236 GEOLOGICAL HISTORY OF LAKE LAHONTAN. 



RfiSUMlE OF CHEMICAL HISTORY. 



The fluctuations of Lake Lahontan, so far as we have been able to 

 determine from the study of its chemical records, may be briefly summar- 

 ized as follows: 



The waters first formed a fresh-water lake having approximately the 

 outline represented on the accompanying map''"' — which indicates the 

 extent of the lake at the highest stage of all — and then evaporated away 

 with many minor oscillations, until a greater degree of desiccation of the 

 basin than the present was attained. During this oscillation the waters 

 were saturated with calcium carbonate and deposited vast quantities ot 

 lithoid tufa. 



Whether the lake evaporated to dryness or not during the time inter- 

 vening between the formation of the lithoid and thinolitic tufa remains un- 

 determined. The contrast in the character of the tufa formed before and 

 after this event is thought to indicate a partial evaporation, or perhaps 

 complete desiccation, with the burial of the less soluble salts This is the 

 inter-Lahontan period of desiccation. 



The waters next rose to about the level of the thinolite terrace, with 

 many fluctuations, and formed at least two and probably three independ- 

 ent water-bodies, which were more highly charged with saline matter than 

 during the first expansion. From this solution, which was probably nearly 

 identical in the various basins, the mineral after which the thinolite is a 

 pseudomorph was crystallized. 



The thinoHtic stage was closed by a rise of the lake. The waters 

 were diluted, but probably stilj contained a larger per cent, of saline matter 

 than during the lithoid stage, and the third or dendritic variety of tufa was 

 deposited on an immense scale, but did not attain as great an elevation on 

 the sides of tlie basin as the first formed tufa. 



During these three major oscillations there were many minor fluctua- 

 tions of level, as is proven by the large number of variations in the tufas 

 formed. 



After the precipitation of the dendritic tufa the lake rose higher than 

 ever before, the evidence being furnished by gravel embankments, terraces, 



"■In pocket at ciul of vohiiiui. 



