EfiSUMfi OF CHEMICAL HISTORY. 237 



and sedimentary deposits (see Chap. IV), and then evaporated away prob- 

 ably to complete desiccation. 



During this last subsidence a thin coating of coral-like tufa was 

 formed, followed by the crystallization of a comparatively limited quantity 

 of thinolite. 



When the lake approached complete desiccation after the post-den- 

 dritic rise, it became divided into a number of independent areas, as during 

 the inter-Lahontan subsidence. It is presumed that all these basins 

 became completely desiccated, probably for a long term of years, and that 

 the salts precipitated were buried beneath playa deposits so completely that 

 when some of the basins were partially refilled, the salts were not redis- 

 solved. This period of desiccation — as determined by calculating the time 

 that would be required for the existing lakes to attain their present de- 

 gree of salinity — is tliouglit to have terminated not more than three liun- 

 dred years since. 



Fig. St.— Curve exhibiting tlie rise and fall of Lake Lahontan: a a, deposition of lithoid tufa; b, deposition of thinolitic 

 tofa ; c, deposition of dendritic tufa. Tbo tigm'es indicate, in feet, the tluttuai kma nf the ancient lake above and bo- 

 low the 1882 level of Pyramid Lake. 



If we project the fluctuations of Lake Lahontan in a curve (Fig. 31), 

 the ordinates representing depths of the lake at various stages, and the 

 abscissas succession in time, we find there are two maxima and two minima. 

 We know that the first of the two high-water periods was the longer con- 

 tinued, for the terraces the waves then cut in the rocks are broader and 

 more strongly marked than the terraces recording the second rise. The 

 second high-water period was of shorter duration, but the lake rose to a 

 higher level than at the first filling. 



The salts impregnating the Lahontan sediments, which are now car- 

 ried to the surface as efflorescences, are believed to have been absorbed 

 from the waters of the ancient lake by the clays and marls forming its 

 bottom, when the lake was greatly concentrated by evaporation. 



