ALPINE 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 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 
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. j>. 169. 
