WHY THE GLACIER MOVES. 173 



find, in immediate contact with those solid materials, more or 

 less of the loose materials of the glacier wall, sometimes five, 

 ten, fifteen or twenty feet in thickness, sometimes as thick as 

 one hundred, two hundred, three hundred feet. The whole 

 prairie is nothing but a glacier bottom. I have spent, this 

 summer, several months in the exploration of that country, 

 with the special object of ascertaining the character of the loose 

 material constituting the Western prairies, and I have satisfied 

 myself that from the Rocky Mountains to the Atlantic Ocean 

 the whole mass of loose materials, sometimes hundreds of feet 

 in thickness, is nothing but an accumulation of glacier wall 

 materials. The whole of that has been ground by the ice 

 rasp, and we owe, therefore, these loose materials, which are 

 the basis of all our agricultural operations, to the work of 

 immense sheets of ice. 



Tliere was one difficulty in accounting for the motion of such 

 a sheet of ice over a flat surface, for from the North Pole to the 

 Gulf of Mexico we have not a natural slope. When I first 

 affirmed that the drift of North America was the result of the 

 working of ice, geologists thought it could not be, and did not 

 adopt my conclusion. They denied it on theoretical grounds. 

 They would recognize the similarity of the accumulations to 

 those observed under the glacier ; they would recognize the sim- 

 ilarity of the polished surfaces to those which are now observed 

 under the glacier ; but they said that glaciers could not move 

 over a level surface, and that therefore some other still unknown 

 cause must have been the agency which produced this result. 



Now is the motion of ice the result of a slope ? I say, no. I 

 say that ice moves, not in consequence of the inclination of the 

 surface on which it rests, but owing to the climatic condition. 

 The slope of the mountain valleys no doubt accelerates the rate 

 of the onward movement of the glaciers, but were it not for the 

 penetration of the snow by water, were it not for the transfor- 

 mation of the snow into ice, and for the constant dilatation of 

 that ice in consequence of the freezing of the water which pen- 

 etrates it, there would be no other motion than subsidence, and 

 therefore even there we should not have the results which we 

 observe. The motion of ice is essentially a climatic and mete- 

 orologic action, and not the mere action of gravitation ; and if 

 we now consider the fact that in different latitudes we have dif- 



