212 GEOLOGICAL UISTORY OF LAKE LAHONTAN. 



removed, leaving the waters in the coiulitiou of a mother-liquur, character- 

 ized by the presence of the more deliquescent salts. Let us consider the 

 probable eflfect of each of these conditions on the character of the calcium 

 carbonate subsequentl_y deposited. 



If the first case, we should expect that there would be but a slight, 

 if any, deposition of tufa in the rising lake. If precipitation of calcium 

 carbonate did take place, however, it would be expected to have the same 

 characteristics as the tufa formed during the first high-water period, but as 

 the thinolite is markedly different from the lithoid tufa we may disregard 

 this postulate. 



If the second were true, and all the j)recipitated salts were redissolved, 

 the previous condition of the lake would be practically re-established, and 

 the ensuing deposits of tufa coidd not be expected to diff"er from that pre- 

 viously precipitated. 



If the third were true, a change in the nature of the lake when the 

 basin Avas partially refilled would result. The first salts to be deposited 

 from a brine obtained by the concentration of waters like those found in 

 the rivers of the Lahontan basin would be calcium carbonate, calcium sul- 

 phate, and sodium chloride. As calcium carbonate had already been depos- 

 ited in immense quantities, and the per cent, of calcium sulphate was prob- 

 ably small, the principal salt that would be precipitated upon a partial 

 crystallization of the substances dissolved in the concentrated lake water 

 would be sodium chloride. If the precipitation and burial of the mineral 

 substances in solution had been stopped at this stage and the basin partially- 

 refilled, the resulting lake would have been characterized by the presence 

 of more soluble salts among which the alkaline carbonates would have pre- 

 dominated. The calcium carbonate subsequently contributed by streams 

 and springs as the lake rose would have been precipitated under different 

 conditions than had previously obtained, and this might have caused it to 

 assume a diff"erent crystalline form. Thus the postulate of a partial desic- 

 cation of the basin agrees best with the facts observed. 



As a qualification of the second hypothesis, we miglit assume, that 

 during the time intervening between the formation of the lithoid and thino- 

 litic tufas concentration was continueil for a long period without the depo- 



