FISHES, I.AHONTAN SYSTEM OF NEVADA AND NORTHEASTERN CALIFORNIA. 39 



calcium carbonate, and the precipitation of great quantities of compact stony tufa took place. Deposits 

 of tufa were formed on rocky slopes throughout the basin and are not especially abundant at the mouths 

 of streams. This is thought to indicate that although the waters were satiu'ated with calcium carbonate 

 they were not highly charged with other chemical substances. This conclusion is sustained by obser- 

 vation of conditions under which a similar tufa is being deposited in existing lakes, and also by the 

 presence of gasteropod shells in the lithoid tufa in great abundance. 



The time of low water, and perhaps of complete desiccation, that succeeded the first rise of Lake 

 Lahontan is recorded by stream channels carved in lacustral beds and by current-bedded gravels and 

 sands superimposed upon previously formed strata. Sections of inter-Lahontan gravel deposits have 

 been observed wherever the material filling the lake basin is well exposed, and furnish indisputable 

 evidence that the lake was greatly lowered before the gravels were deposited. These gravels were in 

 turn covered by a second lacustral deposit, thus forming a tripartite series, a counterpart of which exists 

 in the Bonneville Basin. The first formed tufa deposit was exposed to subaerial erosion during the 

 inter-Lahontan period of low water and became broken and defaced. 



The character of the next succeeding tufa deposit indicates that a change had taken place in the 

 chemical conditions of the waters of the lake when the basin was again partially flooded. The alteration 

 in the composition of the salts dissolved in the lake is thought to have been brought 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 thino- 

 lite; 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 was 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 terraces carved by the waters 

 of the ancient lake. 



After the crystallization of thinolite had been carried on for an indefinite 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 abundance of 

 gasteropod shells obtained from the sediments and tufas accumulated diuring this period 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 compara- 

 tively brief time at the highest level and then slowly subsided. The increase in depth after the depo- 

 sition of dendritic tufa is shown by the presence of lacustral sediments upon that deposit. The struc- 

 ture of the higher bars and embankments about the border of the old lake basin proves conclusively 

 that the greatest lake expansion was diying the second rise. 



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

 action, and the tufa deposits sheathing its interior were broken and the fragments swept away by 

 currents and built into embankments 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 mol- 

 luscan fauna which took place since the last high-water period. The duration of this post-Lahontan arid 

 period is unknown, but it was finally terminated — probably less than 300 years since — by an increase in 

 humidity. The present lakes then commenced their extstetu:e. 



It is evident, then, that Russell found, first, that I/ake Lahontan had no outlet 

 and, second, that there was a pre-Lahontan and a post-Lahontan period of complete 

 desiccation, the latter prevailing to within the last 300 years. 



The fish fauna of the Lahontan system may be termed characteristic, most of its 

 species being distinct from related forms found elsewhere. One genus, Leucidius, is 

 restricted to the system. These circumstances argue in favor of long isolation. Fur- 

 thermore, the Lahontan species do not appear to be related to others of any particular 

 system, contrary to what would be expected if the relatively recent Lake Lahontan 

 had discharged its waters into another basin." It thus appears that Russell's conclu- 



o The cases of Lake Bonneville and Malheur Lakes may be cited. 



