320 PROF. W. MORRIS DAVIS OX [Aug. I9°95 



the ice, than to ascribe it to slender, light, nimble water-streams 

 in association with weathering on so much of the surface as rises 

 above the water. But, after all is said in favour of glacial erosion, 

 it is still difficult to understand how cwms are excavated and 

 enlarged under the glacier-heads, and how channels are deepened 

 and widened under glaciers. Here we can only say, as was said 

 on closing the consideration of the alternative theory, that if, in 

 the face of these difficulties, the theory that glaciers are destructive 

 should lead to consequences which are supported by facts, we 

 should necessarily accept this theory as adequate, even though some 

 of its processes are beyond present explanation. 



XXII. A Personal Seciion. 



Despite the efforts that I have made to present the alternative 

 theories and their consequences impartially, it may be only too 

 apparent that, in my own mind, the theory that glaciers are 

 protective is essentially unsuccessful, and the theory that glaciers 

 are destructive is, on the whole, eminently successful in leading to 

 consequences that accord with observed facts. This opinion was 

 not reached until after I saw in 1899 that the lateral valleys of 

 the Alps prevailingly hang over the wide open main valleys, and 

 that the piedmont Alpine lakes cannot be reasonably explained by 

 the warping of normal pre-Glacial valleys. Since then, my own 

 observations in Norway, the Tian Shan, and the Rocky Mountains 

 have only served to confirm the opinion gained in the Alps ; and at 

 the same time similar evidence has been presented by observers 

 from various glaciated mountains as far separated as Alaska (where 

 Gilbert's studies are most convincing) and Xew Zealand (where the 

 discussion by Andrews is no less unanswerable). But, however fully 

 an author may be convinced of the success of one theory or another, 

 it is always appropriate for him to revise his opinions in the presence 

 of other theories : and that revision I have here tried to make, in 

 view of the acceptance given to the theory of the protective action 

 of glaciers by some geologists whose experience in the glaciated 

 mountains of Europe is far greater than mine. In such a revision, 

 the method here adopted seems logically correct and safer than any 

 other as a means of solving the problem in hand, or any similar 

 problem in which effort is made to determine the nature of invisible 

 facts — invisible because of minuteness, or of remoteness in time or 

 space, or of any other quality. It might, indeed, have been more 

 satisfactory to have the consequences of the theory of the pro- 

 tective action of glaciers set forth by one of those who is convinced 

 of its truth ; but, as I have not been able to find from such a source 

 a thorough attempt to carry the theory to its consequences and to 

 confront the consequences with the facts, I have had to make that 

 attempt in a more or less prejudiced manner myself. 



Let me here emphasize a matter to which, as it would seem,. 



