TtELATlON OF LAKES TO CLLMATIC CONDITIONS. 73 



holding a number of salts in solution, when slowly evapor^ited, do not 

 deposit them in a homoL'-eneous mass, but in successive layers of varying 



i composition. .Vs the order in which different salts are deposited varies 

 with tlie composition of the waters, it is safe to say that in no two lakes 

 is th^ succession of saline deposits formed on evaporation apt to be the 



I same. Disregarding for the present the reaction of the various salts upon 

 each other, it is evident that in the evaporation of natural waters the 

 order in Avhich the contained salts will be precipitated is invei-sely as the 

 order of their solubility. For example, a salt which requires a large 



I amount of water for its solution, or, in other words, is sparingly soluble, 

 will reach its point of saturation and commence to crj-stallize out as 

 evaporation progresses, previous to the deposition of a more soluble salt. 

 To illustrate : it has been found that calcium carbonate requires about 

 10.000 times its weight of water, saturated with carbon dioxide, for its 

 solution ; while calcium chloride is deliquescent, and dissolves in about 

 its own weight of water. In an enclosed lake to which streams and 



I springs are bringing these two salts in equal quantities, and in which 



[ evaporation equals or exceeds the supply of fresh water, it is evident that 

 the calcium carbonate would reach its point of saturation and commence 

 to separate long before the waters had become rich in calcium chloride. 



\ In fact, owing to the deliciuescent nature of the cliloride, natural evapora- 

 tion seldom proceeds far enough to cause its preci[)itation. The early 

 (le[)Osition of calcium carbonate, when natural waters are concentrated by 

 exaporation, is rendered the more certain for the reason that it is by far 



I the most abundant salt found in surface waters. 



The fact that various salts are dei)Osited in a regular succession when 

 mineral waters are evaporated, is of great service in separating certain 



t ones in a pure state Ijy the method known as fractional crystallization. 

 In evaporating the brines of Syracuse, New York, the precipitation of 

 ferric oxide and of calcium sulphate, or gypsum, is tirst secured by mod- 

 erate concentration ; the brine is then conducted to lower vats and evapo- 

 ration continued until the sodium chloride, or common salt, has mostly 

 crystallized and fallen to the bottom ; the mother-liquor, rich in magnesium 

 and calcium, is then allowed to go to waste. A similar i)rocess freijuently 

 takes [)lace in nature. l)ut the salts [)recipitated ct)llect in the same basin 

 in alternating layers. 



In tlie S(»da lakes near Kagtown, Nevada, a douljle carbonate of so(Uum 

 and calcium, known as the mineral gaylussite, forms on the bottom, owing 

 to natural concentration. When the waters aiv still farther evaporated. 



