600 TRANSACTIONS 0£ SECTION C. 



The Devonian Sea also encroached south of James Bay and along the south- 

 west side of the shield from Clear Lake, in Saskatchewan, to Great Bear Lake. 



What took place on the Archaean continent while the coal forests flourished on 

 the lowlands to the south and to the far north is unknown, since no carboniferous 

 rock has been found on its surface. 



Mesozoic and Cenozoic History. 



Early Mesozoic times are a blank, but a few small outcrops of Cretaceous 

 rocks resting on the Archaean toward the south-west show that portions of its 

 rim were once more under water. Dr. Wilson believes that an important facet 

 of the peneplain should be dated from the Cretaceous, since planation was going 

 on in parts of the United States at this time; but no positive evidence of this 

 is at hand. 



Nor is there any evidence as to its history in the tertiary before the oncoming 

 of the Ice Age of the Pleistocene, when its whole surface was scoured more than 

 once by great glacial sheets. The mantle of decayed rock which must have accumu- 

 lated during the long dry land stage was almost completely swept away, leaving 

 the rounded surfaces of ancient rock fresh and clean beneath the boulder clay. 



In an important inter-glacial interval and in post-glacial times much of the 

 morainic material was assorted in great lakes whose shore and deep water 

 deposits cover large parts of the surface. With the departure of the ice the sea 

 once more transgressed upon the lower parts of the shield, but the land has been 

 rising since, leaving a belt of marine deposits up to about 500 feet around the 

 shores of Hudson Bay, the St. Lawrence, and the Atlantic. 



How much of the Shield has been Covered ? 



It is generally stated that the Canadian Shield has been dry land since the 

 Archaean, and hence that erosion has been taking place ever since that time. 

 This is probably true for part of the north-eastern portion of the shield and 

 perhaps also the north-western, hut much of the area, especially toward the south, 

 was buried in early days under Palaeozoic sedimentary rocks, and so protected 

 from further destruction. These sediments are still being slowly stripped from 

 the Archaean in many places. 



This may account for the greater proportion of Huronian and Keewatin rocks 

 in the south as compared with the north. It is probable that in the unprotected 

 northern parts weathering agencies have eaten the higher Archaean rocks com- 

 pletely away from the Laurentian gneiss beneath. Before asserting this positively, 

 however, it may be well to await more thorough exploration of the little known 

 north. 



It is possible, but not very probable, that the whole area was at one time 

 covered with Ordovician or Silurian shale and limestone. If so, all traces of 

 this capping have been removed from hundreds of thousands of square miles of 

 its surface. 



There is one very impressive feature of the Archaean as found beneath the 

 later rocks. The peneplain, with its rounded, hummocky surface, seems exactly 

 the same when one strips from it recent boulder clay, early Palaeozoic shale or 

 sandstone or limestone, Keweenawan eruptives, or even Lower Huronian tillite. 

 where this has remained undisturbed. It is as though all the millions of years of 

 destruction since the Middle Palaeozoic had made only unimportant changes in 

 the pre-Cambrian peneplain. When it is recalled that peneplanation took place 

 twice in the pre-Cambrian, before the Lower Huronian and before the Animikie, 

 one is almost driven to think that pre-Cambrian time is far longer than post- 

 Cambrian. 



Relation of the Shield to the Palaeozoic. 



Except toward the east, the Canadian Shield sinks gently beneath Palaeozoic 

 beds, in most cases retaining its character as a peneplain. How far does it 

 continue to the south and west beneath the sedimentary rocks, and to what depth 

 does it extend 1 l 



