HUMIDITY OF THE QUATERNARY. 259 



judgment, as to confirm the hypothesis that an arid period of long dura- 

 tion preceded the first rise of Lake Lahontan of which we have definite 

 knowledge. 



The variations and fluctuations of the pre-Lahontan arid period are 

 unknown, but, from the general teachings of meteorology, we may reason- 

 ably conclude that, like the present climate of the Great Basin, it was 

 marked by many fluctuations in precipitation and evaporation, which at 

 times gave origin to lakes of greater or less extent. As the arid period 

 drew to a close and more humid conditions prevailed, it is most reasonable 

 to suppose that the change was gradual. Tlie phenomena do not call for a 

 sudden break in the processes of nature. 



Mumidity of the Lahontan period. — Inclosed lakes may be considered as 

 representing the net balance between precipitation and evaporation. As 

 the relations of these two climatic elements are complex and independent, 

 their resultant will be inconstant and variable; their mutual neuti'alizatiou, 

 so far as the lakes of a region are concerned, must, therefore, be a matter 

 of delicate adjustment. It follows from this that the diff"erence in climate 

 between a time of expanded lakes and a time of desiccation might be com- 

 paratively moderate. 



In a given area, like the Great Basin, we may safely say that a lower- 

 ing of the mean annual temperature will increase precipitation and decrease 

 evaporation, thus affording the climatic conditions favorable to the expan- 

 sion of the lakes. On the other hand, a rise in the mean annual tempera- 

 ture would increase evaporation and decrease precipitation, thus favoring 

 the contraction and extinction of inclosed lakes. The existence of a large 

 number of lakes in the Great Basin during the Quaternary is seemingly 

 good evidence that the climate of the region during the time of their greater 

 expansion was more humid than at present; unless it can be shown that 

 there was a very great decrease of evaporation without a corresponding 

 increase of precipitation, a phenomenon only to be observed at the present 

 time in the arctic latitudes. 



That many of the lakes did not overflow is equally positive evidence 

 that precipitation within their hydrographic basins could not have been 

 excessive. Had the rainfall been even moderately copious, it seems self- 



