264 GEOLOGICAL HISTORY OF LAKE LAHONTAN. 



distant regions. In fact, a stndy of the climate of a given region, to be 

 complete, must contemplate the atmospheric plienomena of the world. Nei- 

 ther can we postulate an alteration of a single element in the climatic envi- 

 ronment of a region without altering the relations of all the remaining ele- 

 ments. Hence the interpretation of geological records in terms of climate 

 become more and more difficult. 



Our conclusions, therefore, in reference to the climate of the Quater- 

 nary are at the best somewhat arbitrary and are open to controver?y. The 

 weight of evidence and the impressions which one receives from the study 

 the phenomena in question are such as to lead to at least a well-grounded 

 opinion, even if some of the facts observed might be interpreted differently 

 by diffei'ent observers. 



The present arid climate of the Great Basin cannot be explained b}' 

 saying that the temperature is high and consequently the water that is 

 precipitated is rapidly evaporated. On the contrary, evaporation is rapid, 

 probably for the reason that precipitation is moderate, or, perhaps more accu- 

 rately, because the mean annual humidity of the atmosphere is low. In 

 explanation of the present aridity some writers have attempted to show tlmt 

 as the prevailing winds blow from the Pacific, and consequently are obliged 

 to cross the Sierra Nevada before reaching the Great Basin, the mountains 

 condense their moisture, and hence they reach the region to the eastward 

 as drying winds. In this explanation it is forgotten that the Sierra Nevadn 

 is scarcely, if at all, more humid than the Wasatch or some of the higher 

 of the basin ranges, and that much of the Pacific slope is also an arid coun- 

 try, although situated between the ocean and the mountains that are sup- 

 posed to rob the winds of their moisture. Other explanations of the aridity 

 of much of the region west of the Rocky Mountains have been advanced, 

 but it remained for Captain' Button to present the view that apparently has 

 the strongest foundation.^^ This writer explains the aridity by peculiarities 

 of the currents of the Pacific. In brief, this theory assumes that the cur- 

 rents from the north which follow the western border of the continent cool 

 the air that is carried over them towards the land, this being the prevailing 

 direction of the air currents of the region ; consequently, on reaching the 



83 American Journal of Science, Vol. XXII, 1881, pp. 247-250. 



