368 '  W. H. Niles—Krosion of Valleys. 
If this is granted, it must then be admitted that the abrading 
power of glaciers is much less than if the fragments of rock 
were usually firmly set in the ice. This is one of the many 
reasons which I have for believing that the erosive power of 
laciers is not sufficient, in itself alone, to account for the ex- 
cavation of those valleys in which they are found. 
mong the phenomena which attract the attention and ob- 
struct the progress of the explorer under a glacier, is the 
abundance of streams. A short distance below the edge of the 
glacier the ice is constantly melting, and in every place acces- 
sible to the observer the water falls, usually in large drops but 
sometimes in streamlets. Thus the surfaces not covered by the 
ice are exposed to a constant fall of water, which, first forming 
numerous rivulets, soon collects in small and rapid streams. 
The dropping of the water and the rushing of the torrents, the 
frequent slipping of smaller fragments of stone which have been 
started by the rivulets and the occasional tumbling or plunging 
of a larger mass, the incidental cracking of the glacier and the 
frequent crash of pieces of falling ice, all unite in impressing 
upon the listener that this is a busy place. Where the glaciers 
rest upon the upper portions of the roches moutonnées, the streams 
are formed in the hollows between them which the ice does not 
fill; therefore, under such conditions their erosive power 1s 
exercised upon those lower portions of the rock-surface which 
are not effected by the movements of the glacier. 
In estimating the erosive power ofa stream we must take 
into consideration, not only its volume and velocity, but also 
the more important factor of the materials with which it 1s 
charged. The importance of this is well illustrated by the 
modern appliance called the sand-blast, in which it is not the 
violence of the current of air or steam but the sand which it 
d’Argentiére. I do not learn from what he has written, however, that he saw 
the ice flowing over stones in the manner I have here described. It is, therefore, 
my pleasure to have witnessed what I consider to be a proof of the accuracy of 
the conclusion which Professor Bonney ably drew from other sources. 
