON, R. S. (1943). Geology and ore deposits of the Metaline Quadrangle, Washington; U.S. Geol. Surv., Prof. Paper 202. RANSOME, F. L., and CALKINS, F. C. (1908). Geology and ore deposits of Cœur d'Alêne district; U.S. Geol. Surv., Prof. Paper 62. RASETTI, F. (1951). Middle Cambrian stratigraphy and faunas of the Canadian Rocky Mountains; Smith, Misc. Coll.. vol. 116, no. 5, pp. 1—121. REESOR, J. E. (1954a). Dewar Creek, British Columbia; Geol. Surv., Can., Paper 53-25. (1954b). Findlay Creek map-area, British Columbia; Geol. Surv., Can., Paper 53-34. RICE, H. M. A. (1937). Cranbrook map-area, British Columbia; Geol. Surv., Can., Mem. 207. (1941). Nelson map-area, east half; Geol. Surv., Can., Mem. 228. RICE, H. M. A., and JONES, A. G. (1948). Salmon Arm map-area, British Columbia; Geol. Surv.. Can., Paper 48-4. ROOTS, E. F. (1954). Geology and mineral deposits of Aiken Lake map-area, British Columbia; Geol. Surv., Can., Mem. 274. SCHOFIELD, S. J. (1915). Geology of the Cranbrook map-area, British Columbia; Geol. Surv., Can., Mem. 76. (1922). Relationship of the Precambrian (Beltian) terrain to the Lower Cambrian strata of southeastern British Columbia; Geol. Surv., Can., Mus. Bull. no. 35.

POSSIBLE PROTEROZOIC OCCURRENCES IN BRITISH COLUMBIA, THE YUKON AND NORTHWEST TERRITORIES H. C. Gunning, F.R.S.C. THE LATEST COMPILATION of the geology of British Columbia ( 1 ) shows no Proterozoic (Beltian) strata north of the 51st parallel or west of the area discussed in this volume by Dr. Reesor. However, several areas are shown as "Lower Cambrian and/or older" and these should, for the sake of completeness, be referred to briefly. All the areas are in the eastern part of the province, along or within some 50 miles of the Rocky Mountain Trench. In addition, in the south central part of the province, in the Omineca country and in a few more restricted patches near the 60th parallel or within the Coast Range, there are masses of metamorphic rocks, the southern bodies of which have in the past been referred to as "Shuswap terrain." They are so recrystallized to gneisses and schists, and of such complex structure, that their age is in doubt within wide limits. The Lower Cambrian and/or older strata are as follows, from south to north: Cariboo Area (2) Dr. Reesor has referred to these strata briefly. They are known as the Cariboo group and have been placed in the Proterozoic because they are said to conformably underlie strata of Lower Cambrian age in one locality. However, as Reesor has pointed out, Sutherland-Brown has found Lower Cambrian fossils in the Cariboo group, so the problem of assigning an age to the entire group is similar to that of the Windermere series to the south. The Cariboo group consists of a minimum of about 2,000 feet of conformable beds of limestone, quartzite, phyllite, slate, micaceous or chloritic schist, grit and pebble conglomerate intricately folded. The base of the group is not known to be exposed. The source of the Cariboo group is not known. It probably continues northwest past the Fraser River in an area that has not been studied in detail. In Jasper Park region rocks of supposed Precambrian age form the bases of thrust-fault blocks. They have been named the Jasper series by Allan, Rutherford and Warren, and the Miette series, in Mount Robson Park, by Walcott. The rocks consist of argillites, slates, quartzites, breccias, and con178

