It is natural that the great ridges should present the wildest forms—not on account of their dimensions, but by reason of their positions. They are exposed to the fiercest heat of the sun, and are seldom in shadow as long as it is above the horizon. They are entirely unprotected, and are attacked by the strongest blasts and by the most intense cold. The most durable rocks are not proof against such assaults. These grand, apparently solid—eternal—mountains, seeming so firm, so immutable, are yet ever changing and crumbling into dust. These shattered ridges are evidence of their sufferings. Let me repeat that every principal ridge of every great peak in the Alps amongst those I have seen has been shattered in this way; and that every summit, amongst the rock-summits upon which I have stood, has been nothing but a piled-up heap of fragments. The minor ridges do not usually present such extraordinary forms as the principal ones. They are less exposed, and they are less broken up; and it is reasonable to assume that their annual degradation is less than that of the summit-ridges. The wear and tear does not cease even in winter, for these great ridges are never completely covered up by snow,(160) and the sun has still power. The destruction is incessant, and increases as time goes on; for the greater the surfaces which are exposed to the practically inexhaustible powers of sun and frost, the greater ruin will be effected. [Illustration: PART OF THE NORTHERN RIDGE OF THE GRAND CORNIER.] The rock-falls which are continually occurring upon all rock mountains (such as are referred to upon pp. 29, 55) are, of course, caused by these powers. No one doubts it; but one never believes it so thoroughly as when the quarries are seen from which their materials have been hewn; and when the germs, so to speak, of these avalanches have been seen actually starting from above. These falls of rock take place from two causes. First, from the heat of the sun detaching small stones or rocks which have been arrested on ledges or slopes and bound together by snow or ice. I have seen such released many times when the sun has risen high; fall gently at first, gather strength, grow in volume, and at last rush down with a cloud trailing behind, like the dust after an express train. Secondly, from the freezing of the water which trickles, during the day, into the clefts, fissures, and crannies. This agency is naturally most active in the night, and then, or during very cold weather, the greatest falls take place.(161)