206 GEOLOGICAL HISTORY OF LAKE LAHONTAK 



In the valley of the Humboldt, near Humboldt Lake, a thin deposit of 

 yellowisli, coral-like tufa has been observed coating domes of the dendritic 

 variety, which is quite different from any of the lower tufas occurring' in 

 the basin. 



From their position at the top of the series we refer both of the varie- 

 ties of tufa described above to the last evaporation of Lake Lahontan. The 

 coral-like form was deposited when the lake filled its basin up to within 

 80 or 100 feet of the Lahontan beach; and the thinolite was crystallized 

 when evaporation had lowered its surface so greatly that it became divided 

 into separate basins, one of which, the Smoke Creek Desert, was completely 

 desiccated. 



About Pyramid Lake, where the records of Lahontan history are most 

 complete, these evidences of the last recession of the waters have not been 

 satisfactorily observed. The absence of the second deposit of thinolite in 

 this basin is possibly due to the depth of the waters that occupied it, which 

 did not reach a sufficient degree of concentration to admit of the formation 

 of thinolite crystals, at least not in that portion of the basin now open to 

 inspection. 'Observation has thus far been unable to show conclusively 

 that coral-like tufa similar to that fomied at a recent date in the Humboldt 

 Valley, occurs at other localities, but an outer coating on many of the 

 domes about Pyramid Lake is very similar and is probably of the same 

 date. Its general absence may perhaps be accounted for in many locali- 

 ties by assuming that it would be the first of the tufas to be removed by 

 erosion, after the evaporation of the waters in which it was deposited. Our 

 information regarding the later-formed varieties of tufa is but fragmentary, 

 and does not afford as complete a record of the post-dendritic oscillations 

 of the lake as could be desired. 



Carrying our study of tufa deposits one step nearer the present, we find 

 that the rocks and tufa-crags about Pyramid Lake are coated with a thin de- 

 posit of calcium carbonate, in the form of compact gray tufa, up to the height 

 of about 12 feet above the level of the lake in 1 882. This sheathing also de- 

 scends beneath the lake surface and, judging from its freshness, it is evident 

 that it is still in process of formation. The similai'ity between the tufa now 

 forming and the variety first deposited from the waters of Lake Lahontan^ 



