THE WORK OF STREAMS 13 5 



is in the Black Rock desert and is 450 to 500 square miles in area, 

 although seldom more than a few inches deep. Such temporary 

 desert lakes are called play as. Their beds, when dry, are covered with 

 fine clay and sand, and sometimes with gypsum and salt. On the 

 mud of ancient playa beds the footprints of extinct animals have 

 been preserved (p. 379). 



Salt Lakes. — A salt lake may be formed (1) by the cutting off 

 of an arm of the sea by a delta, as in the case of the Salton Sea, 

 California (p. 132), or by an elevation of the sea bottom, which 

 isolates a body of water. Under such conditions, the water will, 

 at first, have the same composition as sea water. If, however, the 

 water flowing into the lake exceeds the evaporation of its surface, 

 it will gradually be freshened. Such was the history of Lake Cham- 

 plain. If, on the other hand, such a lake has been formed in a 

 desert region where evaporation is excessive, the water will become 

 more salty as time goes on. The Caspian Sea was formerly con- 

 nected with the Black Sea, but is now isolated and is growing more 

 salty. 



(2) Salt lakes are also formed by the concentration of fresh water. 

 Basins in arid regions which do not receive enough water to cause 

 them to overflow may, in time, become saturated with salts of various 

 kinds. The streams bring in common salt (NaCl), gypsum 

 (CaS0 4 '2H 2 0), Epsom salt (MgS0 4 -7H 2 0), and calcium carbonate 

 (CaC0 3 ), which they obtain from the rocks over which they flow. 

 These salts may accumulate in the lake as evaporation proceeds, until 

 the water becomes so concentrated that they are precipitated. Iron 

 oxide and calcium carbonate will be deposited first; upon further 

 concentration, gypsum, which is insoluble in strong brine, will be 

 precipitated ; then common salt and Glauber salts (Na 2 S04), in 

 the order of their solubility. This order is often interfered with 

 under certain conditions. Cold weather, for example, will cause the 

 precipitation of Glauber salts (Na 2 S04) before the common salt has 

 all been precipitated. If the evaporation of the surface of the salt 

 lake does not equal the amount of water received during a wet season, 

 the deposition of gypsum and salt will cease, and the beds of salt 

 may be covered by the sediment brought in by the streams. With 

 the recurrence of the dry season the deposit of gypsum and salt will 

 commence again. Many alternations of mud and salt are encoun- 

 tered in wells sunk on the margins of salt lakes. In some salt lakes 

 most of the salt has been deposited, and the liquid remaining, called 



