102 



GEOLOGICAL HISTOEY OF LAKE LAHONTAN. 



Between the dendritic terrace and the surface of Pyramid Lake there 

 is a broad platform, which is the strongest and best defined of all the La- 

 hontan water-lines. It marks the upper limit of a third variety of tufa, 

 known as " Thinolite"; we call it, therefore, the " Thinolite Terrace." Its 

 elevation is about 110 feet above the level of Pyramid Lake in 1882. This 

 terrace has l)een found to extend entirely around the valleys occupied by 

 Pyramid and AVinnemucca lakes, and may also be followed, though with 

 less certainty, along the borders of Black Rock, Smoke Creek, and Carson 

 deserts. 



The terraces we have named, together witli the present level of Pyra- 

 mid Lake, furnish four definite horizons that will be found convenient refer- 

 ence plains in tracing the Quaternary history of the basin. It is onl}' at 

 exceptional localities, however, that these terraces can be followed for anjr 

 considerable distance, and at only a few points could a sequence like the 

 one shown below be obtained from actual measurements. Our diag'ram is 

 a generalized profile of the Lahontan shores. 



Feet. 



Lahontan boacli 530 



Litboid terrace 500 



Dondritic terrace 320 



Tbinolitf terrace 110 



Suilaceof Pyramid Lake (1882)--.. 



Fig. 14 Generalized profile of Labontan shores. 



The relative age of the various water-lines shown in the diagram will 

 be discussed in connection Avhen the chemical history of the lake is consid- 

 ered. 



The highest terrace of all, the Lahontan, is an inconspicuous feature 

 in itself, but it is important as forming the boundary between subaerial and 

 subaqueous sculpture on the sides of the valleys. It usually appears as a 



