ALPINE SCULPTURE 257 



of which the unfissured rock can be traced from side to 

 side. 



The typical instance just sketched follows as a natural 

 consequence from the theory of erosion. Before either ice 

 or water can exert great power as an erosive agent, it must 

 collect in sufficient mass. On the higher slopes and pla- 

 teaus in the region of cols the power is not fully devel- 

 oped; but lower down tributaries unite, erosion is carried 

 on with increased vigor, and the excavation gradually 

 reaches a maximum. Lower still the elevations diminish 

 and the slopes become more gentle; the cutting power 

 gradually relaxes, until finally the eroding agent quits the 

 mountains altogether, and the grand effects which it pro- 

 duced in the earlier portions of its course entirely dis- 

 appear. 



I have hitherto confined myself to the consideration 

 of the broad question of the erosion theory as compared 

 with the fracture theory ; and all that I have been able to 

 observe and think with reference to the subject leads me 

 to adopt the former. Under the term erosion I include 

 the action of water, of ice, and of the atmosphere, including 

 frost and rain. Water and ice, however, are the principal 

 agents, and which of these two has produced the greatest 

 effect it is perhaps impossible to say. Two years ago I 

 wrote a brief note "On the Conformation of the Alps," ' 

 in which I ascribed the paramount influence to glaciers. 

 The facts on which that opinion was founded are, I think, 

 unassailable; but whether the conclusion then announced 

 fairly follows from the facts is, I confess, an open ques- 

 tion. 



The arguments which have been thus far urged against 



1 PhiL Mag., vol. xxiv. p. 169. 



