Ruskin has pointed out that mountains are of the greatest possible use to man in three ways in connection with air, water and earth.
The mountains have a great deal to do with keeping the air circulating, and renewing its purity. The snow-covered summits of high mountains make the air in contact with them very cold; and as cold air tends to sink.
It descends to the plains and lowlands in cooling winds; while the hot air of the plains rises up to the higher altitudes to be cooled and descend purified again to the lower levels.
The air can, therefore, never remain stagnant in one place, but is always moving and being refreshed by the cool rocks and snows of the mountains.
Mountains, again, are the great reservoirs of the earth’s fresh water, and are the source of the rivers and streams, without which men could not live. They catch the rain and store it up, in the following way. Warm moisture-laden winds are cooled down when they blow against a lofty mountain range, and in summer condense in rain, and in winter in snow.
All the winter, the high mountains are storing up water in the from of snow; and when the summer comes, a great deal of this snow melts and pours down in torrents and streams of water to feed the great rivers.
Much of the snow, too, descends from the higher levels, where it never melts, in the form of glaciers, which at a lower level melt, and are the source of rivers; and a good deal of the rain that falls on mountains finds its way through crevices in the rocks to underground caves, which become reservoirs of water to feed perennial streams.
Lastly, mountains provide the material that forms the fertile soil of the plains. The rocks at high levels on the mountains are constantly being split and broken by the intense frost.
The fragments of rock fall into the valleys, and the smaller ones are carried down by the rushing torrents, and gradually broken and rubbed and ground down into sand and gravel and mud.
The mountain torrents carry this sand and mud into the big rivers; and the rivers when they are in flood deposit it on the land and thus enrich it. Even a small stream will bring down tons of sand and mud in one year. We therefore owe to the mountains, fresh air, fresh water and fertile soil.