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glomerates. Their aggregate thickness is unknown but probably amounts to several thousand feet. The Jasper series is overlain by quartzites believed to be of Lower Cambrian age (3). Okulitch (personal communication) considers that the Miette sandstone may be Lower Cambrian in the Mount Robson area and that the lower boundary of the Cambrian may be at the top of the underlying, more argillaceous, Jasper series. In the Sunwapta and Southesk area, about half way between the Field and Mount Robson areas, R. D. Hughes (information supplied to V. J. Okulitch from unpublished Ph.D. thesis) has described a Precambrian section consisting of 6,000 feet of Hector formation shales, quartzites and conglomerates overlain by 5,000 feet of Jones Creek quartzite. Both formations are unfossiliferous. The latter is overlain conformably by Lower Cambrian beds of the Mount Whyte formation. Okulitch is of the opinion that the Jones Creek formation should be considered basal Lower Cambrian and the Hector, here as in other localities to the south, should be considered equivalent to part of the Windermere series. In the Northern Rocky Aiountains, south of latitude 59 degrees, two areas of strata of possible Precambrian age have been reported upon by M. Y. Williams (4) and by Landon and Chronic ( 5 ) . The first is on Toad River, at and south from the Alaska Highway. Williams reports quartzites, argillites and slates, with ripple-marks and mud-cracks cut by basic dykes and overlain with angular discordance by Silurian strata. It is said that a series of tan quartzites, sandstone and limestone, of possible Cambrian age, lies unconformably upon the supposedly Precambrian strata and is overlain unconformably by the Silurian beds. In the Cassiar district and along and west of the Rocky Mountain Trench from latitude 55 to 58 degrees, great thicknesses of moderately to extremely metamorphosed strata have been mapped by many geologists. The latest report, by E. F. Roots (6), is on the Aiken Lake area. He recognizes two groups of rocks separated by a possible erosion interval and places the lower Tenakihi group in the Proterozoic. This groups consists of upward of 13,000 feet of quartzite and schist, in part altered to gneisses of the so-called Wolverine complex. The rocks are highly folded, in places overturned, and "the general relations are such that the schistosity planes are parallel or nearly parallel to the major bedding structures, but cut across the bedding of the smaller folds, although exceptions to this generalization have been noted." The rocks are interpreted as the metamorphic equivalents of alternating beds of sandstones and shales that were deposited in shallow water. No evidence was found as to the direction of the source area. Foliated granitic pebbles in two known conglomerate layers in the group and other compositional features suggest to Roots that the source terrain was granitic or metamorphic in character. The age is considered to be probably Proterozoic because the group underlies the Ingenika group in which some Lower Cambrian archaeocyatha have been found in a small limestone lens over 4,000 feet above the base. The overlying Ingenika group consists of not less than 18,000 feet of

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slate, argillite, quartzite, quartzitic conglomerate, limestone and mica or chlorite schist that occur in belts up to 5 miles wide on either side of the northwesttrending anticlinorium of Tenakihi rocks. The original rocks are judged to fall into four dominant groups—greywacke and subgreywacke, sandstone, finegrained conglomerate and limestone that was in part oolitic, in part algal and in many places containing detrital non-carbonate material. Some of the quartzite is crossbedded. Roots concludes that the Ingenika strata were laid down under typical géosynclinal conditions. Okulitch has correlated the fauna from the limestone lens with that of the Lower Cambrian Donald formation of southeast British Columbia. The upper age limit of the group is not known and it is not known whether the lower part, below the fossil horizon, should be placed in the Cambrian or the Precambrian. The problem of the age of the Ingenika and Tenakihi groups is similar to that of the Windermere series. Some light on the possible Proterozoic rocks of Cassiar and other parts of British Columbia is provided by the work of H. Gabrielse (7) in the McDame area (8). Here, some 14,000 feet of sedimentary strata, including white and pink quartzite, limestone and argillaceous faciès, have been found to contain fauna of Lower and Middle Cambrian age. The Atan group, to which these strata belong, is overlain by black slate and limestone of Ordovician age. The base of the Cambrian has not been recognized. An adjoining group of rocks, called the Horseranch, consists of quartzite, gneiss, schist and other metamorphic assemblages, plus igneous masses. It is interpreted in part as metamorphosed Atan rocks and, of course, resembles the "Shuswap" rocks of southern British Columbia and Omineca. The western part of Yukon Territory contains large areas underlain by highly deformed sedimentary and volcanic strata, penetrated by a wide variety of intrusives and in part altered to gneisses and schists that are referred to the Yukon group. To date no indisputable proof of Precambrian strata has been presented and parts of the group have been assigned with varying degrees of uncertainty to Paleozoic and/or Precambrian. Bostock (9) discussed the matter at some length and concluded that a thick succession of schist, gneiss, quartzite and limestone in the southwest part of Carmacks area may be of Precambrian age. More recent work by W. H. Poolc and A. Aho (personal communication from V. J. Okulitch) in the Wolf Creek (10) and Ketsa River areas respectively has revealed Lower Cambrian archaeocyatha in strata for which stratigraphie details are not yet available. The Wolf Creek area may contain northwesterly extensions of the strata described by Gabrielse in the McDame area, B.C. D. D. Cairnes (11) introduced the term Yukon group as a result of his work along the Yukon-Alaska boundary, north of Yukon River. He considered the overlying "Tindir Group" to be pre-Middle Cambrian. Later work by Mertie and others (12) led to the conclusion that the Tindir group may be "Algonkian" because "there is apparently a marked stratigraphie hiatus between the top of the 'Tindir Group' and the known Middle Cam-

