224 GEOLOGICAL HISTORY OF LAKE LAHONTAN. 



THE FRESHENIlSrG OF LAKES BY DESICCATION. 



The apparently anomalous phenomena of the desiccation of a great 

 lake without leaving a surfoce deposit of salt, seems explicable in only one 

 way. Adopting the suggestion advanced by Mr. Gilbert in explanation of 

 some portion of the history of Lake Bonneville, the absence of saline 

 deposits is accounted for by the hypothesis that they were buried and 

 absorbed by lacustral clays and playa deposits during periods of desiccation. 



The freshening of a lake by desiccation may be illustrated in all its 

 stages in the various basins that have been examined in the Far West. 

 A lake after a long period of concentration becomes strongly saline, and 

 finally evaporates to dryness, leaving a deposit of various salts over its 

 bed. During the rainy season the bottom of the basin is converted into a 

 shallow lake of brine which deposits a layer of sediment; on evaporating 

 to dryness, during the succeeding arid season, a stratum of salt is deposited 

 which is, in its turn, covered by sediment during the succeeding rainy 

 season. This process taking place year after year results in the formation 

 of a stratified deposit consisting of salts and saline claj's in alternating- 

 layers. The saline deposits may thus become more and more earthy until 

 the entire annual accumulation consists of clays. The site of the former 

 lake then becomes a playa. A return of humid conditions would refill a 

 basin of this character, and might form a Iresh-water lake, the bottom of 

 which would be the level surface of the submerged playa. 



The larger lakes of the Lahontan basin, as well as a number of less 

 importance in eastern Nevada and southern Oregon, are Avithout outlet 

 Thev occur in basins that in almost all cases were occupied by much 

 larger water-bodies during the Quaternary, which, like their modern repre- 

 sentatives, never overflowed. From the long period of evaporation that 

 has taken place, one would expect the existing lakes to be dense mother- 

 liquors. The fact is, however, that tJiey are but slightly charged with 

 saline niatter, and in some instances are sweet to the taste and sufficiently 

 fresh for all culinary pur})oses. In many localities the lacustral beds sur- 

 rounding and underlying the present lakes are highly charged with soda 

 salts, which rise to the surface during the dry season as efflorescences. 



