Sec. 16. Holding fast this clue, we have next to take into consideration another fact of not less importance,--that over the whole of the rounded banks of lower mountain, wherever they have been in anywise protected from the injuries of time, there are yet visible the tracks of ancient glaciers. I will not here enter into detail respecting the mode in which traces of glaciers are distinguishable. It is enough to state that the footmark, so to speak, of a glacier is just as easily recognizable as the trail of any well-known animal; and that with as much confidence as we should feel in asserting that a horse had passed along a soft road which yet retained the prints of its shoes, it may be concluded that the glaciers of the Alps had once triple or quadruple the extent that they have now; so that not only the banks of inferior mountains were once covered with sheets of ice, but even the great valley of the Rhone itself was the bed of an enormous "Mer de Glace," which extended beyond the Lake of Geneva to the slopes of Jura.[55] Sec. 17. From what has already been noted of glacier action, the reader cannot but be aware that its universal effect is to round and soften the contours of the mountains subjected to it; so that a glacier may be considered as a vast instrument of friction, a white sand-paper, applied slowly but irresistibly to all the roughnesses of the hill which it covers. And this effect is of course greatest when the ice flows fastest, and contains more embedded stones; that is to say, greater towards the lower part of a mountain than near its summit. [Illustration: FIG. 24.]