But the glacier which comes out of the snow-fields is itself made not of snow, but of ice. The snow, as it slides downward, is pressed together into ice. You have learned that each snowflake is made of little crystals of ice. A mass of snow is thus only a mass of minute crystals of ice with air between. Hence when the snow gets pressed together, the air is squeezed out, and the separated crystals of ice freeze together into a solid mass. You know that you can make a snowball very hard by squeezing it firmly between the hands. The more tightly you press it the harder it gets. You are doing to it just what happens when a glacier is formed out of the eternal snows. You are pressing out the air, and allowing the little particles of ice to freeze to each other and form a compact piece of ice. But you can not squeeze nearly all the air out, consequently the ball, even after all your efforts, is still white from the imprisoned air. Among the snowfields, however, the pressure is immensely greater than yours; the air is more and more pressed out, and at last the snow becomes clear transparent ice. A glacier, then, is a river, not of water, but of ice, coming down from the snow-fields. It descends sometimes a long way below the snow-line, creeping down very slowly along the valley which it covers from side to side. Its surface all the time is melting during the day in summer, and streams of clear water are gushing along the ice, though, when night comes, these streams freeze. At last it reaches some point in the valley beyond which it can not go, for the warmth of the air there is melting the ice as fast as it advances. So the glacier ends, and from its melting extremity streams of muddy water unite into a foaming river, which bears down the drainage of the snow-fields above. A river wears down the sides and bottom of its channel, and thus digs out a bed for itself in even the hardest rock, as well as in the softest soil. It sweeps down, too, a vast quantity of mud, sand, and stones from the land to the sea. A glacier performs the same kind of work, but in a very different way. When stones fall into a river they sink to the bottom, and are pushed along there by the current. When mud enters a river it remains suspended in the water, and is thus carried along. But the ice of a glacier is a solid substance. Stones and mud which fall upon its surface remain there, and are borne onward with the whole mass of the moving glacier. They form long lines of rubbish upon the glacier, and are called moraines. Still the ice often gets broken up into deep cracks, opening into yawning clefts or crevasses, which sometimes receive a good deal of the earth and stones let loose by frost or otherwise from the sides of the valley. In this way loose materials fall to the bottom of the ice, and reach the solid floor of the valley down which the ice is moving; while at the same time similar rubbish tumbles between the edge of the glacier and the side of the valley.