Most beautiful things; there are flowers and trees, And bevies of birds, and swarms of bees, And cities, and temples, and towers, and these All pictured in silver sheen. CHAPTER XXV. GLACIERS. Glaciers are rivers of ice, and, like other rivers, some of them are small and some very large. They flow down the gorges from high mountains, whose peaks are always covered with a blanket of eternal snow. Summer and winter the snow is precipitated upon these mountains, and from time to time the heat of the sun's rays softens the snow, when by its great weight it packs more closely together until it is, in many cases, formed into solid ice-cakes. If we take a quantity of snow or a quantity of granulated ice and put it under a sufficient pressure we can produce clear solid ice, and it is by this process that ice is formed out of the snow and hail that falls continually upon the tops of these glacial mountains. We have seen that ice possesses certain viscous or semi-fluidic properties and that it will yield to pressure, but if we put it under sufficient tensional strain it snaps like glass or any other brittle substance. As the snows upon these mountains pile up higher and higher the pressure becomes greater and greater until it reaches a point where the mass begins to move gradually down the mountain side, following the gulches and defiles that furnish a path of least resistance to its flow. At the sides and bottom, where there is contact with the earth, the movement is slower than it is at the surface and in the middle of the ice stream. If there were no curves in the ravine or gulch through which it flows the point of greatest movement would be confined to the middle of its width. But in flowing through a winding gulch the most rapid flow follow the lines of greatest pressure, and this line is deflected from side to side, so that the line of greatest flow is more winding than is the bottom of the valley through which it flows. (The movement is called a "flow," but it is very sluggish, only a few inches in a day, as will appear later.) If the bottom and sides of the valley were straight the surface of the ice would be comparatively even; I say comparatively, for as compared with a smooth surface it would be very rough; but there would be none of the great crevasses or openings now to be found in the ice, which sometimes are very large and extend to a great depth. If in its downward course the bottom of the ravine suddenly becomes steeper, the top of the ice is put under a tensional strain which causes it to break, thus forming the crevasses.