Work of Lakes are the result of imperfect drainage, and may be due to original depressions in the land surface as it emerged from the sea; to later obstructions of drainage lines by uplift athwart their course; to filling by glaciers; or to other factors. In humid climates most lakes have outlets which tend to lower by erosion, and this is a factor in their filial destruction. More important is the filling of the basin. All lakes, because the water flows through them very slowly, act as settling basins and retain most of the sediment brought to them by streams. The depression soon becomes filled with mechanical, chemical or organic deposits and the lake ceases to exist. The mechanical sediments include clays, sands and gravels. As in the oceans, the coarser deposits form nearest shore, the finer out in deeper and more quiet water. Chemical deposits are characteristic of arid regions where few lakes have outlets. All the dissolved matter brought in by streams remains in solution until the water becomes so concentrated that the salts must be precipitated. Common salt, gyp sum and limestone are usual chemical precipi tates from lakes. (See GYPSUM; SALT). Many important beds of gypsum and much salt in the United States have been formed in this manner. Organically certain small plants known as Chara cause the deposition of calcium car bonate. Such accumulation are the most com mon source of marl in our northern lakes. (See MARE). Many water plants, especially sphagnum moss, accumulate and form peat (see COAL; PEAT; PEAT Boas; SWAMP; etc.), which may ultimately become coal. Bog iron ore, chiefly limonite, accumulates, partly as a chemical precipitate, partly as the result of so called iron bacteria. All deposits laid down in lakes are known as lacustrine in contrast to glacial, alluvial and eolian deposits. During geologically recent times lakes have existed that are now extinct. Their boundaries have been traced by the old beaches. The more noted of these lakes in the United States are lakes Agassiz, Bonneville and Lahontan. See LAKE; GREAT SALT LAKE; LAKE AGASSIZ; LAKE LAHONTAN, etc.
Work of Plants and Animals.— Organ isms are important geological agents. ically, roots grow in cracks and wedge . apart, trees blow over and expose fresh sod at the surface; earthworms, moles, and other burrowing animals bring soil to the open air.
Vegetation also acts as a cover 'preventing drifting of sand, erosion by running water, and the rapid heating and cooling of rocks. Chem ically, decaying organic matter yields carbon dioxide and acids that act as solvents for mineral matter, and that aid in weathering. Accumulating vegetation forms peat and finally coal. Coral polyps secrete lime and build whole
islands of coral rock. Limestone consists largely of the skeletons of minute lime-secreting animals and many other deposits are formed by organisms of one sort or another. Plant and animal remains embedded in the rock as fossils form the basis of most of our efforts to corre late rock layers and to divide geologic time into units. Man, by means of deforestation, irrigation and other activities, is coming to have control over geologic processes.
Diastrophism.— Diastrophism may be con sidered as the deformation of the earth by in ternal forces, the origin of which is not well understood but which are believed by many to be the result of the cooling and shrinking of the earth in its passage from the molten to the solid state. Others believe they are due to the settling of heavy wedge-shaped seg ments of the earth's crust (oceanic blocks) and the forcing aside and upward of lighter segments (continental blocks), since actual experiment with a pendulum seems to show that the specific gravity of the rocks under the oceans is greater than that of those under the continents. Be that as it may, it is a well known fact that the earth's crust is undergoing almost constant deformative movements which are usually divided into two classes, epeiro genic and orogenic. See I SOSTASY.
Epeirogenic movements are those which involve the uplift of whole continental masses very slowly without appreciable folding or dis location except gentle warping. Most of the great interior of the United States between the Appalachian Mountains and the Rocky Moun tains consists of flat lying rocks varying from 500 to 2,000 feet above sea level. These rocks show conclusively by their character and fossils that they were laid down beneath the sea, and that they now occur at this elevation proves a broad extensive uplift. That such changes of level are still going on is best shown along sea coasts where elevated beaches and wave cut terraces and caves not yet obliterated by weathering testify to recent uplift. Such ele vated beaches are known at several different levels along the coast of California. On the other hand, off the Atlantic coast the channel of the Hudson River can be traced many miles along the bottom of the shallow ocean across the continental shelf. There is not sufficient current to account for the erosion under water, and it seems practically certain that the channel was carved at a time when this shelf was land, and that the coast has since been submerged. There is abundant evidence that certain regions have been thus elevated and again depressed many times during their history. See BEAcHEs;