366 W. H. Niles—Erosion of Valleys, 
Art. XLV.—Upon the Relative Agency of Glaciers and Sub- 
Glacial Streams in the Erosion of Valleys ;* by Professor W. 
H. NILEs. 
particularly upon the right side of the Great Aletsch Glacier 
where it way 
the inequalities of surface, has been made so well known that 
additional description is unnecessary here. Under these condi- 
tions the glacier does not act upon the lowest surfaces of rock 
beneath it, and these show by their roughness and irregularity 
that they were not shaped by its action. It, therefore, become 
evident that in such places some power must have acted or 18 
now at work lower than the surfaces upon which the glacier 
oves. 
Under the edge of the Great Aletsch Glacier I observed in a 
few places, that pieces were being broken from the lee edges of 
Y 
the roches moutonnées by the pressure concentrated upon certain 
as the strike of the rock and nearly parallel with the direction 
of the motion of the glacier. A longitudinal section of one of 
these ridges gave an outline like that of an elongated roche 
moutonné, while a transverse section showed quite a regularly 
P See Societ , ol. xix, 
pp. 330-336, March 20, rs. ee sok Bact 
+ Proceedings of the Boston Society of Natural History, vol. xv, pp. 378-381. 
