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The Western Rocky Mountains. — In contrast to the 

 outer or eastern ranges, the mountains near the watershed 

 stand in isolated peaks, carved from a large block of older 

 rocks. Less folding and fewer faults occur. More massive 

 bases and higher summits, to which cling many glaciers, 

 give a more Alpine aspect to the scenery. Small cirques, 

 such as the gap called White Man's pass at Canmore, 

 in the outer mountains, give place in the inner ranges 

 to great amphitheatres such as those in the vicinity of 

 Laggan. 



The first appearance of this part of the Rocky Moun- 

 tain system may have occurred shortly after the Jurassic 

 sediments were laid down, and then only as low ranges. 

 Periods of subsidence and elevation may have followed 

 and recurred throughout Cretaceous times. 



HISTORICAL GEOLOGY. 



The wide depression, in which the sediments of the 

 central part of the continent were deposited, was at its 

 greatest marginal extent probably in Devonian time. 

 Earlier deposits appear on the southeastern margin and 

 again in the mountains to the west. 



A great series of ancient sediments, some probably 

 Pre-Cambrian in age, is found in the area occupied by the 

 western part of the Rocky mountains and the adjoining 

 ranges to the west. This thick series shows in its upper 

 part the existence of marine conditions during Cambrian 

 time. The downwarp, which was here partly filled by 

 coarse sediments, may not have extended far to the east 

 from the continental margin of that time, and was probably 

 separated from the main ocean by a barrier. A general 

 subsidence before the close of the Cambrian is indicated 

 by patches of rocks of this age on other parts of the con- 

 tinental area. Deep sea deposits, magnesian limestones 

 of the Castle Mountain series, were formed before the 

 recovery of elevation which closed the period. 



Considering only the area east of the Rocky mountains, 

 it is not clear, that during Ordovician time, the marine 

 invasion shown by the character of the sediments at the 

 top of the Castle Mountain series was other than by an 

 arm of the Pacific. In the eastern part the invasion from 

 the south in early Trenton time is marked by the deposition 



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