They show that if the usual view of glacial erosion be correct, the great canyons, so far from being _formed_, were only very _slightly modified_ by glacial agency.

But I am 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_.