CLASSIFICATION OF MOUNTAIN RANGES. 673 



many ranges in various parts of the world have been formed 

 in this way, their sites being now marked by profound faults 

 which are clearly traceable, though the tilted mountains of the 

 upheaved side of- the faults have long since passed through youth, 

 maturity, and old age, leaving no topographic evidence of their 

 former existence. 



Many stages of mountain-building have left their impress on 

 the great Cordilleran belt of the western part of the United States 

 and the Dominion of Canada. The Gold and Selkirk ranges of 

 British Columbia, according to Dr. George M. Dawson, consist 

 of Archa3an, Cambrian, and Silurian formations, which were 

 pushed up into mountain folds before the close of these very 

 ancient divisions of geologic time. The auriferous slates of the 

 Sierra Nevada, as Becker has shown, were similarly built up in a 

 folded mountain range at the close of the Gault epoch in the 

 Cretaceous period. During the ensuing long lapse of time to the 

 end of the Tertiary era, this precursor of the Sierra Nevada range 

 had been worn down to only a moderate elevation by the gnaw- 

 ing frosts, heat, rains, and running streams ; but the beginning 

 of the Quaternary era, according to Le Conte and Diller, brought 

 revolutionary changes. The previously base-leveled region which 

 now forms the Great Basin was then upheaved as a high plateau ; 

 intense volcanic activity was manifested in many parts of this 

 area, and especially from the vicinity of Lassen Peak and Mount 

 Shasta northward to the Columbia River and eastward along the 

 Snake River to the Yellowstone National Park ; and long faults, 

 running mostly from north to south, divided the distended region 

 into a multitude of orographic blocks, which, being soon allowed 

 to sink, became tilted in their subsidence and form the present 

 Basin ranges. 



If we attempt to correlate these events with the Quaternary 

 glaciation of the northern part of our continent, they seem to 

 have been contemporaneous with the maximum extension of the 

 ice-sheet of the first Glacial epoch. The ice accumulation I have 

 attributed, on evidence derived from fiords and from river-chan- 

 nels now deeply submerged by the sea, to former great elevation 

 of the glaciated areas, probably three thousand to four thousand 

 feet higher than now. But the glacial and modified drift show 

 that toward the end of each of our two principal Glacial epochs 

 the land on which the ice lay was depressed nearly to its present 

 level or in part lower. This depression of the earth's crust I 

 believe to have been caused by the vast weight of the ice-sheets ; 

 and, in the first Glacial epoch, we have the correlative somewhat 

 sudden elevation of a contiguous area, with outpouring of lava 

 and formation of tilted mountain ranges, in the Great Basin and 

 north to the Columbia. During the long interglacial epoch very 



TOL. XXXIX. 49 



