IVA VE-LIKE PR O GRESS OF AN EPEIR O GENIC UPLIFT. 395 



remains to be briefly noticed. Between the epochs of mountain- 

 building by plication, the diminution of the earth's mass pro- 

 duces epeirogenic distortion of the crust, by the elevation of 

 certain large areas and the depression of others ; and these 

 effects have been greatest just before relief has been given by 

 the formation of folded mountain ranges. Two epochs have 

 been preeminently distinguished by extensive mountain plication, 

 one occurring at the close of the Paleozoic era, and the other 

 progressing through the Tertiary and culminating in the Quater- 

 nary era, introducing the Ice age. During the last, besides pli- 

 cation and overthrust faulting of the Coast range, the St. Elias 

 range, the Alps, and the Himalayas, a very extraordinary devel- 

 opment of tilted mountain ranges, and outpouring of lavas on an 

 almost unprecedented scale, have taken place in the Great Basin 

 and the region crossed by the Snake and Columbia rivers. With 

 the culminations of both of these great epochs of mountain- 

 building, so widely separated by the Mesozoic and Tertiary eras, 

 glaciation has been remarkably associated, and indeed the ice 

 accumulation appears to have been caused by the epeirogenic 

 and orogenic uplifts of continental plateaus and mountain ranges. 

 These processes are well consistent with Dana's doctrine of the 

 general permanence of the continents and oceanic basins ; for 

 upheaval of an ocean bed would not diminish but increase the 

 earth's volume. The late glacial and postglacial uplift of North 

 America from its Champlain depression, by the wave-like move- 

 ment which has been here described, seems an effort of the earth 

 to regain the state of isostasy, or flotation of the crust on the 

 heavier mobile interior, which is capable of flow, whether it be 

 solid or molten. 



Warren Upham. 



