268 



GEOLOGICAL HISTORY OF LAKE LAHONTAN. 



it follows that both series of phenomena resulted from a common climatic 

 change. In the case of Lake Lahontan we have attempted to demonstrate 

 that the change which caused the expansion of the lake was a lowering in 

 the mean annual temperature, and that the periods of desiccation indicate 

 a relative rise of temperature. This interpi'etation is in harmony with the 

 verdict of the preat majority of writers in reference to the prevailing ele- 

 ments in the climate of the Glacial epoch. In former times, as at present, 

 the climate of various regions in the same latitude differed widely in refer- 

 ence to humidity. Tlie more humid regions were the areas of greatest 

 glaciation. 



The discussion of tlie ultimate cause of the cold of the Glacial epoch is 

 beyond the scope of the present report. 



A summary of the writer's conclusions in reference to the climatic oscil- 

 lations indicated by the fluctuations of Lake Lahontan is embodied in the 

 following schedule : 



1. Pre-Lahontan arid period. 



Probable climatic comlitioDS- 



A time of aridity ; precipitation sniall ; evaporation 

 rapid ; temperatnre bigb. 



Kesnlta . 



r Lake 

 • i glai 



[ Lakes small, at times desiccated; mountains free from 

 glaciers. 



2. First rise of Lake Labontau . . i 



I 



f Probable climatjc conditions. 



I 



3. Inter-Lahontan arid period . . . 



(.Results 



Probable climatic conditions. 



Keaolts 



f Precipitation moderatt^; evaporation decreased ; tem- 

 ( peratare low. 



f Large lakes in the valleys and glaciers in the monnt- 

 ■ \ ains. 



4. Second rise of Lake Lahontan ■{ 



I 



f Probable climatic conditions. 



I 



{ Kesults 



C Probable climatic conditions. 



5. Poat-Lahontan arid period < 



I Kesalts . 



e Decreased precipitation ; evaporation rapid ; tempera- 

 ( ture high. 



(Lakes smaller than at present, and at times possibly 

 desiccated ; glaciers contracted and possibly com- 

 t pletely melted. 



( Precipitation moderate, but probably more copious than 

 " -J during the first rise j evaporation decreased ; temper- 

 *■ ature low. 



. Broad lakes and large glaciers. 



f A time of great aridity ; precipitation small ; mean 

 \ temperature higher than at present. 



Lakes desiccated and glaciers melted. 



6. Present time . 



i Climatic conditions f Precipitation small ; evaporation rapid; mean temper- 

 * atnre about 50° Fahr. 

 r Country arid ; rivers sniall and fluctuating ; lakes and 

 ^«^°"^ \ glaciers small. 



