84 GEOLOGICAL HISTORY OF LxVKE LAHONTAN, 



An examination of a sample from another playa gave less than 3 per 

 cent, soluble in water, consisting principally of sodium carbonate, calcium 

 carbonate, and common salt. 



It is not to be expected tliat all deposits of this character would have 

 even approximately the same composition, but the conclusion arrived at in 

 the field, that they are the result of both mechanical and chemical processes, 

 is strengthened by the analyses that have been made. In some instances 

 easily soluble salts form a large percentage of the deposit, which then 

 becomes a salt-field, a bed of gypsum, or is largely composed of other simi- 

 lar salts. At times these deposits become covered with mechanical sedi- 

 ments, and perhaps buried so deeply that they are not again dissolved when 

 the basin is reoccupied by a lake. All stages in this process, which, in fact, 

 is the closing chapter in the history of many lakes, may be observed in the 

 arid region of the Far West. 



Playas in which the mechanical deposits greatly predominate are the 

 most common, and may be studied in a large number of the desert- valleys 

 of Utah and Nevada. Examples of salt-playas are numerous, especially 

 in Southern Nevada, where they are of economic importance, and, besides 

 common salt, frequently contain large quantities of sodium sulphate and 

 carbonate, borax, etc. In some instances the lower portions of earthy 

 playas are saturated with brine — as is the case in Diamond Valley, Nevada — 

 which, when raised to the surface and evaporated, is capable of supphnng 

 an almost unlimited quantity of salt. One of the most instructive playas 

 in the Great Basin is situated in Utah, a few miles southward of Fillmore. 

 In this instance the water entering the basin and partially flooding it during 

 the rainy seasons is probably charged with calcium sulphate in excess of 

 all other salts, and on evaporating leaves a deposit of crystallized g3^psum, 

 or selenite, which is now approximately 12 square miles in extent, and has 

 been penetrated to the depth of 6 feet without revealing its entire thick- 

 ness. The salts more soluble than gypsum, which nuist have been 

 deposited by the waters covering the playa at various times, have appa- 

 rently been flooded out b)" an overflow of the basin, thus leaving the sele- 

 nite in a remarkably pure condition. The small crystals of selenite swept 

 from the surface of the deposit liy the wind have been accumulated in im- 



