CHEMICAL PRECIPITATES 1 49 



of by evaporation from the surface of the lake. In either case an 

 arid climate is requisite to maintain the salinity; in a moist region 

 the large rainfall and slower evaporation would cause the lake to 

 rise until it found an outlet, and then the water, if originally salt, 

 would become fresh. The history of Lake Bonneville exemplifies 

 the change from fresh to saline conditions. As long as the water 

 level was maintained above the outlet, the lake was fresh, but when 

 the advancing aridity of the climate diminished the rainfall and 

 increased the rate of evaporation, the water level sank until it fell 

 below the outlet. Then the lake became saline, reaching its 

 maximum salinity in the intensely bitter waters of Salt Lake, which 

 is the remnant of the large lake. 



All river water contains greater or less quantities of dissolved 

 matters, and of these one of the commonest is ordinary salt (NaCl). 

 When such waters are evaporated, the solids remain behind, and 

 thus the water becomes more and more saline till it reaches satu- 

 ration. Other substances occur also, as will be seen below. 



The mechanical deposits formed in salt lakes do not differ in 

 any very important manner from those of fresh lakes. The finer 

 clays settle more rapidly in brine than in fresh water, which makes 

 strongly saline lakes extraordinarily clear and limpid. The organic 

 deposits of salt lakes are practically nothing, for brackish water is 

 not favourable to many organisms and in dense brines very few 

 animals or plants can exist, and those that can are not the kinds 

 which give rise to peat, or to siliceous or calcareous deposits. For 

 the same reason, the deposits of whatever kind, laid down in salt 

 lakes, are almost barren of fossils, except of land animals and 

 plants, such as are washed into the lake by flooded streams. 



The chemical deposits are much the most interesting and char- 

 acteristic 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 in 

 different lakes, but, in general, it follows the inverse order of 



