The Chemical Deposits are much the most interesting and characteristic of the accumulations gathered in salt lakes. These chemical precipitates differ much in the various lakes, according to the nature of the rocks which form the drainage basins, but while some of the substances are rare and restricted in extent, others are extremely common and wide-spread. Several changing factors combine to vary the order of precipitation of the salts, which is a highly complex process, but, in general, it follows the inverse order of solubility, the least soluble material being deposited first and the most soluble last. Comparatively little chemical reaction appears to take place in these lakes; the substances arej for the most part, thrown down merely by the evaporation of saturated solutions and are the same as those carried in very dilute solutions by the tributary springs and streams. If the precipitation of salts is slow and occasional, the chemically and mechanically formed deposits are mingled together; but if such precipitation be rapid, then thick and nearly pure masses of the salts are thrown down in their proper order, as the concentration by evaporation proceeds.
The first substances to be deposited from solution are the carbonate of lime and oxide of iron (CaC03 and Fe203), and in moderately saline lakes this is about the limit of precipitation. These same materials are thrown down in fresh lakes also, and their deposition is principally due to the loss of the solvent C02. The ancient Lake Lahontan, which formerly occupied part of northwestern Nevada, was the seat of calcareous deposition on a magnificent scale, and every crag and island which its waters touched is sheathed in thick masses of calcareous sinter. Pyramid Lake, a remnant of Lahontan, has a remarkable island of calcareous tufa; and Mono Lake, California, is famous for similar deposits, which have assumed curious and whimsical shapes.
As the concentration of the lake waters proceeds, the next substance to be precipitated is gypsum (CaS04, 2 H20), which, though much more soluble than the carbonate of lime, is yet only sparingly so. After all the gypsum in solution has been thrown down, there follows a pause in the deposition, until a further stage of concentration has been reached, and then common salt is precipitated, which deposition continues steadily as concentration proceeds, but at an advanced stage the salt is mingled with the sulphate of magnesia (MgS04), should that be present. The highly soluble salts, such as the chlorides of magnesium and calcium (MgCl2, CaCl2), remain in solution until the water is completely evaporated to dryness, hence they are rarely found in beds of rock salt; yet when they do occur, as at Stassfurt in Prussia, they are mingled with salt, which is thus precipitated till the very end of the process.
Fig. 105. - Island of calcareous tufa, Pyramid Lake, Nevada. (U. S. G. S).
Fig. 106. - Salt deposit, El Paso, Texas. (U. S. G. S).
Various circumstances may change the order of precipitation just given. In seasons of high water the flooded rivers dilute the waters of the lake, checking the chemical precipitation and, at the same time, increasing the mechanical deposition; thus beds of sand and mud are thrown down upon the beds of gypsum and salt, alternating with them, as the influx of fresh water or evaporation predominates. Changes of temperature also have an effect upon the order of precipitation. Thus, in cold weather, Salt Lake Q washes up on its shores quantities of sulphate of soda (Na2S04), which is formed at low temperatures by the double decomposition of NaCl and MgS04.
Besides the chemical deposits already mentioned, others occur on a smaller scale. On the western side of the Great Basin, in Nevada, California, and Oregon, are several lakes which contain large proportions of carbonate.of soda, and in some of them the concentration is sufficiently advanced to cause precipitation, while others form deposits of borax.
Fig. 107. - Deposits in an alkali lake. (U. S. G. S).
Much the most abundant of the chemical deposits made in salt lakes are gypsum and rock salt, and the enormous scale on which the latter was formed in past ages of the world's history is demonstrated by the vast bodies of rock salt which are found embedded in the rocks in so many parts of the world. Near Berlin, at Sperenberg, an artesian well was sunk through such a deposit for nearly 4000 feet, without reaching the bottom. In various regions of the United States, notably in New York and Kansas, large bodies of salt are found, but not on such a scale as in Europe.
Salt bodies of such immense extent and thickness, with little or no interstratified, mechanically deposited material, are not explained by the usual operations of a salt lake as above described. The key to the understanding of these enormous deposits of salt appears to be given by a gulf from the eastern part of the Caspian Sea, known as the Karibogas. This gulf, which is connected with the Caspian only by a very narrow channel, is situated in an extremely hot and almost rainless region, and evaporation is very rapid. On the bottom masses of salt have been deposited uninterruptedly for a long time past, and in a very deep or a slowly subsiding basin the process might continue almost without a limit.
It should be noted that the chemical deposits made in salt lakes are crystalline and at the same time stratified. This association is not the usual one, as stratified rocks are ordinarily not crystalline, and crystalline rocks are mostly Unstratified.