The third great use of mountains is to cause perpetual change in the _soils_ of the earth. Without such provision the ground under cultivation would in a series of years become exhausted, and require to be upturned laboriously by the hand of man. But the elevations of the earth's surface provide for it a perpetual renovation. The higher mountains suffer their summits to be broken into fragments, and to be cast down in sheets of massy rock, full, as we shall see presently, of every substance necessary for the nourishment of plants; these fallen fragments are again broken by frost, and ground by torrents, into various conditions of sand and clay--materials which are distributed perpetually by the streams farther and farther from the mountain's base. Every shower that swells the rivulet enables their waters to carry certain portions of earth into new positions, and exposes new banks of ground to be mined in their turn. That turbid foaming of the angry water,--that tearing down of bank and rock along the flanks of its fury,--are no disturbances of the kind course of nature; they are beneficent operations of laws necessary to the existence of man, and to the beauty of the earth. The process is continued more gently, but not less effectively, over all the surface of the lower undulating country; and each filtering thread of summer rain which trickles through the short turf of the uplands is bearing its own appointed burden of earth to be thrown down on some new natural garden in the dingles beneath.