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brian formation in the vicinity of Tatonduk River." Mertie concluded that the evidence is too weak to justify the definite appellation Precambrian. The rocks of the Tindir group include practically unmetamorphosed dolomite and limestone, shale, slate, quartzite and diabase and basalt. Cairnes' Yukon group would include rocks that in adjoining central Alaska are

FIGURE 1

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known as the Birch Creek schists and which Smith has concluded are probably older than the unmetamorphosed Tindir group. Northwest Territories M. Y. Williams (13) and G. S. Hume (14) have discussed possible late Precambrian strata in the Franklin Mountains and in the lower Mackenzie River areas respectively. In the former, 375 feet of dark shales underlie the Lower Cambrian Mount Clark formation. A disconformity is assumed on the basis of conglomerates in the base of the Mount Clark but an unconformity has not been demonstrated. Williams tentatively placed the dark shales in the Beltian(?). Hume has recorded the presence of unfossiliferous black shales and quartzites in the Katherine group on the upper Carcajou River and near the Canol pipeline road. The Katherine group strata underlie with apparent conformity Middle or Upper Cambrian beds of the^Macdougal group, and, accordingly, may be Cambrian in age. It is reported (W. H. Mathews, personal communication) that large areas in the Mackenzie Mountains contain exposures of strata that may be Proterozoic but the proof is not available. The accompanying map shows the distribution of Proterozoic and possible Precambrian strata in British Columbia. REFERENCES (1) B.C. NATURAL RESOURCES CONFERENCE (1956). Resources map, no. 3; B.C. Atlas of Resources, p. 8. ( 2 ) HOLLAND, S. S. (1954). Yanks Peak-Round Top Mountain area, B.C.; B.C. Dcpt. Mines, Bull. 34. (3) (1947). Geol. Surv., Can., Econ. Geol. Series, no. 1. (4) WILLIAMS, M. Y. (1944). Geological investigations along the Alaska Highway from Fort Nelson, B.C., to Watson Lake, Yukon; Geol. Surv., Can.. Paper 44-28. (5) LAN-DON and CHRONIC (1947). Bull. A.A.P G., vol. 31, pp. 1608-1618. (6) ROOTS, E. F. (1954). Aikcn Lake map-area, B.C.; Geol. Surv., Can., Mem. 274. (7) GABRIELSE, H. (1954). McDame, B.C.; Geol. Surv., Can., Preliminary Paper 54-10. (8) OKULITCH, V. J. (1955). Archaeocyatha from the McDame Creek area; Trans. Roy. Soc. Can., Ser. Ill, vol. 49, Sec. IV. (9) BOSTOCK, H. S. (1936). Carmacks district, Yukon; Geol. Surv., Can., Mem. 189, p. 18. (10) POOLE, W. H., and AHO, A. (1955). Wolf Lake area, Yukon; Geol. Surv., Can., Paper 55-21. (11) CAIRNES, D. D. (1914). Yukon-Alaska boundary; Geol. Surv., Can., Mem. 67. (12) SMITH, P. S. (1939). Areal geology of Alaska; U.S. Geol. Surv., Prof. Paper 192, pp. 8-10. (13) WILLIAMS, M. Y. (1922). Geol. Surv., Can., Sum. Rept., pt. B, p. 69 et seq. (14) HUME, G. S. (1954). Lower Mackenzie River area, N.W.T. and Yukon; Geol. Surv., Can., Mem. 273.

