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Lakes

Lakes are bodies of water occupying hollows of the land, either with an outlet, when their waters are fresh, or without one, when they are salt. They vary in size from the salt Caspian, 170,000 square miles, to the fresh water Superior, over 30,000 square miles, and Victoria Nyanza, but little less. In elevation they range from Sir-i-kol, the source of the Oxus, 15,600 feet above sea-level, and Titicaca, 12,800 feet, to the Caspian, the surface of which is 85 feet, and the Dead Sea, the surface of which is 1,272 feet below sea-level. In depth, Lake Baikal, with an area of 9,000 square miles - about that of Lake Erie - the largest body of fresh water in Asia, exceeds all others, being 4,080 feet, with its surface 1,360 feet above, and its bottom 2,720 feet below, sea-level. But the bottom of the Dead Sea is 2,580 feet, and that of the Caspian 3,685 feet below sea-level. Lake Como is rather deeper than the Dead Sea, but Tanganyika, Geneva, Superior, and the deepest Scottish lakes are about 1,000 feet in depth. Lakes have originated in a variety of ways. Some are the craters of dormant volcanoes, such as Lake Albano, near Rome. Others, known as lagoons, are formed on low sandy coasts by storm-beaches, and are commonly brackish, as in the Landes of Bordeaux. The upheaval of surrounding land has formed many large lakes, such as the Caspian, the great Equatorial lakes of Africa, and those of Switzerland and Italy, in which animals closely allied to marine forms suggest a former connection with the ocean. The great series of lakes in the St. Lawrence basin, the greatest area of fresh waters on the globe, have been compared to an elevated Baltic. The depression of a plain has been another cause, as in the Jordan valley, with the fresh-water Tiberias 600 feet below sea-level, and the Dead Sea 100 miles farther south and nearly 700 feet lower. The waters of a lake may be held up by ice, like the Merjelen See on the Aletsch glacier, or may have been dammed back by a landslip, a lava-stream, a glacial moraine, or the work of beavers. Hollows produced by irregularities in boulder-clay left on the melting of an ice-sheet, and rock-basins scooped out of solid rock on the lower slopes of once glaciated mountains, form the tarns of northern mountains. The subsidence of rock-basins along a coast-line has produced fjords (q.v.). Lakes in the course of a river act as filters, and so tend to become choked by the fans of growing deltas carried into them, and they also act as flood-regulators during heavy rain. Salt lakes vary much in salinity and in composition. Aral has less than 11 grams of salt per 1,000, the open Caspian less than 13, Van over 17, the Dead Sea 221, Urumiah nearly 223, and Karaboghaz Bay in the Caspian 285 grams. In the Caspian, Dead, and Urumiah Seas the salt is mainly chlorides of soda and magnesia, with notable proportions of chloride of potash and lime in the Dead Sea; but Van contains a large proportion of carbonate and sulphate of soda, and is thus alkaline rather than saline. Other smaller lakes in Tibet, California, etc., contain borax (q.v.).

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