370 W. H. Niles—FErosion of Valleys. 
of glacial valleys so often have. If it is objected that such 
water-worn surfaces are rarely met with upon the sides of val- 
leys from which the glaciers have retreated, it must be remem- 
bered that the ice above the streams and the atmospheric 
agencies modify these surfaces after they have been left by the 
streams, hence the rocks have the features which they received 
from the last agent which acted upon them. 
Still lower and quite underneath the side of the glacier there 
are larger and often much longer lateral streams, which are 
much more important agents in the excavation and formation 
of the valleys. These, flowing in channels of their own forma- 
tion in the rock and quite below the ice, tend to deepen the 
valley along its edges and to give it that cafion-like form so 
often seen. 
[I have not space in the narrow limits of this article to consider the valuable 
d dingl tributions of others to the subject of glacial action-] 
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