SUMMARY AND DISCUSSION J. E. Gilí, F.R.S.C. THE DESCRIPTIVE PAPERS provide an excellent review of what is known of rocks classed as Proterozoic in Canada. Only a few small areas have been missed, such as isolated occurrences in the Hudson Bay lowland, the Belcher Islands, and north of Lake Melville. The inclusion of a considerable amount of material not published elsewhere adds substantially to the value of the volume. There seems to be no reason to attempt a summary of the descriptive papers, since these are themselves summaries and a further summation would only obscure the diversity that is an important feature of the rock groups classed by the authors as Proterozoic. Rather I will direct attention to the criteria that have been used in classifying rocks as Proterozoic and the problems that result. Some attention will also be given to suggestions offered by J. T. Wilson1 and by Harrison and Eadc for dealing with these problems and a few observations will be made on the history of the Proterozoic as now known. J. T. Wilson has noted in his historical summary (p. 12) that Proterozoic was introduced by Chamberlin and Salisbury to refer to that part of Precambrian time starting at the beginning of deposition of the "Huronian" and continuing until the beginning of Paleozoic time. This usage was confirmed in 1934 by the National Committee on Stratigraphical Nomenclature and "Archean" was adopted for the earlier part of Precambrian time (Alcock, 1934). Most of the contributors to this volume have apparently intended to follow these recommendations. Recent results from isotope studies have raised questions about the classification of certain rock groups and have given rise to suggestions for revising the definitions of Proterozoic and Archean. However, the need for names for two or more divisions of Precambrian time is recognized by all. Confusion has arisen primarily through the attempts of field men to classify rocks as Proterozoic or Archean without reliable criteria for so doing. Before discussing the reasons for conflicts I will attempt to summarize the criteria used for classifying rocks as Proterozoic by authors of descriptive papers in this book. iMention of a name without a reference or with only a page reference indicates a paper, by the author named, included in this symposium. Other references are by name and date, correlated with the list at the end. 183

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Most of the authors of descriptive papers do not discuss their reasons for including the rock groups described as Proterozoic. Generally it is evident that either a classification previously made by the Geological Survey of Canada or some other authoritative body has been accepted, or lithology and local structural relations have been the determining considerations. In spite of the reference to life in the derivation of the word, fossils have not been used. Age determinations based on radioactivity have had an important influence on thinking about rocks in some areas as, for example, in the Grenville province, but have not, as yet, been given much attention by field men generally. From the descriptive papers it appears that, to be classed with some certainty as Proterozoic, a stratified rock group should not be highly metamorphosed and preferably not folded. If it is folded, it is classed as Proterozoic if it includes or can be shown to be contemporaneous with sediments of a particular kind now known to be formed characteristically in shallow epicontinental seas or shallow seas along continental margins. These sedimentary groups show moderate to fairly complete sorting in the clastic fraction, giving clean quartzites and mudstones, instead of greywacke. Other features are crossbedding, ripple-mark, intraformational conglomerate and other internal structures characteristic of shallow-water environment. Limestones, dolomites and iron-formations are characteristic members of many groups. These assemblages will be referred to as the shelf faciès or shelf-type sediments, though some may have formed in géosynclinal troughs near continental margins—so-called miogeosynclines. If the strata rest with sharp angular unconformity on highly deformed volcanics, gneisses, or batholithic intrusives, or if Paleozoic rocks rest on them conformably (Windermere system, Conception group) or with only modest discordance (Purcell system), the Proterozoic age is regarded as more certain. Intrusives that cut "Proterozoic" strata and nothing known to be younger, are Proterozoic; those that are unconformably beneath "Proterozoic" strata have ordinarily been classed as Archean. These specifications are traceable back to C. R. Van Hise (1909), and were based on ideas about Precambrian time and geologic processes that have since been shown to be incorrect. Looked at critically today all the criteria used, except the conformable relations to Cambrian strata, cannot be accepted as reliable evidence. At best the others may provide a basis for a guess that, on statistical grounds, may have better than an even chance of being correct. First, consider metamorphism. There are plenty of examples of Paleozoic and younger rocks that have been highly metamorphosed, yet the gneisses of western Newfoundland and of New Brunswick, the Grenville gneisses and many others in the Shield and the Monashee of southern British Columbia have been classed as Archean mainly because of the degree of metamorphism.

