PRESIDENTIAL ADDRESS. 595 



Keweenawan 



Unconformity 



Upper (Animikie) 



Unconformity 

 Huronian -i Middle 



Unconformity 

 "■ Lower 

 Unconformity 

 Keewatin 



Eruptive Contact 

 Laurentian 



This compromise system is now generally in use in Canada, though if Canadian 

 relationships alone were considered the Animikie would be separated from the 

 Huronian and placed closer to the Keweenawan, and the Laurentian would be 

 treated as consisting of eruptive rocks frequently later in age than the Lower 

 Huronian. 



The most natural classification for Canada would be as follows : — 



Keweenawan 



Unconformity 

 Animikie 



Great Unconformity 

 •a- ■ i Upper 

 Huroman { Lower 



Great Unconformity 

 Keewatin 

 Laurentian = Post-Keewatin or Post-Huronian granite and gneiss. 



The laccolithic domes described on previous pages were formed partly in the 

 interval between the Keewatin and the Lower Huronian, but mostly later than 

 the Lower Huronian. Over much of the shield, however, our knowledge of the 

 relations is not sufficient to separate the mountain structures of the two ages. 



Let us now consider the history of the region during the successive periods 

 suggested above. 



Conditions during the Keewatin. 



One naturally asks what the conditions were in Keewatin times before the 

 earliest known laccolithic mountains were raised. The granitic texture of the 

 eruptives implies very slow cooling under great pressure. The old interpretation 

 of these rocks, following the usual conception of the nebular hypothesis, made 

 them parts of the 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 

 Grenville 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 attention, but sedimentary 

 rocks occur in it at all levels and with thicknesses of hundreds or thousands of 

 feet. They include Lawson'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 Grenville 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 de- 

 posited on the Keewatin sea bottoms, and one type unlike modem 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 



