GEOLOGICAL SEASONS. 197 



Period to have terminated 720,000 years ago, the denuda- 

 tion in the intervening time must have amounted to 120 

 feet, which Mr. Croll thinks would imply the removal of 

 all the detrital deposits of the continental glacier. They 

 have not been thus removed, and consequently 720,000 

 years is too high a figure. If we assume the Glacial 

 Period to have terminated 80,000 years ago, then 13 feet 

 of rock, or 18 feet of drift, must have been removed from 

 the whole face of the continents ; and this, according to 

 good authority, is all that has been done. My own judg- 

 ment of the evidences is that the rate of denudation is 

 greater than has been assumed ; and hence I must con- 

 sider 80,000 years as abundantly adequate for all the 

 post-glacial erosions. This opinion is confirmed by what 

 we have observed of changes in progress before our eyes; 

 in the recession of glaciers, the transportation of soils, 

 the filling of lakes, and the shifting of river-channels, as 

 well as in the disappearing relics of the continental 

 glaciers, hidden in mountain gulches and rocky crevices, 

 or slowly wasting beneath accumulations of common 

 drift.* 



I stated that northern oscillations of level are to be 

 regarded rather as consequences than as causes of north- 

 ern glaciation; and I have alluded already to a submer- 

 gence of northern lands as an accompaniment of the next 

 general glaciation of the north temperate zone. Let us 

 return to this for a moment. The formation of an ex- 

 tensive ice-cap about either pole, and its relative diminu- 

 tion about the other, must have a tendency to displace 

 the earth's center of gravity toward the loaded pole. 

 Beneath a film of water free to adjust itself as the ocean 



*The author has more fully considered this subject in Preadamites, ch. xxvii. 



