•304 PEOF. W. MOEEIS DAVIS ON [-^- u o' I 9°9» 



It has been mejitioned above tbat examples of young banging 

 lateral valleys in normal relation to narrow young main valleys are 

 well known, though rare, in non-glaciated regions; but it is an 

 unbroken rule that lateral valleys and maturely open main valleys 

 in non-glaciated regions come together at accordant levels. It 

 would, therefore, seem hardly permissible to believe in the pro- 

 duction of hanging lateral valleys over broadly open main valleys 

 in glaciated mountains as a result of normal erosion, until some 

 examples of such a hanging relation are pointed out in maturely 

 sculptured, non-glaciated mountains. Indeed, if the association of 

 hanging valleys with glaciated mountains is doubted, and if the 

 occurrence of hanging valleys is explained by normal processes as 

 above indicated, then the whole discussion is transferred from the 

 problem of glacial action to the problem of stream-action ; and as 

 such it must be settled by the study of normally sculptured 

 districts, not by the study of glaciated districts. No study of a 

 non-glaciated district has yet shown it to be characterized by 

 hanging valleys ; but numerous studies attest the prevalence of 

 accordant valley-junctions in such districts. Hence, in the entire 

 absence of positive evidence that hanging lateral valleys are 

 associated with wide-open main valleys in non-glaciated mountains, 

 it seems reasonable to regard the abundant evidence as to the 

 accordant relation of lateral and main valleys in such districts as 

 settling the case, and to conclude that hanging valleys are (except 

 in connexion with narrow gorges of young main rivers) to be 

 explained in some way in connexion with glaciation. 



It is, however, not so much the occurrence of any one abnormal 

 feature, such as hanging lateral valleys, in glaciated mountains as 

 it is the systematic combination of various abnormal features that 

 is so strongly persuasive of the dependence of such forms in some 

 way or other on glacial action. There is, indeed, to-day a general 

 agreement among geologists as to the association of abnormal 

 features, like cwms and hanging valleys, with glaciation. A study 

 of Penck & Bruckner's ' Die Alpen im Eiszeitalter ' would be 

 pertinent in this connexion. But there is still some disagreement as 

 to the manner in which the abnormal features have been produced. 



XVII. Vaeiotjs Methods of Discussing Glacial Eeosion. 



The possibility that the abnormal features of glaciated mountains 

 might be the result of glacial erosion has been much discussed. 

 The chief methods of discussion are as follows : — 



(a) Observation of existing glaciers. — It has been 

 sought to determine, by the direct observation of existing glaciers, 

 whether the greater glaciers of the Glacial Period were effective 

 eroding agents. Most observers have been led to the conclusion that 

 existing glaciers slowly grind and scour the rock-surface on which 

 they move : but some have held that the total scouring must be 

 very small. It is, on the other hand, well known that existing 



