REVIEWS 569 



that portion immediately below the Unter-Aar glacier, is a familiar 

 and striking example. 



Although these phenomena have long been noticed and some 

 appreciation of their signification has been felt, it remained for a mas- 

 ter in modern physiography to see and to set forth their fuller mean- 

 ing. It is assumed that in the pre-glacial times the tributaries joined 

 the trunk streams in the normal way with well-adjusted gradients, and 

 that the discordance now shown is the result of the superior erosion of 

 the trunk valleys by the glacial tongues that occupied them. The 

 amount of the discordance is, therefore, taken as a rough measure of 

 the superior erosive efficiency of the glaciers. In the Alps this is 

 recorded in hundreds of feet, and in Norway it reaches into the lower 

 thousands, and gives an impressive illustration of the erosive power 

 of glaciers. 



There are, however, several qualifications to be applied to this 

 rough measure, and these are rather more complicated, and perhaps 

 more important, than one might apprehend from reading the paper, 

 though they do not seriously affect its method or its conclusions. In 

 the broadening of the main valley by glacial erosion the mouths of 

 the tributaries were cut back and the present points of intersection lie 

 at higher levels than the original axis of the main valley. This is 

 theoretically restored by projecting the gradient lines of the tributaries 

 till they meet in the center of the mam valley. For the main pur- 

 poses of a general view, such as is sought by the paper, it is, doubt- 

 less, sufficiently near the truth to project the present lines of the 

 tributary valleys without modification, but in stricter studies it is neces- 

 sary to recognize the changes which the tributary valleys have under- 

 gone while the main valleys were being deepened by glacial erosion. 



If unobstructed erosion was in progress in the tributary valleys 

 while the main ones were being excavated by glaciers, the discordance 

 between the two at the close would measure the difference in the rates 

 of erosion of glaciers and of ordinary agencies, and a plus correction 

 would be necessary to secure the absolute glacial erosion. During a 

 part of the glacial period the tributary valleys were smothered in a 

 general mantle of ice and suffered glacial modification as well as the 

 main valleys, but not in just the same way. The main valleys lay in 

 the chief direction of ice movement, for they determined it. The 

 tributaries in the main lay more or less athwart the ice movement. 

 They must hence be presumed to have suffered more or less of rasping 



