September 15, 19 10] 



NATURE 



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Ihe Lower Huronian ; but the break does not seem very 

 profound, and the rocks do not differ much from those 

 just described. 



The least changed parts of the Huronian extend as a 

 wide band for 200 or 300 miles north-east of Lake Huron, 

 and in this area the uneven surface of Laurentian and 

 Kcewatin beneath the Lower Huronian Boulder Clay pre- 

 serves for us a portion of the earliest dry land, the 

 earliest peneplain known in America, and possibly in the 

 world. This band has remained comparatively stable, 

 while, so far as our information goes, all other parts of 

 the Canadian Shield have undergone violent changes. 



Rise of ihe Late Laurentian Mountains. 



The Lower Huronian tillite has been found in many 

 places throughout the .^rcha-an region, over a stretch of 

 1000 miles from east to west, and 700. miles from north 

 to south, so that in all probability deposits like the 

 Pleistocene till covered most of the surface. 



Everywhere, however, except in the band extending 

 north-east from Lake Huron, it seems to have been in- 

 volved in later mountain building, and has been so sharply 

 folded in with the Keewatin as to destroy the appearance 

 of unconformity. It is instructive to note that so long 

 and momentous an interval was entirely overlooked by 

 geologists or treated as of small importance until a few 

 years ago. There is usually no angular discordance to 

 be observed, and the secondary schistose structures of 

 Keewatin and Huronian are similar and parallel. The 

 Huronian boulder conglomerate has often been rolled out 

 to a schist in which only the harder boulders can be 

 recognised as lenses ; and sometimes even they are lost 

 entirely, so that no evidence of discordance remains. 



It is evident that the invasion of the later Laurentian 

 granites and gneisses was accompanied by very important 

 dynamic and metamorphic effects. Most of the batholithic 

 domes of North-western Ontario are post-Lower Huronian, 

 and date perhaps from the Middle Huronian or the interval 

 between it and the Upper Huronian (.\nimikie). 



The granites and gneisses of this second time of moun- 

 tain building have not been distinguished in mapping from 

 those of. the first in most places, and as they are both of 

 precisely the same habit, it will probably never be possible 

 to separate them completely. Thus far both have been 

 included under the name Laurentian, which must be con- 

 sidered as representing a lithological facies rather than a 

 geological period. It may be, however, that the forma- 

 tion of batholithic mountains never really ceased from the 

 end of the Keewatin to the end of the Lower Huronian. 

 .As the rocks called Laurentian are entirely eruptive, they 

 should not be limited to a definite time, but only to a 

 definite set of conditions as to composition, rate of cool- 

 ing, and amount of pressure. 



.As in the earlier cvcle, the period of mountain-building 

 was followed by a period of destruction, ending in a 

 peneplain of very wide extent. 



The Animikie or Vpper Huronian. 



The interval between the lower formations and the 

 .Animikie is of great magnitude, perhaps even greater than 

 that between the Keewatin and the Lower Huronian, and 

 Lawson has suggested for it the name of the Eparcha;an 

 Interval. The Animikie has not been found resting on the 

 Middle Huronian in Canada, so that this formation may 

 partly bridge the chasm. Unless the Middle Huronian 

 quartzites include part of the products of erosion, we have 

 no evidence as to the disposal of the many thousands of 

 cubic miles of materials removed from the later Laurentian 

 mountains. 



The Animikie begins in most places with a thin basal 

 conglomerate lying almost horizontally on the upturned 

 edges of the previous schists and gneisses. Above this 

 come chert, black slate, and other sediments, sometimes 

 to the extent of 8000 or 10,000 feet. The slate often 

 contains carbon enough to make an important coal region 

 if collected in definite beds. 



The whole no doubt implies a transgressing sea, which 

 ultimately must have covered a very large part of the 

 Canadian Shield, since rgcks of this age are found over 

 wide surfaces north-west of Lake .Superior, near Lake 

 Mistassinni, in the heart of Labrador, on the east side of 



Hudson Bay, and near Great Bear and Dubaunt lakes. 

