ALPINK SCULPTURE. 185 



In ascending one of the larger valleys, we enter it where 

 it is wide and where the eminences are gentle on either 

 side. The flanking mountains become higher and more 

 abrupt as we ascend, and at length we reach a place where 

 the depth of the valley is a maximum. Continuing our 

 walk upward, we find ourselves flanked by gentler slopes, 

 and finally emerge from the valley and reach the summit 

 of an open col, or depression in the chain of mountains. 

 This is the common character of the large valleys. Cross- 

 ing the col, we descend along the opposite slope of the 

 chain, and through the same series of appearances in the 

 reverse order. If the valleys on both sides of the col were 

 produced by fissures, what prevents the fissure from pro- 

 longing itself across the col? The case here cited is 

 representative; and I am not acquainted with a single 

 instance in the Alps where the chain has been cracked in 

 the manner indicated. The cols are simply depressions, 

 in many of which the uufissured 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 

 plateaus in the region of cols the power is not fully 

 developed; but lower down tributaries unite, erosion is 

 carried on with increased vigor, and the excavation gradu- 

 ally 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 produced in the earlier portions of its course 

 entirely disappear. 



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," * 



* Phil. Mag. vol. xxiv. p. 169. 



