LAKES WITHOUT OUTLETS. In all countries there is evaporation from the surfaee of lakes, so that less water flows out than enters; there may also be loss through seepage into the earth. On account of this, even in moist climates, lakes with a small drainage area may not rise to the point of outflow. This is true of small basins in sand-dune regions, or in sandy glacial de posits, where the water speedily soaks into the loose soil. Slowly, however, the washing in of clay and the growth of vegetation forms a more impervious bottom, so that first swamps, then ponds, are caused. Little kettle-shaped basins in terminal moraines often have such a small drainage area that they do not rise to overflow, or possibly overflow only in periods of heavy rains or melting snows. Deep lakes in volcanic craters, like Crater Lake in Oregon, also fail to reach the point of overflow, because the rainfall and drainage are not sufficient to fill the basins. Where the climate is very arid, the lack of abundant rain, the rapid evaporation, and the seepage, commonly keep the lake waters below the rim of the basin; and in deserts these condi tions may completely dry up the basins, or permit them to have lakes or swampy bottoms only dur ing rains. These effects of aridity are illustrated
in the Great Salt Lake, the Dead Sea, and the Caspian Sea. As the streams entering lakes bring a load of mineral matter in solution, and as this is not carried off in the vapor, lakes without outlet have a steadily increasing load of mineral matter. Among the mineral substances thus brought, salt, gypsum, and carbonate of lime are usually the most abundant. In time, therefore, these substances may be present in such quan tity that no more can be held by the water, :aid then some of the mineral load must be de posited. Thus carbonate of lime is being precipi tated on the bottom and shores of the Great Salt Lake, and salt and gypsum have been pre cipitated in many lakes in recent times and in past ages.