262 GEOLOGICAL HISTORY OF LAKE LAHONTAN. 



Temperature of the Lahontan Period. — Considering temperature as the 

 controlling climatic element — in reference to a restricted region, as in the 

 preceding discussion — and knowing that a high temperature promotes evap- 

 oration and hence tends to decrease the volume of lakes, and that a low 

 temperature produces a contrary result, we should apparently be justified in 

 concluding that as the Quaternaiy lakes of the Great Basin were larger than 

 the present water-bodies of the same region, the former climate must have 

 been colder than the present. It may be said, however, that had the cold 

 been intense and produced arctic conditions, precipitation would have been 

 retarded. This postulate, however, is not applicable to the area in question, 

 whei'e the climatic oscillations were of moderate intensity even in compari- 

 son with the present prevailing arid conditions, thus indicating that if the 

 periods of desiccation were times of arctic cold, the lake periods must have 

 been at least sub-arctic. On this assumption the Great Basin to-day should 

 have a climate resembling that of circumpolar lands. The absence of "ice 

 walls " about the smaller of the Quaternary lakes of the Far West is nega- 

 tive evidence, perhaps of some value, in opposition to the above liypothesis. 

 Moreover, the character of the abundant molluscan fauna of the Lahontan 

 basin precludes the hypothesis of an arctic climate. 



If we postulate sub-tropical or tropical conditions of the Lahontan ba- 

 sin during the Quaternary, we must, from the analogy of tropical countries 

 in general, conclude that the region would jJi'obably have been humid as 

 well as warm, and consequently productive of abundant faunas and floras. 

 The absence of fossils indicative of such conditions is sufficient evidence 

 that they did not prevail. In the chapter devoted to the life history of the 

 former lake it has been shown that its shores must have been at least as 

 desolate and lifeless as the borders of the existing lakes of the same region. 



The alternation of humid and arid conditions during the Quaternary 

 finds, perhaps, its best analogue in the present annual climatic changes of 

 the same region. The seasons in the Great Basin are two, an arid and a 

 humid, the former being of the greater length. In the winter precipitation 

 is abundant in comparison with the summer; in fact nearly the entire rain- 

 fall of the year takes place between December and March. During these 

 months the skies are clouded, and rain and snow are not infrequent ; the 



