The river Colorado of the West, which runs from the Rocky Mountains to the Gulf of California, flows for nearly three hundred miles at the bottom of a profound chasm, or cañon, being hemmed in by vertical walls which in some places are more than a mile in depth. The tributary streams flowing into the river run through smaller ravines forming side cañons; and there is no doubt that these wonderful chasms have been, in the course of ages, slowly carved out by the river Colorado and its numerous tributary streams. Sometimes the walls of the cañon are not more than fifty yards apart, and in height they vary from three thousand to six thousand feet. Far above the level of the highest floods patches of gravel are found here and there on the sides, which must have been left there by the river when it had not cut its way so far down. These cañons afford striking testimony to the erosive power of running water, of which they are the most wonderful illustration in the world. But water, even when in the form of ice, has more or less power to wear away solid rock; and the glaciers that we see in Switzerland, Norway, and other countries must slightly deepen the rocky valleys down which they flow. Let us see how this can be accomplished. The snow that falls in the High Alps, impelled by the weight of fresh layers of snow overlying it, and by the slope of the mountain-sides, gradually creeps down into the valleys. Owing to the pressure thus put upon it, and partly to the melting power of the sun's rays, it assumes the form of ice; and glaciers are composed of solid ice. The downward motion is so slow that a glacier appears quite stationary; and it is only by putting in stakes and watching them change their positions that it can be shown to be moving. In all respects except speed, glaciers flow like rivers, for ice is a viscous body, behaving partly like a fluid and yet partly like a solid substance; but it will not endure a sharp bend without snapping. Hence, a glacier in traversing a valley frequently gets split. The cracks thus formed widen by degrees until they expand into chasms, or "crevasses." Like rivers, glaciers transport a large amount of rocky matter to lower levels, and at the same time wear away and deepen their rocky channels. Let us see how they do this twofold work of transportation and erosion. In the first place, a large amount of débris falls onto the sides of a glacier from the peaks, precipices, and mountain-side along which it flows. Some stones, however, fall down crevasses, and so reach the bottom, where they become cemented in the ice. In this way they are slowly carried down over the rocky floor of the valley, until at last they reach the end of the glacier, where in the warmer air the ice melts just as fast as it creeps down; and there they will be left to form a heap of stones, sand, and mud.