MODIFICATION OF THEORY EXAMINED. 125 



chief causes which led to the country being so deeply 

 covered with ice. In fact, had it not been for some 

 such conservative power in the ice, a glacial epoch 

 resulting from the causes which I have been advocating 

 would not have been possible. This conservative 

 tendency certainly renders it more difficult for the 

 physical agencies to get rid of the ice during inter- 

 glacial periods; but we evidently have no grounds for 

 assuming that it will defy their melting-powers. 



