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 level of the whole flank of the mountain being now so much reduced, the glacier has brought itself by its own work into warmer climate, and has wrought out its own destruction. It would gradually be thinned away, and in many places at last vanish, leaving only the barren rounded mountains, and the tongues of ice still supplied from the peaks above.