PHYSIOGRAPHY. The study of the land forms of the earth, with their present distribution and their mode of origin, is a comparatively new branch of science.
The volume of the land above sea-level is much less proportionally than the volume of the sea; the average level of the solid crust of the earth is estimated at 7500 feet below sea-level. The average altitude of the land lying above sea-level is estimated at 2250 feet, the extreme height being Mount Everest, in the Himalayas, 29,000 feet. See CONTINENT; AMERICA; EUROPE; etc.
The forms of relief of the earth, as they exist to-day, are the resultant of two opposing opera tions, upbuilding and leveling. Chief among the former are earth movements, whose visible re sults are folds and faults, uplifted ridges and blocks of land. Rivers, glaciers, and winds also net as constructive agents, in depositing sand, gravel, and other detritus. Strains in the earth's crust, produced perhaps by shrinking of the in terior on cooling, result in folds and breaks in the crusts. These may be of small extent, producing mountain ridges, or they may involve large parts of the earth, resulting in raising continents above the sea. They may be low and flat, or they may be high and sharp, even to so great an extent that the sides of the fold pass beyond the vertical. (See FAULT; ANTICLINE; SYNCLINE.) Lava flows out from vents, and spreads over great areas, or it may be forced in between beds of stratified rock, or, in a plastic state, be forced up through such beds.
No sooner has a region been uplifted than the agencies of erosion, always at work, attack it with renewed activity. Water percolates into the seams and crevices of the rocks, and freezing, splits them into fragments by its expansion. Water, often with acids in solution, dissolves the soluble portions of the rocks, and thus disinte grates them. Flowing water, glacial ice, and the wind wear the rock away. The rock waste thus produced is transported, always downward, by the winds, streams, glaciers, and its own weight, most of it having the sea bottom as its ultimate destination. On the way, however, some of it is deposited, as in dunes, moraines, and deltas, and thus the agencies of destruction are also con structive agents. Thus there is a constant move ment downward, from the land to the sea. Un less this is offset by elevation movements in the crust of the land, it results eventually in the reduction of the land to a low plain. Fur thermore, if the limits of sea and land remain constant, there is a vast accumulation of sedi ment on the sea bottom, and a corresponding thinning of the solid crust over the land. See PHYSIOGRAPHY; GEOLOGY.