THE WORK OF LAKES. Lakes are formed by some interference with drainage, usually a dam across some stream course, as the growth of a mountain barrier, a lava flow, or a dam of glacial deposit. The lake waves work on the coast-line; cliffs are cut, beaches are formed, and on these the fragments are ground finer. The rivers which enter the lake add still more to the deposit ac cumulated, forming deltas where they enter, but giving the finer material to the currents for trans portation off into the lake. Weathering adds to the supply of sediment, and the wind drifts more rock fragments to the water. In the quiet lake waters, even the finest of this sediment in time settles to the bottom. Given time, then, the fate of lakes is to be filled; and the truth of this has been graphically stated in the remark that "rivers are the mortal enemies of lakes." But rivers are not allowed to do the entire work of lake destruction, as has been shown. Aside from the agencies of lake-filling mentioned, the influ ence of organisms is effective. The shells of ani mals and the accumulations of plant remains are also factors of importance. In the last stages of lake destruction water-loving plants—the reeds, rushes, and sphagnum mosses—are effective, both by their own accumulation, and by their in terference with waves and currents, thus aiding in the deposit of rock fragments. Many filled
lakes have been transformed by plant growth to bogs in the northern climates, where the sphag num moss grows readily. By the processes of lake-filling important accumulations of sedimen tary rocks are made; and in some countries, as the Western United States, where large lakes were formed behind mountain dams in recent geological periods, there are extensive areas occupied by lake-formed strata. Coal-beds, repre senting the stages of organic influence, are a part of these lake beds. In arid climates, where evaporation exceeds the rainfall, the lake waters are lowered below the outlet; then, year by year, the mineral substances brought in solution by the incoming water, and left behind in the lake as the water is evaporated, become more and more concentrated. Such lakes become salt, and, if the process continues, deposit layers of salt, gypsum, and other substances. Before this stage is reached, however, the precipitation of car bonate of lime takes place because salt water dissolves this substance less easily than fresh water. Beds of these precipitated rocks are com mon in the West, where they have been recently formed, and, in fact, are in some cases still forming; they are also found among the strata of earlier ages when similar conditions existed: See LAKE.