596 TRANSACTIONS OF SECTION C. 



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 

 rudiments 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 predominantly 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 processes were at work at the very beginning of 

 known geological time, before the Archaean 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 probably sea- 

 weeds throve in the mud. In the Grenville 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 conti- 

 nents 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 sediments 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 foun- 

 dation on which they were laid down. Though the floor on which they once rested 

 has nowhere been found, one may be certain that its materials included silica, 

 alumina, and alkalies in the right proportions 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 Archaean 

 surface. Possibly the earth was already shrinking 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 mountain rows ; or it may be 

 that some other cause must be sought for the somewhat haphazard 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, ascending to form the wide-based domes of the Lauren- 

 tian 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 as it sank. It is possible that Sederholm's Bottnian in Fin- 

 land may partially fill the gap. 



Whatever disposal was made of the debris, several thousands of feet must 

 have been carved from the mountains 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 



