Passing from the chemical to the mechanical agents of Denudation, it is worth while to notice the immense power which is often generated by the agency of frost, especially in those countries that are subject to great vicissitudes of heat and cold. During a thaw, water finds its way into the clefts and joints by which all rocks are traversed, and when it is afterward converted into ice, it expands with a mechanical force that is almost irresistible. The hardest rocks are burst asunder, great blocks are detached from the mountain side, and sent rolling down its slopes, or tumbling over crags and precipices, until at length they come to rest in shattered fragments at the bottom of the valley. In this condition they await but the coming of the winter’s torrent to be borne still further on their long journey to the sea. The fearful havoc done in this way by the alternate action of sun and frost contributes in no small degree to the fantastic and picturesque forms assumed by the mountain peaks of Switzerland. Huge masses of rock have been literally hewn away, until nothing has remained behind but those splintered obelisks and tapering pinnacles so familiar to the eye amidst the sublime scenery of the Alps. Indeed one of the greatest perils encountered by the adventurous spirits whose ambition it is to rival one another in the danger of their exploits, and to climb whatever was before regarded as inaccessible, arises from the enormous fragments of rock which are rent almost unceasingly from the overhanging crags and hurled into the abysses below them. The following incident related by Professor Tyndall is very much to the point. “We had gathered up our things, and bent to the work before us, when suddenly an explosion occurred overhead. Looking aloft, in mid-air was seen a solid shot from the Matterhorn describing its proper parabola through the air. It split to pieces as it hit one of the rock-towers below, and its fragments came down in a kind of spray, which fell wide of us, but still near enough to compel a sharp look out. Two or three such explosions occurred afterward, but we crept along the back fin of the mountain, from which the falling boulders were speedily deflected right and left.”