 These rocks are found in Labrador up to 1575 feet above 

 the sea. This level, if extended in all directions, would 

 submerge three-fourths of the .Archjean peneplain. 



-At present these areas, though large, are widely 

 separated ; and it may be rash to assume that even soft, 

 easily weathered rocks, like the -Animikie slate, could 

 have been completely removed from the intervening spaces. 

 It is probable, however, that less than. half of the .Archsean 

 then remained as dry land. 



The Keweenawan. 



There is an interval marked by a small discordance and 

 a basal conglomerate between the -Animikie and the 

 Keweenawan, but the break in time was apparently not 

 great. The two groups of rocks often occur together, 

 though in many places the Keweenawan sediments overlap 

 on to the Archaean, as in the neighbourhood of Lake 

 Nipigon. Most of the Keweenawan sedimentary rocks are 

 of shallow-water varieties, such as sandstone and con- 

 glomerate. .At various places on the north-east shore of 

 Lake Superior a coarse basal conglomerate is found as 

 remnants preserved in small valleys or ravines in the 

 granite. The ancient surface is now in process of resur- 

 rection by erosion, and the boulders once rolled on a 

 Keweenawan shore are being freed from their matrix and 

 once more set in motion by the waves of Lalce Superior. 



The Keweenawan, like the Keewatin, was a time of 

 vigorous volcanic activity, and in post places the lava- 

 sheets and laccolithic sills of diabase connected with their 

 eruption far surpass the sediments in amount. The 

 volcanic rocks are generally basic in character, and prob- 

 ably most of the diabase dykes widely found in almost 

 all parts of the Canadian -Archsan are of this age. The 

 important deposits of copper, nickel, and silver in Northern 

 Canada are closely bound up with the Keweenawan basic 

 volcanic rocks or with deeper-seated diabases, probably of 

 the same origin. 



Here, as in the Keewatin, we are confronted with floods 

 of basic lava coming up from unknown sources through 

 the acid Laurentian gneiss. Do these basic lavas repre- 

 sent heavier segregations settling to the bottom during the 

 slow movements of the granitic magma as it climbed into 

 the -Archaean batholiths? One might imagine these 

 heavier and more liquid parts sinking beneath the lighter, 

 more viscid, magmas of the domes, and remaining fluid 

 until the mountain masses above had become completely 

 solid. The supposed thrust from the -Atlantic basin to the 

 south-east might then bring strains to bear on the solid 

 crust, more or less shattering and shifting its masses, 

 squeezing up the still molten diabase through all the 

 fractures and pores. 



Several remarkable basins were formed in the -Arehasan 

 peneplain by the ascent of these lavas, permitting the 

 massive roof which formerly covered them to collapse by 

 block faulting or by the formation of an irregular syncline. 

 The basin of Superior seems to be of this nature. It is 

 still rimmed by the Keweenawan lavas, sometimes accumu- 

 lated to the thickness of .:;o,ooo feet. Just to the north is 

 the smaller basin of Lake Nipigon, with its edges and 

 islands of biabase sheets, and to the east, near Sudbury, 

 is the extraordinary synclinal basin, with which the great 

 nickel mines are connected. These basins seem to have 

 resulted from the collapse of the solid crust because of 

 the removal of support when basic eruptives ascended 

 from beneath. 



Palaeozoic History. 



The exact relation of the Keweenawan to the Cambrian 

 is somewhat in doubt, though most geologists make it 

 pre-Cambrian. The St. Mary's, or Lake Superior, sand- 

 stone, which rests upon the Keweenawan with a slight 

 discordance and overlaps upon the -Archaean, is generally 

 called Cambrian ; it contains no fossils, and occurs only 

 along the shores of Lake Superior and St. Mary's River, 

 so that its position in time is uncertain. 



Potsdam sandstone, either Lpper Cambrian or Lower 

 Ordovician, rests upon the planed-down -Arch.-ean surface 

 at the Thousand Islands and other points in Eastern 

 Canada, often with a conglomerate at its base ; and un- 

 doubted Ordovician limestones feather out upon the 



NO. 2133, VOL. 84] 



