204 



GEOLOGICAL HISTORY OF LAKE LAHONTAK 



SUCCESSION" OF TUFA DEPOSITS. 



The relation of the three varieties of tufa to eacli other, and the man- 

 ner of their occurrence on the interior of the Lahontan basin, are indica- 

 ted in the followinon ideal section of the lake shore : 



Fig. 28.- — Diagram sbowiug succession of tufa deposits. 



The first formed deposit, lithoid tufa, extends upward about 500 feet 

 above the horizontal lake-beds occupying the l:)ottoni of the basin. The 

 second deposit, thinolitic tufa, finds its upper limit about 100 feet above the 

 present level of Pyramid Lake, The third and last, dendritic tufa, extends 

 npwards to witliin approximately 200 feet of the highest shore-line. The 

 lower limits of the tufas cannot he determined with accuracy, as they are 

 concealed by lacustral sediments. 



Considering these deposits by themselves, we learn that Lake Lahon- 

 tan rose to about the level of the lithoid terrace, and then evaporated away 

 to a horizon certainly somewhat lower than the present level of Pyramid 

 Lake. During this evaporation the lithoid tufa was deposite'd, and evi- 

 dently owes its precipitation directly to the concentration of the lake waters. 

 The lake was then refilled to the level of the thinolite terrace, where it 

 must have maintained a nearly constant horizon for a long time. Concen- 

 tration by evaporation continiied and the deposition of the crystals after 

 which thinolite is a pseudomorph, took place. From this horizon the lake 

 surface was carried upwards about 220 feet, with many oscillations, and 

 for a long period deposited dendritic tufa. Subsequently the basin was 

 more completely filled, as is shown by the lacustral beds that occur above 