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In considering this and other questions relating to Precambrian history it is instructive to draw up a diagram like Figure 1 to get things in proper perspective. When one compares the length of time involved in a typical younger cycle of orogenesis with the great length of Proterozoic time, it is obvious that a gneiss series unconformably beneath flat-lying shallow-water sediments is not necessarily Archean. Whether Proterozoic time is taken to be 500 million years or 1,500 million years long there was ample time for several erogenic cycles, peneplanations and repeated overlaps by marginal

FIGURE 1. Diagram showing some ages calculated from isotope ratios in uraninite, pitchblende and galena from certain veins and dykes. The short line opposite a name marks the age reported. The long line indicates the range of possible error inherent in the analytical method used. Data mainly from G. L. Cummings, J. T. Wilson, R. M. Farquhar and R. D. Russell, Proc. Geol. Assoc. Can., vol. 7, pt. 2, pp. 27-80, 1955.

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and widespreading epicontinental seas. Gneisses of several different ages could, therefore, have formed during the Proterozoic and numerous unconformities representing different periods of erosion are to be expected. The classification of shelf-type sediments as Proterozoic was based originally on the idea that during the earlier part of Precambrian time rocks did not break down as readily into mineral fractions as they did later, but by the beginning of Proterozoic time it was thought that conditions of weathering had changed so that sediments resembling those of Paleozoic and later age could form. There may well be some truth in this idea. Changes in the compositions of the atmosphere and oceans must have occurred with the passage of time and changes in the shelf faciès should have developed along with them. However, the critical features marking rocks as Proterozoic have still to be defined. This matter will be discussed later. Shelf-type sediments are easily destroyed by erosion because they stand high. With the passage of time, more and more of the older shelf groups would have been completely removed, so there should be more Proterozoic shelf groups than Archean shelf groups in the Shield today. The Archean shelf groups should show crossbedding, ripple-mark and other internal structures due to wave action in shallow seas, as in the Proterozoic shelf faciès, and they should rest unconformably on older rocks, but they may lack clean quartzites, limestone and iron-formation because of incomplete rock breakdown. Such sediments would probably be greywacke-slate-conglorncrate assemblages differing from the géosynclinal assemblages only in their internal structures and structural relations. None has been recognized to date, but the greywacke groups have not ordinarily been scrutinized with this distinction in mind. It should be noted that the exclusion of the greywacke-.slate-conglomerate groups from the Proterozoic has been based on the false premise that such rocks formed only in Archean time. Similar assemblages are found in much younger rock groups and could, no doubt, form in certain situations today. All that is required for their formation is rapid mechanical disintegration and deposition. They occur most characteristically with volcanics in folded belts and could have formed at almost any stage in earth history in rapidly sinking troughs associated with volcanic arcs. Shelf sediments of similar composition could also have formed in certain arctic areas at almost any time. If there is anything truly distinctive about Archean sediments it has still to be defined. The above observations lead to the conclusion that many folded greywacke groups now classed as Archean could just as well be Proterozoic as Archean. The abundance of volcanics in the oldest rock groups found in different parts of the Shield early suggested to geologists working there that volcanism was especially active and widespread in the early stages of earth history. Radioactivity ages show that many of these rock groups are indeed very ancient, so the idea of large-scale volcanic activity in early Precambrian

