PLAYAS AND SALINAS 603 



and the southern United States), the tak\r or schala of Asia, or the 

 sebcha of Africa. Russell has described a number of such tempo- 

 rary playa lakes from the«western United States (49:50). He finds 

 them a characteristic feature of the greater part of the valleys of 

 Nevada, the largest being in the Black Rock Desert in the north- 

 western part of the state. It forms during the winter months and 

 reaches an area of from 450 to 500 square miles, but is seldom over 

 a few inches in depth. Often after storms it is a vast sheet of 

 liquid mud, a characteristic of many playa lakes. In a few hours 

 or a few days the water of the lake may all evaporate, leaving a 

 hard, dry and absolutely barren surface, cracked in all directions 

 as the surface contracts in drying. "The lake beds then have a 

 striking resemblance to tesselated pavements of cream-colored mar- 

 ble, and soon become so hard that they ring beneath the hoof-beats 

 of a galloping horse, but retain scarcely a trace of his foot-prints." 

 Around the margin of the lakes is a belt of plain with desert vege- 

 tation, the transition to which is formed by a marshy tract which 

 in summer is marked by an abundant efflorescence of salts. 



Mechanical analysis has shown that the material of the playa 

 may be 100 per cent, clay, and that laterally it will gradually pass- 

 through the addition of sands into the surrounding eolian deposits. 

 In limestone regions where siliceous rocks are wanting the malie- 

 rial of the playa will be largely lime mud, and this may be the 

 origin of some of the finely bedded, sun-cracked calcilutytes of the 

 American Siluric, where the percentage of lime and magnesium 

 carbonates is seventy or less. 



If the playa lake exists for some time it may become stocked 

 with certain forms of organisms, especially types whose eggs or 

 larvas can be transported by wind or by birds. The small crusta- 

 ceans Estheria, Daphnia, and Cypris are characteristic of desert 

 lakes, the first having been found in ponds which are dry for 

 eleven successive months. (Fischer, quoted by Walther-57 :p^.) 

 When, as is frequently the case, salts are present in the sediment, 

 these effloresce on the surface, and from their hygroscopic charac- 

 ter keep the surface of the playa sufficiently moist to prevent the 

 removal by the wind of the accumulated material, and further to 

 catch all dust particles carried across the . surface by the winds. 

 Thus the surface of the playa becomes dusted over with a fine coat- 

 ing of sand or dust, this process being repeated as the salts rise to the 

 surface of the newly added layer. Where salt is present in great 

 abundance a moist, slippery surface with incrustations of salt re- 

 sults, thus forming salinas. When wet, their surface is impassable, 

 but when drv a crust of hard salt of dazzling whiteness characterizes 



