4 GEOLOGICAL UlSTOKY OF LAKE LAHONTAN. 



Chiipter V (on the chemical history of Lake Lahontan) is also divided 

 into sections. 



Section 1 treats of. the general clieniistry of natural waters as they 

 occur in streams, springs, lakes, oceans, and inclosed lakes or seas, and is 

 an introduction to the chemical liistory of Lake Lahontan. 



Section 2 is an account of the tufas precipitated from tlie water of the 

 lake. These present three main divisions, named, respectively, "Lithoid," 

 "Thinolitic," and "Dendritic." The first is a compact, stony variety, and is 

 the oldest of the principal calcareous deposits that sheath the interior of the 

 basin. It occurs from a horizon thirty feet below the Lahontan beach all 

 the way down the sides of the basin to the lowest point now exposed to view 

 (page 190). ThiiKilite is composed of crystals, and was formed in the ancient 

 lake when it was greatl}^ reduced by evaporation ; its upper limit is about 

 400 feet below the Lahontan beach (page 192). Dendritic tufa has a branch- 

 ing or dendritic structure, whence its name; it is superimposed upon the 

 ]:)reviously-formed varieties. Its upper limit is 180 feet below the Lahontan 

 beach (page 201). The aggregate thickness of the tufa deposits is from 

 thirty to perhaps fifty or seventy-five feet. Chemical analyses show that 

 all the varieties are composed of somewhat impure calcium carbonate. Fol- 

 lowing the description of these deposits is a discussion of the conditions 

 favoring the deposition of calcareous tufa from lake waters (page 210). 



Section 3 considers the salts precipitated from the waters of the lake 

 when evaporation took place, and discusses the manner in which lakes may 

 be freshened by desiccation (])age 223). 



Section 4 contains an account of tlie efflorescences now forming on the 

 surface of the deserts in the Lahontan Basin, and presents a brief descrip- 

 tion of the more valuable salt-works of the region, which ai*e all supplied 

 by the salts contained in Lahontan sediments (page 230). 



Chapter VI presents the life history of the ancient lake as determined 

 from the abundant molluscan remains and other fossils that have been found. 

 The shells show that the lake was fresh throughout its higher stages. During 

 the period when thinolite was formed it seems to have been too concentrated 

 to admit of the existence of molluscan life, as no fossils have be6n found in 

 that deposit. A chi])i)C(l ini[)lement discovered in the upper lacustral beds 



