There cannot, however, be a doubt but that, after a certain lapse of time, the bulk of the matter removed becomes less and less.
For these reasons. The rock that is removed is taken away by friction. Of two kinds. The first, of the foreign matter imbedded in the bottom of the glacier (or rolling underneath it) against the bed-rock, which foreign matter it has been already stated (§9) is derived from two sources—viz. from the rock-bed itself, and from masses which have fallen on to the surface of the glacier, and afterwards worked their way down.
It is obvious, as the rocks which are being operated upon by the glacier become more and more smooth, that the supply from the first of these sources must constantly diminish. It is equally certain that when the rock-bed has lost many of its asperities, and the glacier—so to speak—fits more closely to it, the matter which falls from above has greater difficulty in getting between the ice and the rock-bed. Here are two ways of accounting for the fact that striations are rare or wanting upon roches nivelées, and it will now be perceived why it was said (§9) that striations "indicate a comparatively early and coarse stage of glacier action."
There remains to be considered the friction of the ice itself against the rock-bed. This, too, must diminish as the surfaces over which the glacier passes become smoother and flatter. The more thoroughly parallel the bottom of the glacier and the bed-rock are to each other, the less friction will there be, and the less abrasion.
There is therefore good reason to believe that not only is the depth of rock removed from any given place less and less year by year, but that the total amount of matter removed by the glacier constantly diminishes. Just as a smoothing-plane, that is set fine, will take shaving after shaving from a plank (each shaving being thinner than the last), and at length glides over the wood without producing any effect except a kind of rude polishing; so a glacier, passing over rocks, takes shaving after shaving (in the form of sand or mud), and at length glides on, and puts the finishing touches, by