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time is confirmed. I cannot, however, agree that this is justification for calling all greenstone-greywacke assemblages Archean. The widespread occurrence of similar volcanics in Paleozoic and younger orogens makes it appear almost certain that volcanism continued, though probably on a reduced scale, through the Proterozoic. Some of the greenstone belts classed as Archean should, therefore, be Proterozoic in age. There is one good example in the Shield today of a shelf-type assemblage grading into and therefore roughly contemporaneous with a géosynclinal assemblage. This is in the Labrador "Trough" where on the southwest side, clean quartzites, slates, dolomites and iron-formations rest with sharp angular unconformity on granitic gneiss, while farther eastward basic volcanics appear in abundance interlayered with sediments. If erosion had cut a few thousand feet deeper, only the greenstone group would have been left and these rocks would almost certainly have been called Archean. Fahrig and Bergeron agree that the gneisses still farther to the northeast were formed by metamorphism of extensions of the same rock groups, under higher temperature conditions. All these contrasting groups are now classed as Proterozoic, presumably because the idea that differentiated shelf-type sediments could not form in Archean time has prevailed. If we should decide to continue to use the base of the Huronian in the Original Huronian area as the surface of reference marking the beginning of Proterozoic time, rock groups not in contact with the Original Huronian could be placed in the Archean or Proterozoic only by comparing dates provided by radioactivity methods with a similarly determined age for the base of the Huronian. Some assemblages similar to the Huronian appear to be older than the Huronian, so the distinction cannot be based on lithology. Some shelf-type sedimentary groups must, on this basis, be placed in the Archean and some greenstone-greywacke assemblages in the Proterozoic. This brings up the question of the reliability and accuracy of radioactivity dating and the prospects for the future. Examination of published lists of dates as determined by various methods leaves one with a feeling that great age is certainly indicated for some rocks and that useful information about relative ages of rocks is supplied by determinations by the same method, but much confusion may result from comparing results obtained in one area by one method and in another area by another method. In this book, Farquhar and Russell (p. 27) emphasize the fact that, so far, sediments can only be dated by indirect means, but they finish their paper with a note of optimism based on recent work on glauconite and other minerals in sediments using the potassium-argon and rubidium-strontium methods. This is an interesting possibility, but the results cannot be regarded as reliable unless a number of different samples give essentially the same results. It is virtually impossible to be sure that the mineral or minerals used are not contaminated by clastic particles either by straight mechanical inclusion or by incorporation in part in new minerals formed during diagenesis or metamorphism. Such contaminating particles could carry con-

