33^ 



NATURE 



[Settembek 15, 1910 



earth's original crust, which cooled under the tremendous 

 weight of an atmosphere including everything volatile at 

 red heat, an atmosphere 200 or more times heavier than 

 at present. We know, however, that this cannot apply 

 to the Laurentian gneisses of Canada, since they push up 

 eruptively through great thicknesses of older rocks — the 

 Keewatin in the north and west, and the GrenviUe series 

 in the east, including large amounts of water-formed 

 deposits. Though these older rocks are now found only 

 on edge in synclines protected on each side by domes of 

 gneiss, there can be no doubt that they once spread out 

 wide and flat on the surface of the earth. 



The eruptives of the Keewatin have received most atten- 

 tion, but sedimentary rocks occur in it at all levels and 

 with thicknesses of hundreds or thousands of feet. They 

 include Lawsun's Couchiching, with its great areas of 

 mica schist and gneiss formed from what were originally 

 muddy and sandy sediments. In other places quartzites 

 and arkoses, slates and phyllites, represent less meta- 

 morphosed clastic materials. The slate is often black with 

 carbon. In the north-west there is little limestone or 

 dolomite, but the (irenviUe and Hastings series of the 

 east, which are probably in part of Keewatin age, contain 

 thousands of feet of limestone. All the ordinary types of 

 sedimentary rocks were being deposited on the Keewatin 

 sea-bottoms, and one type unlike modern sediments — the 

 banded silica and magnetite or hematite of the " iron 

 formation." The rock last mentioned belongs to the top 

 of the Keewatin, and is very widespread. Its crumpled 

 jaspers have attracted much attention because of their 

 association with iron ore, but in reality the other varieties 

 of sedimentary rocks are present in far greater amount 

 both as to thickness and extent. 



In almost every part of the western region there are 

 associated with the sediments great sheets of basic lavas, 

 agglomerates, and ash rocks, as well as smaller amounts 

 of quartz porphyry, &c., showing that the Keewatin was 

 one of the periods of great volcanic activity in the world's 

 history. It is somewhat puzzling to find these pre- 

 dominantly basic volcanics in the Keewatin, while all the 

 underlying eruptives of the Laurentian are decidedly acid, 

 chiefly granite or syenite in composition. 



The extensive sedimentary and eruptive rocks of this 

 earliest formation imply that the ordinary geological pro- 

 cesses were at work at the very beginning of known 

 geological time, before the Archa-an mountains came into 

 existence. There must have been broad land areas where 

 rocks like granite or gneiss weathered to mud and sand, 

 probably under a cool climate, for the greenish arkoses 

 and slates charged with carbon suggest cold rather than 

 heat. 



In the north-west volcanoes were active, but the east 

 was comparatively free from eruptions. Both volcanic ash 

 and ordinary clay and sand seem to have been spread out 

 on the sea bottom in the Lake Superior region, and prob- 

 ably seaweeds throve in the mud. In the GrenviUe region 

 the waters seem to have been clearer, and limestones were 

 deposited on a very large scale, sometimes pure, but often 

 muddy and mixed with a good deal of carbon, so that 

 fucoids probably flourished here also. 



If we reconstruct the conditions of the Keewatin, we 

 must then assume continents which have entirely vanished, 

 on which weather, rain, and rivers worked, sweeping 

 sediments down to the shallow or deeper seas to be spread 

 out on a bottom which has also disappeared. The sedi- 

 ments and lavas and tuffs may be said to rest on nothing, 

 for the once fluid or plastic Laurentian gneiss, cradling 

 their synclines and pushing up from beneath them, could 

 not have been the foundation on which they were laid 

 down. Though the floor on which they once rested has 

 nowhere been foimd, one may be certain that its materials 

 included silica, alumina, and alkalies in the right propor- 

 tions to fuse into a granitic magma, and this is practically 

 all that is known of the pre-Keewatin world in Canada. 



Rise and Fall of the Early Laurentian Mountains. 

