80 THE ICE AGE IN CANADA. 



snow and ice o" the latter. lUit if tlie pressure of sucli 

 snow and ice was sullicient to depress the lulls, it nuist 

 necessarily at the sunie time elevate the plains, and this 

 change, by diminishing evaporation and by increasing 

 continental warmth, would at once cause the ice-caps to 

 melt away. Thus subsidence pro(hiced by accunndations 

 of ice would at once accomplish the destruction of such 

 accumulations, while it would remove the high lands 

 necessary to account for any extensive movement of 

 glacial ice. In other words, as elsewhere urged in this 

 volume, the facts of dynamical geology and physical 

 geography are fatal to hypotheses of polar ice-caps and 

 continental ice-sheets, and if one were to admit all that 

 has been alleged in reference to depression of land by 

 l)ressure of ice, these ditliculties would not be removed in 

 the slightest degree. 



//. — Causes of Glaciatioa and Distribation 

 of Erratics. 



We now come to consider the probal)le causes of 

 glaciation and boulder-dei)osits, and first the agenc) of 

 " continental " and local glaciers. 



1.— GLACIER ACTION, ETC. 



1. With res})ect to the agency of land ice, I have no 

 hesitation in saying that, as I have maintained for thirty 

 years, a sheet of ice covering the wide surface of the 

 American continent, and pilhig up a " moraine " such as 

 that which extends from the northern end of the Missouri 

 coteau and south of the great lakes to the Atlantic coast, 

 is a physical impossibility. It is so, firsi, because the 

 only possible gathering-ground of sullicient snow to form 

 glaciers is on high lands sufliciently cold and sufficiently 



