HUMIDITY OF THE QUATERNAKY. 



261 



Considerations of this character miglit be multiphed, Init it is presumed 

 that enough has ah-eady been written to show that the climatic change 

 which gave origin to Lake Lahontan but did not permit it to overflow, must 

 have been one of moderate precipitation in comparison, for example, with 

 the present rainfall of the region of the Laurentian lakes, even if we consider 

 the rate of evaporation in the Great Basin to have been the same during 

 the Quaternary as now. An increase in the annual rainfall of a region vaay 

 safely be considered as causing a decrease in the mean evaporation, thus 

 indicating that the rainfall in the region of Lake Lahontan during the Qua- 

 ternary, could not have been greatly in excess of the present mean annual 

 precipitation in the same area. It will be seen from this that the his- 

 tory of Lake Lahontan is decidedly in opposition to the hypothesis that 

 the climate of the Glacial epoch was characterized b}- a marked increase 

 in precipitation. 



A safe conclusion seems to be that the change from arid to more humid 

 conditions which produced the Quaternary lakes of the Great Basin was not 

 sudden or excessive, but consisted in gradual climatic oscillations of moder- 

 ate range. 



Considering the question of humidity alone, we venture to correlate 

 periods of lake expansion with an increase in mean annual precipitation; 

 and periods of contraction and desiccation with decrease of rainfall We 

 therefore use the diagram representing the fluctuations of Lake Lahontan, 

 as an expression of the humidity element in the climate of the region during 

 the Quaternary. Interpreting the curve representing the oscillations of the 

 lake in terms of lumiidity we have: 



Fio. 34. — Curve of Labontan climate. Wet versut dry. 