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siderable amounts of the significant isotopes. A single determination could, therefore, be very misleading. If the same methods can be applied to lavas, more reliable results should accrue. The results reported by Farquhar and Russell from galenas in the Cobalt series and in "Archean-type" rocks are of special interest. Those from the Cobalt series include six ranging from 19±2 X 108 years to 20±2 X 108 years, giving a total possible range of 17 to 22 X 10s years = 500 million years. Three determinations are lower. Turning to those from "Archeantype" rocks, there are sixteen determinations ranging from 22±2 X 108 years to 27±2 X 108 years or a total possible range of 20 to 29 X 108 years = 900 million years. There are two younger ages, and one termed anomalous. These samples are from widely separated areas, so such a wide range is not surprising. The low figures instead of being looked upon as inaccurate may well be as meaningful as any of the others. However, the two determinations on material from Golden Manitou, showing a possible range of 700 million years, should warn us that none of these figures can be accepted as more than very rough approximations. Should not the range of possible ages for the "Archean-type" rocks of the southern Shield be, therefore, 1,600 to 2,900 million years, based on these data, rather than 2,300 to 2,500 million years, as given on the map? Average figures mean little in an area of this size. Farquhar and Russell imply that the results from the Cobalt series are meaningless and that those from the "Archean-type" rocks are worthy of serious attention. From the results listed, my own inclination would be to take the six determinations from the Cobalt rocks giving 19±2 X 108 and 20±2 X 108 more seriously than, for example, the two from Golden Manitou giving 23 ±2 X 108 and 26±2 X 10s years. "Reworking" would not be expected to produce six out of nine specimens giving such closely similar results if much change was involved. From an unbiased examination of the results to date, it can be concluded that the rocks classed as Proterozoic represent a substantial part of Precambrian time, possibly more than half, and a succession of many warpings, foldings and intrusions is indicated, but the dates are not yet accurate enough to establish time relations between sedimentary groups differing in age by less than 200 million years for the younger groups and 400 million years for the oldest groups. The contributions made to date by workers in this field are very important, and they promise even more important revelations in the future. Work toward perfection of methods, the devising of new methods and routine determinations by the best methods now available should therefore receive the support and co-operation of all geologists. To get the most out of age determinations by these methods it is important that material for analysis be collected carefully and that detailed information be supplied as to the nature of the material and its occurrence, along with the analytical results. Such information is lacking for many

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published results. It is also important that every determination be reported unless it is known to be wrong owing to laboratory errors. I have suggested elsewhere that clean quartzites and limestones may have been absent from the oldest rock groups (Gill, 1952). It was considered probable that quartzite could not form because there were no coarse-grained quartz-bearing rocks to supply the quartz sands and that limestones could not be deposited until the primitive oceans were saturated with CaCOs. Another reason for the absence of clean quartzites may have been less active breakdown of quartz-bearing rocks under weathering, as discussed above. Quartzites should have appeared locally in a small way at first and more abundantly later. Limestones could have appeared widespread and in abundance almost from the start. Since these two types are the commonest easily identified shelf-type sediments, it may be that in selecting this faciès as Proterozoic we have tended to include rocks formed at various times back to these beginning dates. Stretching the Proterozoic :o include all such rocks may be a practical way to avoid too severe disturbance of the status quo and at the same time to fix a time for the beginning of the Proterozoic. This is, in effect, what Harrison and Bade have suggested. To encompass all the rocks previously classed as Proterozoic they suggest a beginning about 2 billion years ago. In another part of their paper, Harrison and Eade refer, apparently with approval, to use of Archean and Proterozoic in the sense of older Precambrian and younger Precambrian, applied locally in any map-area and without implication of absolute age or to age relations to rocks termed Archean or Proterozoic in other areas. To attempt to use these terms in both an absolute and relative sense would lead to endless confusion, so I assume that they have abandoned this idea in favour of the one given above. This is, I think, wise. These terms have been used with absolute time connotations so long that to use them in a purely local relative sense would create more confusion than is necessary under the circumstances. It would be far better to abandon them entirely. A serious objection to defining Proterozoic in any of the ways suggested up to now is the lack of accord with the meaning implied by its roots— earlier life. It was invented to conform with Paleozoic, Mesozoic and Cenozoic and to extend the division of time based on the evolution of life back to an earlier date. If some basis in fossil forms cannot be found for its delimitation it would be better to abandon it in favour of a non-zoic term. There does, however, appear to be one possibility of justifying its use without disturbing past usages more than would be necessary in any case, for other reasons. This is to define the beginning of Proterozoic time as the time of first appearance of algal structures in rocks. It is my impression that the majority of paleontologists today recognize the forms variously described as Collenia, Cryptozoon or algal structures to be organic in origin. It is true, as Alice Wilson points out (p. 22), that similar forms can be produced mechanically and it is probably true that certain small occurrences have been mistakenly called algal structures, but

