tail 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.]

Suppose now a chain of mountains raised in any accidental form, only of
course highest where the force was greatest,--that is to say, at the
centre of the chain,--and presenting any profile such as _a_, Fig. 24;
terminated, perhaps, by a broken secondary cliff, and the whole covered
with a thick bed of glacier, indicated by the spotted space, and moving
in the direction of the arrows. As it wears away the mountain, not at
all at the top, but always more and more as it descends, it would in
process of time reduce the contour of the flank of the hill to the form
at _b_. But at this point the snow would begin to slide from the central
peak, and to leave its rocks exposed to the action of the atmosphere.
Supposing those rocks disposed to break into vertical sheets, the summit
would soon cleave itself into such a form as that at _x_; and the
flakes again subdividing and falling, we should have conditions such as
at _y_. Meanwhile the glacier is still doing its work uninterruptedly on
the lower bank, bringing the mountain successively into the outlines _c_
and _d_, in which the forms _x_ and _y_ are substituted consecutively
for the original summit. But the le