170 GEOLOGICAL HISTOEY OF LAKE LAHONTAN. 



chemical questions connected with the present study, it may be well to see 

 how far our observations relating to the physical history of Lake Lahontan 

 can be correlated. 



The presence of vast alluvial slopes of pre-Lahontan date, on which 

 the water-lines of the old lake are traced, leads to the conclusion that the 

 climate of the region was arid for a long time previous to the first filling 

 of the basin of which we have any definite record, viz., the earlier high- 

 water stage of Lake Lahontan. The discussion of this question, however, 

 falls more properly in the chapter devoted to the consideration of Quater- 

 nary climate. We assume, for the present, that a change from arid to 

 more humid conditions caused the Lahontan basin to be filled to the level 

 of the lithoid terrace, and to remain at that horizon long enough to enable 

 its waves to excavate u broad shelf in the rocky shores. The terraces 

 above the lithoid are of subsequent date, as is shown by the section of the 

 higher water-lines given on page 103; as there indicated the lithoid terrace 

 is frequently a shelf cut in the rock, on which rest the built terraces that 

 define the Lahontan beach. At other localities the lithoid terrace is rep- 

 resented by gravel embankments that are overplaced by much smaller 

 structures of the same character at the level of the highest water-line. 

 Cumulative evidence of this nature shows that the lake lingered at the 

 horizon of the lithoid terrace for a much longer time than at the higher 

 levels. The lithoid terrace and the Lahontan beach thus record two 

 independent high-water stages The fluctuations of the lake during the 

 interval between the formation of these water-lines cannot be determined 

 from the physical records alone, but are not difficult to sketch, at least in 

 outline, when the tufas that were precipitated from the waters of the lake 

 are considered. Turning to the stratified beds accumulated in the lake 

 basin, we find two series of fine lacustral sediments separated by a widely- 

 spread sheet of water-worn and current-bedded sands and gravels which 

 were evidently deposited in shallow water. This record of two lake 

 periods, with a time intervening when the basin was at least as nearly 

 desiccated as at the present day, is perhaps the most positive of all the 

 chapters of Lahontan history. That the formation of these two deposits 

 of lake sediments may be correlated in time with the formation of the 



