I. There is an upper stratum of the atmosphere characterized by intense cold, and called the region of perpetual snow. It covers the earth like a great arch, the two ends resting, one on the arctic, the other on the antarctic zone, while the centre, being about 16,000 feet above the sea,[2] rises directly over the tropics. Wherever a mountain is sufficiently lofty to pierce this upper stratum, its summit is covered with snow, and, as the snow never melts, it is plain that, from the accumulations of fresh snow-drifts, the mountain-tops, by gradually increasing in height and width, would become the supporting columns of vast hills of ice, which, breaking up at last from their weight and width, would roll down the mountain-sides and cover vast areas of country with a ruin and desolation more terrible than that of any avalanche. Olympus would really be superposed upon Ossa. By a beautiful arrangement this undue growth is prevented, so that the hill-tops never vary much in height above the sea. The cone of ice and snow which covers the higher part of the mountain, sends down into each of the diverging valleys a long sluggish stream of ice, with a motion so slow as to be almost imperceptible. These streams are called glaciers. As they creep down the ravines and gorges, blocks of rock detached by the frosts from the cliffs above, fall on the surface of the ice, and are slowly carried along with it. The bottom also of the glaciers is charged with sand, gravel, and mud, produced by the slow-crushing movement; large rocky masses become eventually worn down into fragments, and the whole surface of the hard rock below is traversed by long parallel grooves and striæ in the direction of the glacier's course. Among the Alps, the lowest point to which the glacier descends is about 8500 feet. There the temperature gets too high to allow of its further progress, and so it slowly melts away, choking up the valleys with piles of rocky fragments called moraines, and 'giving rise to numerous muddy streams that traverse the valleys, uniting at length into great rivers such as the Rhone, which enters the Lake of Geneva turbid and discoloured with glacial mud. [Footnote 2: The average height of the snow-line within the tropics is 15,207 feet, but it varies according to the amount of land and sea adjacent, and other causes. Thus, among the Bolivian Andes, owing to the extensive radiation, and the ascending currents of air from the neighbouring plains and valleys, the line stands at a level of 18,000 feet, while, on mountains near Quito, that is, immediately on the equatorial line, the lowest level is 15,795.--See Mrs. Somerville's _Physical Geography_, 4th edit. p. 314.]