THE PLEISTOCENE OR GLACIAL PERIOD. 463 



to the extent of more than 300 feet; that is, some parts of the Bonne- 

 ville shore line are more than 300 feet higher than others (Fig. 540). 

 This deformation affects even the later and lower shore lines, and 

 stands in no intimate relation to the faulting of the region. 



Lake Lahontan. 1 — Farther west, but still in the area of the Great 

 Basin, were other lakes, probably contemporaneous with Bonneville. 

 The largest was Lake Lahontan, a lake of most irregular outline (Figs. 536 

 and 547), the history of which was similar to that of Lake Bonneville. 

 The basin of Lake Lahontan is thought to have been due to the dis- 

 placement of faulted blocks. As in the case of Bonneville, a condi- 

 tion of aridity preceded the lake. When increased humidity brought 



^ J; 





Fig. 544. — Section of Lahontan sediments, near Agency Bridge, Truckee Canyon 

 Nev. (Russell, U. S. Geol. Surv.) 



the lake into being, its waters rose until they covered an area of nearly 

 9000 square miles. This stage of the lake, like the first stage of Lake 

 Bonneville, was followed by a period when the lake nearly or quite 

 disappeared. Later, it was restored, and its waters rose about 30 

 feet higher than before, but did not find an outlet. The two stages of 

 high water in Lake Bonneville and Lahontan have been thought to 

 correspond with epochs of glaciation in the adjacent mountain regions. 



At some stages of the lake's history, the condition of the water 

 was such as to allow mollusks to live in it, while at other stages it 

 appears to have been so saline as to have prevented its habitation. 

 These facts point to considerable fluctuations in the climate during 

 the history of the lake. 



The deposits in Lake Lahontan are comparable to those in Lake 

 Bonneville (Figs. 543 and 544), but among the clastic sediments are 

 found thin beds of volcanic ash, and the relative importance of the 

 chemical precipitates is greater. The main precipitate was calcium 



1 Russell, Mono. XI, U. S. GeoL Surv. 



