84 



of limestones in the Lake Winnipeg basin. A more 

 general submergence during Devonian time is represented 

 by beds of magnesian limestone which are exposed along 

 the foot of the Cretaceous escarpment across Manitoba 

 and in a broad sheet northward down the Mackenzie 

 river and in the Rocky mountains throughout their entire 

 length. The absence of the succeeding Carboniferous 

 deposits in the eastern part of this basin, as well as to the 

 north, suggests a retreat of the sea westward. In the 

 mountain region Carboniferous limestones are prominent 

 in southern Alberta, but northward these thin out and 

 are replaced by sandstones and shales. 



A farther retreat during Permian and Triassic time, 

 during which sandy and shaly deposits were laid down, 

 is indicated in a thin series of these rocks in the mountains. 

 They extend northward to Stewart river in the Yukon, 

 and prove that with the shallowing of the Carboniferous 

 sea there was also transgression northward. 



The crustal disturbances of Jurassic time in British 

 Columbia were reflected in the inauguration of another 

 downwarping movement that produced a narrow trough 

 in the belt now occupied by the Rocky mountains. This 

 permitted the entrance of the sea from the north across 

 northern British Columbia. The deposits carried to this 

 basin in general went to form fine grained black shales. 

 Sandstone members appear in the lower part at intervals, 

 but generally the source of the material is believed to 

 have been at some distance. In northern British Columbia 

 volcanic ash is intercalated with the sediments, and vol- 

 canic outflows are found on what were probably land 

 areas. 



At the close of the Jurassic, sedimentation became 

 periodically rapid. Sands were washed into the basin 

 and the surface elevation was maintained at or near sea 

 level, so that the continental drainage replaced the saline 

 water in the basin. Land areas were maintained for 

 long periods during which coal seams were formed from 

 the vegetation. This period, which is generally ascribed 

 to the Lower Cretaceous, was closed by a general subsid- 

 ence to the east, in which the sea advanced again to cover 

 a large part of the centre of the continent. This invasion 

 of the sea submerged the fresh water deposits of the Dakota 

 in the east and also spread in the central part of the basin 

 similiar sandy beds as basal members of the marine series. 



