26 I Prof. Tyndall on the Conformation of the Alps, 



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 ; and in the case of many of them the unfis- 

 sured rock can be traced from side to side. 



The typical instance just sketched follows as a natural conse- 

 quence from the theory of erosion. Before either ice or water 

 can exert great power as an erosive agent, it must collect in suf- 

 ficient 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 vigour, 

 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 

 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 frac- 

 ture 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 u 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, unassail- 

 able; but whether the opinion fairly follows from the facts may 

 be regarded as an open question. The arguments which have 

 been thus far urged against the opinion appear to me to be far 

 from conclusive. Indeed the idea of glacier erosion appeared so 

 daring that its boldness was deemed by many its sufficient refu- 

 tation. It is, however, to be remembered that a precisely similar 

 position was taken up by many respectable people when the 

 extension of ancient glaciers was first mooted. The idea was 

 considered too hardy to be entertained; and the evidences of 

 glacial action were sought to be explained by reference to almost 

 any process rather than the true one. Let those who so wisely 

 took the side of " boldness " in that discussion beware lest they 

 place themselves, with reference to the question of glacier erosion, 

 in the position formerly occupied by their opponents. Looking at 

 the little glaciers of the present day — mere pigmies as compared to 

 the giants of the glacial epoch — we find that from every one of 

 them issues a river more or less voluminous, charged with the 

 matter which the ice has rubbed from the rocks. Where the 

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



