134 THK ICE AGE IN CANADA. 



eitlier slow and ^radnal or by paroxysms, as the weiglit 

 increases beyond the limit of the rigidity of the outer 

 crust. Hence, a great weight of ice placed oi. 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 imywrtant, must ever con- 

 stitute a subsequent ami incidcMital effect of glacial 

 accumulations proceeding from other causes. In this 

 connection it must also be o])served that different portions 

 of the crust must be of unequal tliickness and hardness, 

 and supported on material of different degrees of mol)ility; 

 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 lieen, and are 

 still being, weighed down by superficial accumulations, or 

 arc being buoyed up by the removal of matter l)y denudation 

 or by the lateral ])ressure under tliem of the subterranean 

 forces of the earth, wiiile locallv such effects aie 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 tlie causes which could have 

 led to such climatal conditions as those to wliicii we have 

 referred ; and here, however unreasonal)le this may a])pear 

 to some, I am disposed to content myself witli 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 



