Lakes. 435 



the mountains on each side of them are in their present 

 forms mountains, far less because of denudation, than 

 by reason of operations of fracture and dislocation. For 

 clear demonstrations of such assertions none are given, 

 and I now propose to give a resume of the reasons as 

 originally published by me and since confirmed by 

 others, which show, how it happened that certain rock- 

 bound hollows were scooped out by the agency of glacier- 

 ice. In doing so, I shall briefly go into other subjects 

 than those involved in questions of mere movements 

 of the earth's crust. 



In the first place, consider what is the effect of 

 marine denudation. On the sea-shore, where waves 

 are always breaking, the effect of this, and of the 

 weathering of cliffs that rise above the waves, is to 

 waste back the land. But the sea in this case cannot 

 make a deep hollow below its own average level. 

 What it might do, if there were hollows there, would 

 be to fill them with detritus, for it cannot cut them 

 out. The consequence is, that the chief power of the 

 sea and the weather combined, working on the land 

 and wasting it back, is to act as a great planing 

 machine, wearing down the larger irregularities that 

 rise above its level in the manner shown in the descrip- 

 tion of the first denudation of the Weald at page 343, 

 and of South Wales at page 497, so as in the end to 

 form a plain of marine denudation. 



Again, what is the effect in any country of running 

 water ? Rivers cannot make large basin-shaped hol- 

 lows surrounded by rocks on all sides. All that 

 running water can do upon the surface is to scoop 

 out trenches or channels of greater or less width, form- 

 ing gorges or wider valleys, according to the nature 

 of the rivers and the rocks, and the time employed 



F F 2 



