134 THE ICE AGE IN CANADA. 



either slow and gradual or by paroxysms, as the weight 

 increases beyond the limit of the rigidity of the outer 

 crust. Hence, a great weight of ice placed on mountains 

 or high table-lands must tend to depress them relatively 

 to the plains and sea beds, and the lateral pressure on the 

 under crust may co-operate in raising the latter. Such 

 movements, however, though important, must ever con- 

 stitute a subsequent and incidental effect of glacial 

 accumulations proceeding from other causes. In this 

 connection it must also be observed that different portions 

 of the crust must be of unequal thickness and hardness, 

 and supported on material of different degrees of mobility; 

 and further, 'that there are many fractures in the crust 

 presenting lines of weakness. These differences must 

 materially affect the results of pressure in different 

 localities. 



Movements of the kind above referred to have not ceased. 

 Certain regions have in very recent times been, and are 

 still being, weighed down by superficial accumulations, or 

 are being buoyed up by the removal of matter by denudation 

 or by the lateral pressure under them of the subterranean 

 forces of the earth, while locally such effects are here and 

 there being relieved by igneous eruptions. This is, how- 

 ever, a subject too large to be treated of here. 



///. Climatal Conditions. 



We have now to consider the causes which could have 

 led to such climatal conditions as those to which we have 

 referred ; and here, however unreasonable this may appear 

 to some, I am disposed to content myself with the 

 geographical changes long ago insisted on by Sir C. Lyell. 

 There is the more reason to do this, since the facts 

 established show that great geographical changes actually 



