A Glacier is an enormous mass of solid ice filling up a valley, and stretching from the eternal snows which crown the summits of the mountains, down to the smiling cornfields and rich pastures of the plains. It is constantly fed by the accumulated snows of winter, which, slipping and rolling down the slopes of the mountains, lodge in the valleys below, and are there converted into ice. For it must be remembered that the Glacier properly so called does not commonly extend much higher than 9000 feet above the level of the sea. Beyond that elevation the compact and massive ice gradually passes into frozen snow, called by the French Nevé, and by the Germans Firn. The change which takes place in the condition of the snow as it descends into the valley is chiefly owing to these two circumstances: first, it is closely compacted together by the weight of the snowy masses pressing down upon it from above; and secondly, in the summer months it is thawed upon the surface during the day by the heat of the sun, and frozen again at night. On a small scale this process is practically familiar to every school-boy. When he makes a snow-ball he is practically converting a mass of snow into ice, and that by a series of operations very closely resembling those which Nature employs in the manufacture of a Glacier. In Switzerland the Glacier is often two or three miles in breadth, from twenty to thirty miles in length, and five or six hundred feet in depth. Though so vast in its bulk and so solid in its character, it is not, as might be supposed, a fixed, immovable mass. On the contrary, it is moving incessantly, but slowly, down the valley which it occupies, at the rate of several inches--sometimes one or two feet, and even more--in the day. In Greenland a Glacier explored by Doctor Hayes, in his expedition to the North Pole, was found to move for a whole year at the average rate of a hundred feet a day. It may be thought, perhaps, that this fact requires further confirmation; but at all events it is certain that the language of the poet, when he addresses the Glaciers as “motionless torrents,” though it conveys an accurate and beautiful idea of the appearance they present to the eye, is not rigorously true in a scientific sense. Indeed, it is just because the Glaciers are not motionless that they serve as instruments of Denudation.