14 THE ICE AGE IN CANADA. 



the snow-line ; and that cases in which, in such latitudes, 

 glaciers extend nearly to the sea-level, occur only where 

 the mean temperature is reduced by cold ocean currents 

 approacliing to high land, as for instance in Terra del 

 Fuego and the southern extremity of South America. 

 But the temperate regions of North America could not be 

 covered with a permanent mantle of ice under the existing 

 conditions of solar radiation ; for even if the whole were 

 elevated into a table-land, its breadth would secure a 

 sufficient summer heat to melt away the ice, except from 

 high mountain peaks. 



" 2. It seems physically impossible that a sheet of ice, 

 such as that supposed, could move over an uneven 

 surface, striating it in directions uniform over vast areas, 

 and often different from the present inclinations of the 

 surface. Glacier-ice may move on very slight slopes, but 

 it must follow these [since gravitation, along with the 

 more or less plastic nature of the ice, has been shown to 

 be the cause of its motion] ; and the only result of the 

 immense accumulation of ice supposed, would be to 

 prevent motion altogether by the want of slope or the 

 counteraction of opposing slopes, or to induce a slight and 

 irregular motion toward the margins or outward from the 

 more prominent protuberances. 



" It is to be observed, also, that, as Hopkins has shown, 

 it is only the sliding motion of glaciers that can polish or 

 erode surfaces, and that any internal changes resulting 

 from the mere weight of a thick mass of ice resting on a 

 level surface, could have little or no influence in this way. 



" 3. The transport of boulders to great distances, and 

 the lodgment of them on hill-tops, could not have been 

 occasioned by glaciers. These carry downward the blocks 

 that fall on them from wasting cliffs. But the universal 



