Thus among the rocks which form the dry lands of the globe there occur masses of limestone, sand, marl, and other materials which we know were deposited in lakes, because they contain types of plants and animals like those found in the lakes of our own time. From this kind of evidence we can mark out the places of great lakes that have long ago vanished from the face of Europe and North America. There are also the so-called Salt Lakes to consider. These are generally the lakes that have no outlet and into which a small amount of water now flows, but never enough to cause the lake now to overflow, whatever it may have done in past times. The water that now runs in escapes merely by evaporation. But just as the bottom of a kettle in which hard water is constantly boiled gradually becomes furred, so a lake bottom into which water is continually pouring, bringing dissolved in it all sorts of mineral salts, becomes coated with sediment. The mineral salts are not evaporated, consequently the lakes become gradually more mineral--or, for convenience, let us say, become salter. Among the mineral salts common salt and gypsum are most important; but some bitter lakes contain sodium carbonate or magnesium chloride. The Dead Sea and the Great Salt Lake of Utah show by the deposits round them how they have changed their shape and depth. In the upper terraces of the Great Salt Lake, 1000 feet above the present level of the water, fresh-water shells occur, showing that the basin was at first fresh. The valley bottoms around salt lakes are now crusted with gypsum, salt, or other deposits, and their waters are without sign of life. Such conditions help us to understand how great deposits of salt or gypsum were once laid down in England, Poland, and Germany, and in many other places where now the climate would not permit of the necessary evaporation and condensation of the water. [Illustration: _Stereo Copyright, Underwood & U._ _London and New York_ A Petrified Tree This magnificent fossil is in the Petrified Forest of Arizona; and it affords one of the most striking examples known of the solidification and petrifaction of material by the infiltration of mineral salts. The trunk is now not merely encrusted with stone: it is permeated by silica, and is, in fact, itself a stone as hard as flint.] CHAPTER V RECORDS LEFT BY THE SEA