RESUME. 251 



was the first rise of Lake Laliontan. Like all inclosed lakes it must have 

 fluctuated in depth and extent with the alternation of arid and humid seasons, 

 and risen and subsided also in i-esponse to more general climatic oscillations, 

 which extended through years and perhaps embraced centuries. Finally 

 the climatic conditions which favored lake expansion ceased, and a time of 

 aridity, like that which preceded the first rise, was initiated. The lake 

 slowly contracted until its basin reached a greater degree of desiccation 

 than that now prevailing. This was the inter-Lahontan period of desiccation. 



During the first rise lacustral marls and clays were deposited through- 

 out the basin; the depth of these is unknown, but they certainly exceed 

 150 feet in thickness. The waters were saturated with calcium carbonate 

 and the precipitation of great quantities of compact stony tufa took place. 

 Deposits of tufa were formed on rocky slopes throughout the basin, and 

 are not especially abundant at the mouths of streams. This is thought to 

 indicate that although the waters were saturated with calcium carbonate, 

 they were not highly charged with other chemical substances. This con- 

 clusion is sustained by observation of conditions under which a similar tufa 

 is being deposited in existing lakes, and also by the presence of gasteropod 

 shells in the lithoid tufa in great abundance. 



The time of low water, and perhaps of complete desiccation, that suc- 

 ceeded the first rise of Lake Lahontan, is recorded by stream channels 

 carved in the lacustral beds and by cun-ent-bedded gravels and sands super- 

 imposed upon previously formed strata. Sections of inter-Lahontan gravel 

 deposits have been observed wherever the material filling the lake basin is 

 well exposed, and furnish indisputable evidence that the lake was greatly 

 lowered before the gravels were deposited. These gravels were in turn 

 covered by a second lacustral deposit, thus forming a tripartite series, a 

 counterpart of which exists in the Bonneville basin. The first formed tufa 

 deposit was exposed to subaerial erosion during the inter-Lahontan period 

 of low water and became broken and defaced. 



The character of the next succeeding tufa deposit indicates that a 

 change had taken place in the chemical conditions of the waters of the lake 

 when the basin was again partially flooded. This alteration in the com- 

 position of the salts dissolved in tlie lake is thought to have been brought 



