TEMPERATURE OF THE QUATERNARY. 263 



rivers rise, and many channels that are completely drj' during the summer 

 become flooded ; the inclosed lakes increase in area and many new ones are 

 formed in basins that are jiarched deserts during the summer months. The 

 winter season is, therefore, the humid period, during which evaporation is 

 decreased, and is in every way favorable to the existence of lakes. Could 

 these conditions be continued for a sufficient length of time each year it is 

 evident that the Quaternary lake basin would be refilled. 



On the other hand, throughout the arid season the rain ceases almost 

 entirely, the skies are clear and cloudless for days and perhaps weeks at a 

 time ; the heat in the desert valleys becomes intense, and evajDoration is 

 greatly accelerated. The result is a decrease and failure of the streams and 

 the shrinkage and disappearance of the lakes. These annual changes illus- 

 trate the character of the secular oscillations that took place during the 

 Quaternary. 



The former great extension of the lakes of the Great Basin is, there- 

 fore, considered as evidence that the mean annual temperature was then 

 lower than at present. Interpreting the curve given on page 237, which 

 indicates the fluctuations of the Lake Lahontan, in terms of temperature we 

 have the following as a generalized diagram of this element in the climate 

 of the Quaternary: 



h^ 



0- Cold Period 



E 



c 



a 



c 



2 



Fio. 35. — Cnrve of Lahontan climate. Cold versus warm. 



In the last few pages of this attempt to decipher the prevailing char- 

 acteristics of the climate of the Quaternary in the region of Lake Lahontan, 

 the questions of humidity and temperature have been considered in refer- 

 ence to a restricted area. In treating of such a complicated and far-reaching 

 question, however, it is evident that we should not be confined by geograph- 

 ical limits, but must count the changes of climate in broad and perha^js far- 