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the more extensive occurrences are in a different category. They are very similar in shape, size and distribution to younger examples that have been accepted as organic, so there seems to be little reason to question their authenticity as fossils. If these "algal" forms were to be used as the prime index of Proterozoic age, many of the rock groups heretofore classed as Proterozoic would automatically be confirmed as of that age, because algal structures occur rather widely in these rock groups. Precambrian groups conformably or unconformably above such beds, or intrusives cutting them, would be classed as Proterozoic. Groups beneath them could be Archean or Proterozoic. Groups not in contact with them could not be classified with any precision until dating methods based on radioactivity are greatly improved. Rocks older than the oldest algal structures would be Archean. If it can be shown that any of them bear clear evidence of the existence of life, they could then be called Archeozoic, a term used by Chamberlin and Salisbury for the early part of the Precambrian, and still used in some modern textbooks. It appears probable that the first algal structures were formed near the time of formation of the first carbonate rocks, so limestones and dolomites may serve as a rough index where algal structures are absent. Deposition of limestone on any considerable scale could not occur except under rare circumstances, until the upper part of the ocean was saturated with CaCOs. Initiation of limestone deposition should, therefore, have been an event of world-wide significance. The quantities of Ca, Mg and Fe contributed in soluble form to the oceans must have depended mainly on the breakdown of rock silicates under weathering. The composition of the atmosphere had a great influence, therefore, both on the concentrations of these elements and of CÛ2 in the ocean. The composition of the atmosphere, especially the CÛ2 concentration, must have been profoundly influenced by the development of living organisms, so there should have been a definite linkage in time between life forms and carbonate rock deposition. Some algal cherts may possibly have formed before the first limestone, or all algal structures may have formed considerably later than the first limestone. There is no way at present to determine the relations with any precision. A further problem is much to the fore in both the east and west, namely the location of the top of the Precambrian. In both areas there was sedimentation for a long period without interruption before the arrival of Olenellus. To include all the conformable sediments below the Olenellus zone as Cambrian is to place the beginning of the Cambrian at different times in different areas. Reesor places it somewhere during "Windermere time." There seems to be no way of getting a reasonably uniform time position for the end of the Precambrian except by using the base of the biozone of the trilobite genus Olenellus, as advocated by Wheeler (1947). To summarize, it is clear that nearly all geologists have used Proterozoic to refer to the later part of Precambrian time. The contradictions that are

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developing stem from attempts to classify Precambrian rocks on an absolute time basis without reliable criteria for so doing. Redefinition of Proterozoic to place the top at the base of the biozone of the trilobite genus Olenellus and the base at the time of formation of the oldest sediment with algal structures would conform with the meaning indicated by its roots—earlier life—and at the same time would cause less disturbance of current usage than would result from alternative suggestions. If this is not done, the recommendations of the 1934 National Committee on Stratigraphical Nomenclature should be followed, but a non-zoic term should be substituted for Proterozoic (Alcock, 1934). Regardless of which method is adopted, it should be recognized by field men that, though most greenstone-greywacke assemblages formed during the first half of Precambrian time, some must have formed in the latter half; also, that shelf-type sediments formed throughout Precambrian time and, although most of those preserved should, on laws of chance, be late Precambrian in age, some older ones probably exist. Classification on lithological grounds should be more often right than wrong, but it is better to omit absolute time terms unless reliable age information is available. Relative ages can be indicated by a table and in descriptions by "older" and "younger" or by numbers or letters. REFERENCES AI.COCK, F. J. (1934). Report of the National Committee on Stratigraphical Nomenclature; Trans. Roy. Soc. Can., Ser. Ill, vol. 28. Sec. IV. pp. 113-122. GILL. J. E. (1952). Early history of the Canadian Shield; Proc. Geol. Assoc. Can., vol. 5, pp. 57-68. VAX HISE. C. R. (1909). Principles of classification and correlation of pre-Cambrian rocks; Jour. Geol., vol. 17, pp. 97-104, and 118-122. WHEELER, H. E. (1947). Base of the Cambrian system; Jour. Gcol., vol. 55, no. 3, pt. I, pp. 153-159.