230 GEOLOGICAL HISTOEY OF LAKE LAHONTAN. 



oscillations; while one I'eceiving the drainage from a lofty mountain range 

 may be but slightly lowered by a climatic change that would produce des- 

 iccation in a neighboring valley. From this and other allied reasons, Great 

 Salt Lake may not have been evaporated to dryness during the time that 

 the lakes of western Nevada completely disappeai-ed. 



The only analysis we have of the waters of Sevier Lake gives 8.64 

 per cent, as the total of saline matter in solution. The sample analyzed 

 was collected in 1872 ; ten years later the lake was almost completely des- 

 iccated, and its site converted into a field of salt. We have classed this as 

 a playa lake, and do not consider it of importance in the present discussion) 

 as it not only varies greatly in salinity, owing to variations in volume, but 

 is also so situated that it receives the drainage of a broad desert and must 

 receive large quantities of saline matter from the efflorescences formed each 

 year on the neighboring land surface. 



A comparison of the moUuscan life of Pyramid and Walker lakes with 

 Lahontan fossils indicates that a marked change has taken place in the 

 fauna of the basin since the last high-water stage of the old lake. This 

 question is considered in the chapter devoted to the life history of Lake 

 Lahontan. 



Section 4.— EFFLORESCENCES. 



In the preceding pages we have had occasion to speak of the saline 

 incrustations, or efflorescences, to be seen over large areas in the Lahontan 

 basin. It is now our purpose to describe these accumulations more full3^ 

 They originate in the saline lacustral clays which floor all the valleys once 

 occupied by the ancient lake, and usually occur in greatest abundance on 

 the borders of the larger deserts, where the}^ not uncommonly whiten the 

 surface over many square miles. In tracing their distribution it is notice- 

 able that they occur most abundantly in those portions of the valleys that 

 are underlaid by the clays deposited directly from suspension in the ancient 

 lake, but are not found on the surfaces of many of the more modern playas 

 which occupy the lowest depressions in the various basins, thus showing 

 that the recently formed playa-beds are in many instances less saline than 

 the true lacustral clays. 