 .After the work of the volcanoes, of rain and frost and 

 rivers, of winds and tides and currents, had piled up miles 

 of rock in Keewatin times, there came a great upheaval 

 of mountains over thousands of square miles of the early 

 Archa:'an surface. Possiblv the earth was alreadv shrink- 



ing through loss of volcanic material and of the steam 

 and gases that exhale in eruptions. The Atlantic floor 

 may have been settling down, thrusting inwards from 

 the south-east, pushing up the weakened earth's crust 

 beneath the shield into inountain rows ; or it may be that 

 some other cause must be sought for the somewhat hap- 

 hazard domes which arose over such wide areas. 



It may be suggested that the many thousands of feet 

 of lava and stratified materials had so blanketed the 

 lower-lying rocks that the heat from beneath crept up 

 into them, softening and semi-fusing them, until in the 

 slow lapse of time they began to flow sluggishly, ascend- 

 ing to form the wide-based domes of the Laurentian 

 mountains. The source of the internal heat need not be 

 discussed here. Uranium, with its various progeny, may 

 have been as active then as now, or a more rapid axial 

 rotation may have kneaded the discrete particles of a 

 mass of planetesimals, and so warmed them up to the 

 heat of fusion. 



Then followed the deliberate and almost complete 

 destruction of the great mountain system during a long 

 period of time, which has left no known Canadian record. 

 The sediments derived from this destruction may have 

 been piled on the bed of the Atlantic ;is it sank. It is 

 possible that Sederholm's Bottnian in Finland mav 

 partially fill the gap. 



Whatever disposal was made of the d(5bris, several 

 thousands of feet must have been carved from the moun- 

 tains and swept out of view during the immense interval 

 which separates the Keewatin and early Laurentian from 

 the Lower Huronian, for the next series of rocks rests 

 with a great discordance on the upturned edges of the 

 synclinally disposed Keewatin schists and the truncated 

 domes of Laurentian gneiss. 



The Huronian. 



The Lower Huronian has very different relationships 

 from the Keewatin. Where least disturbed, as north of 

 Lake Huron and in the Cobalt region, the floor beneath 

 it is often well preserved. Dr. Miller has shown that at 

 Cobalt the surface of Keewatin and Laurentian was hilly 

 or hummocky before the basal conglomerate of the Lower 

 Huronian was deposited ; and Prof. Brock, in describing 

 the Larder Lake district to the north, refers to " the 

 clean-swept and often rounded surface of the older rocks 

 on which it is frequently laid down."' 



The basal conglomerate of the Lower Huronian con- 

 tains pebbles and boulders of all the Keewatin and 

 Laurentian rocks that went before, and among them are 

 found beautifully striated stones. It is the oldest knowr> 

 Boulder Clay or tillite. The vast period of subaerial 

 destruction that carved away the early Laurentian moun- 

 tains ended in a glacial period, the ice-sheets of which 

 covered many thousands of square miles of North America, 

 just as the last great period of peneplanation ended with 

 the Pleistocene ice-sheets. 



It is not a little impressive to see modern till resting 

 on the Huronian tillite, and including fragments of it as 

 boulders. It is possible to break out from the modern 

 glaciated surface stones the underside of which received 

 their polish and stria? in the Lower Huronian, while their 

 upper surface has been smoothed and scratched by 

 Pleistocene ice movements. 



At Cobalt the tillite is accompanied by slate,, which may 

 be compared in all essential characters except hardness 

 with the stratified clay of adjoining lake deposits of 

 Pleistocene Age. The most recent and unconsolidated beds 

 make clear the origin of some of the most ancient and, 

 in appearance, most different rocks in the world. 



In the Lake Huron region the action of ice was prob- 

 ably followed by an invasion of the sea, for the tillite is 

 succeeded bv thousands of feet of quartzite, arkose, and 

 conglomerate, and by a few hundred feet of limestone. 

 Possibly much or all of the limestones of the GrenviUe 

 and Hastings series, which Dr. ..\dams reckons amon" 

 the great limestone formations of the world, were formed 

 at about the same time. 



The Middle Huronian (Logan's Upper Huronian") is 

 separated by a basal conglomerate, possibly, glacial, from 

 1 ".Bur. Mines, Or.t.", 19c 5,p. 3'- 



NO. 2133. VOL. 84] 



