CAUSES, STAGES, AJVJD TIME OF THE ICE AGE. 36 \ 



ebbles from near the sources of the Mississippi to the shore of 

 he Gulf of Mexico, Prof. E. W. Hilgard thinks that the pregla- 

 ial uplift, inaugurating the Ice age, was four thousand or five 

 housand feet more in the central part of the continent than at 

 this river's mouth. 



Although the adequacy of the preglacial epeirogenic elevation 

 of this continent to produce its Pleistocene ice sheet was tardily 

 recognized, it was distinctly claimed by Dana in 1870 that the 

 Champlain subsidence of the land beneath its ice load, supposing 

 it to have been previously at a high altitude, must have brought 

 climatic conditions under which the ice would very rapidly dis- 

 appear. The depression would be like coming from Greenland to 

 southern Canada and New England. In Prof. Dana's words: 

 " Such an extended change of climate over the glacier area was 

 equivalent in effect to a transfer from a cold, icy region to that of 

 temperate climate and melting sun. The melting would there- 

 ore have gone forward over vast surfaces at once, wide in lati- 

 ude as well as longitude." 



Such explanations as these, accounting for the gradual accu- 

 mulation and comparatively rapid dissolution of the North 

 American ice sheet, are also found to be applicable to the ice 

 sheets of other regions. The fiords of the northern portions of 

 the British Isles and of Scandinavia show that the drift-bearing 

 northwestern part of Europe stood in preglacial time one thou- 

 sand to four thousand feet higher than now ; while, on the other 

 hand, late glacial marine beds and strand lines of sea erosion 

 testify that when the ice disappeared the land on which it had 

 lain was depressed one hundred to six hundred feet below its 

 present height, or nearly to the same amount as the Champlain 

 depression in North America. Mr. T. F. Jamieson appears to 

 have been the first in Great Britain or Europe to attribute the 

 ice accumulation to altitude of the land, and to hold the view 

 (which I receive from him) that the submergence of glaciated 

 lands, when they were loaded with ice, was caused directly by 

 this load pressing down the earth's crust upon its fused inte- 

 rior, and that the subsequent re-elevation was a hydrostatic up- 

 lifting of the crust by underflow of the inner mass when the ice 

 was melted away. Just the same evidences of abundant and deep 

 fiords and of marine beds overlying the glacial drift to heights 

 of several hundred feet above the sea are found in Patagonia, as 

 described by Darwin and Agassiz. On these three continental 

 areas the widely separated chief drift-bearing regions of the earth 

 are found to have experienced in connection with their glaciation 

 in each case three great epeirogenic movements of similar char- 

 acter and sequence first, a comparatively long-continued uplift, 

 which in its culmination appears to have given a high plateau 



