THE CRUST OF THE EARTH. The cold, outer portion of the earth is composed of rocks, some derived from beneath the surface, whence they have risen in molten condition, others formed by the reassortment of the materials obtained from the disintegration of other rocks. These rocks have been subjected to movements, as a result of which the earth's surface has been made irregular. The cause of these movements of the crust de pends upon the heated condition of the earth's interior ; the results have been to make great downfolds where the ocean basins are situated and upfolds where the continents are located, with numerous minor uplifts and downsinkings along narrow lines, both in the sea and on the land, forming mountain ranges. (See CRUST OF THE EARTH.) By far the greater part of the earth's surface is fairly level. Most of the ocean bottom is a vast series of submarine plains with occa sional mountain ranges and volcanic peaks rising above them. On the land much more than half the surface is also plain or plateau, some of the plateaus rising to elevations of 10,000 to 15,000 feet. See CONTINENT.
In the ocean the deposit of waste from the land, and the accumulation of the solid parts of ani mal remains, has the general tendency to level the sea floor. Agents of erosion are in general ineffective excepting at the contact between land and sea; and consequently the only forces oper ating to make the sea floor irregular are those of uplift or downsinking of the crust. On the land, on the other hand, the action of the forces of denudation carves the mountains, plains, and plateaus, making the surface more irregular. And along the coast-line the work of the waves and tides is added to the dynamic processes by which the land is being irregularly denuded. Thus the land portion of the earth's crust is often deeply scarred and cut, revealing the in ternal structure of the superficial portions of the crust.