TERMINAL MORAINES. 225 



The main evidence of the age of these deposits will come 

 up for discussion when we consider the date of the glacial 

 period in general and of its various episodes. (See chapter XX, 

 pp. 580-592). 



But it will be in place to give expression at this point to 

 some cautions against premature judgments respecting the 

 evidence. It is important to keep in mind (1) the fact that 

 there is no valid objection to the supposition that geological 

 changes proceed much more rapidly at some periods than at 

 others. Indeed it is a fundamental doctrine of evolution 

 that cumulative strains in the earth's crust may go on un- 

 noticed for an indefinite period until the limit of resistance 

 is reached, when there will be a rapid readjustment which 

 may well deserve the name of catastrophe. Preglacial 

 elevation proceeded all through the latter portion of the 

 Tertiary Period until the snows of the Glacial Period pro- 

 duced the ice age, when the very weight of the ice facilitated 

 the subsequent depression and the rapid return to the ocean 

 of the water that had been locked up in the continental 

 glaciers. In this accumulation of ice over the northern 

 hemisphere and its subsequent return to the ocean there is 

 brought to light a force of such incalculable power affecting 

 the elevation and depression of the land that the effects are 

 entirely abnormal to our present experience. We cannot 

 reason back from the rate of present changes to that of 

 the Glacial Period. 



(2) Again, we are never at liberty to lose sight of the fact 

 that a long period of disintegration of the rocks over the gla- 

 ciated region preceeded the ice age. The rocks over that 

 region were then rotted to a great depth as they are now in 

 the region south of the ice limit. This furnished an immense 

 amount of oxidized material for the first grist of the glacial 

 mill, and that was spread widely over the southern margin 

 of the glaciated area. 



