252 GEOLOGICAL HISTORY OF LAKE LAHONTAN. 



about by a partial deposition of the saline matter accumulated during the 

 first high-water stage, at the time of the inter-Lahontan period of desiccation. 

 The tufa superimposed upon the lithoid variety is known as thinolite; it is 

 composed of well-defined crystals and is without fossils. It was evidently 

 precipitated from a more highly concentrated chemical solution than that 

 from which the lithoid variety was deposited. That this was the case is 

 rendered evident, since the crystalline variety occurs only low down in the 

 basin, while the lithoid tufa may be found within 30 feet of the highest 

 terrace carved by the waters of the ancient lake. 



After the crystallization of thinolite had been carried on for an indefi- 

 nite period, the lake rose to within 180 feet of its first maximum, and the 

 heaviest deposit of calcium carbonate found in the basin was precipitated. 

 During this stage the lake was not strongly saline, as is shown by the abund- 

 ance of gasteropod shells obtained from the sediments and tufas accumulated 

 during this portion of its history. 



After the precipitation of the dendritic tufa, the lake continued to 

 rise and at last reached a horizon 30 feet higher than the first maximum. 

 During this expansion the waters lingered but a comparatively brief time at 

 the highest level and then slowly subsided. The increase in depth after the 

 deposition of dendritic tufa is shown by the presence of lacustral sediments 

 upon that deposit. The structure of the higher bars and embankments 

 about the border of the old lake basin, proves conclusively that the greatest 

 lake expansion was during the second rise. 



With the last recession of the lake all portions of its basin were brought 

 within the reacli of wave action, and the tufa deposits sheathing its interior 

 were broken, and the fragments swept away by currents, and built into em- 

 bankments and terraces. The waters continued to fall until the basin was 

 completely dry. All the salts not previously precipitated were deposited 

 as desiccation advanced, and became buried and absorbed by playa clays. 

 The proof of the occurrence of this time of desiccation is furnished by the 

 comparatively fresh condition of the existing lakes of the basin, and by the 

 change in the molluscan faima which took place since the last high-water 

 period. '^Flie dnnition of tlii.s post-Laliontan arid period is unknown, but 



