T 
ACADEMY OF SCIENCES. 4 
nature of glacial erosion. All writers on this subject seem to regard glacial 
erosion as mostly, if not wholly, a grinding and scoring; the débris of this 
erosion as rock-meal; the great boulders which are found in such immense 
quantities in the terminal deposit, as derived wholly from the crumbling cliffs 
above the glacial surface; the rounded boulders, which are often the most 
numerous, as derived in precisely the same way, only they have been engulfed 
by crevasses, or between the sides of the glacier and the bounding wall, and 
thus carried between the moving ice and its rocky bed, as between the upper 
and nether millstone. Ina word, all boulders, whether angular, or rounded, 
are supposed to owe their origin or separation from their parent rock to 
atmospheric agency, and only their transportation and shaping to glacialagency . 
Now, if such be the true view of glacial erosion, evidently its effect in. 
mountain sculpture must be small indeed. Roches moutonneés are recognized 
by all as the most universal and characteristic sign of a glacial bed. Some- 
times these beds are only imperfectly moutonneés, i. e., they are composed 
of broken angular surface with only the points and edges planed off. Now, 
moutonneés surfaces always, and especially angular surfaces with only points 
and edges beveled, show that the erosion by grinding has been only very 
superficial. They show that if the usual view of glacial erosion be correct, 
the great cations, so far from being formed, were only very slightly modified 
by glacialagency. But Iam quite satisfied from my own observations that this 
is not the only nor the principal mode of glacial erosion. I am convinced that 
a glacier, by its enormous pressure and resistless onward movement, is 
constantly breaking off large blocks from its bed and bounding walls. Its 
erosion is not only a grinding and scoring, but also a crushing and breaking. 
It makes by its erosion not only rock-meal, but also large rock-chips. 'Thus, 
a glacier is constantly breaking off blocks and making angular surfaces, and 
then grinding off the angles both of the fragments and the bed, and thus 
forming rounded boulders and moutouneés surfaces. Its erosion is a constant 
process of alternate rough hewing and planing. If the rock be full of fissures, 
and the glacier deep and heavy, the rough hewing so predominates that the 
plane has only time to touch the corners a little before the rock is again 
broken and new angles formed. This is the case high up on the cafon walls, 
at the head of Cascade Lake and Emerald Bay, but also in the cavion beds 
wherever the slate is approached. If, on the other hand, the rock is very hard 
and solid, and the glacier be not very deep and heavy, the planing will pre- 
dominate over the rough hewing, and a smooth, gently billowy surface is the 
result. This is the case in the hard granite forming the beds of all the 
cafions high up, but especially high up the cafion of Fallen Leaf Lake, where 
the cation spreads out, and extensive but comparatively thin snow-sheets have 
been at work. In some cases on the cliffs, subsequent disintegration of a 
glacier-polished surface may have given the appearance of angular surfaces 
with beveled corners; but, in other cases, in the bed of the cavion, and on 
elevated level places, where large loosened blocks could not be removed by 
water nor by gravity, I observed the same appearances, under conditions which 
forbid this explanation. Mr. Muir, also, in his Studies in the Sierra, gives 
many examples of undoubted rock-breaking by ancient glaciers. 
