296 riiirersitij of California Publications in Zoologtj [Vol. 16 



counties the Snake River has cut a deep caiion, reaching in places to 

 a depth of 1500 feet or more. 



The Blue Mountains in southeastern Columbia County form a 

 region of steep slopes and rather sharp ridges, with an elevation of 

 4000 to 6000 feet. The caiions are deep and the valleys narrow. 



GEOLOGICAL HISTORY 



In early Miocene time the greater portion of eastern Washington 

 and also a portion of eastern Oregon and Idaho were flooded by a 

 series of lava flows. These Columbia Lavas have a thickness in places 

 of probably over 4000 feet. All the visible rocks of Walla Walla and 

 Columbia counties are basalts probably formed at the time of these 

 lava flows. The caiion walls of Snake River indicate at least ten suc- 

 cessive flows, between some of which sufficient time elapsed for soil to 

 form and forests to grow. 



Volcanic outbursts continued during the Middle Miocene and much 

 volcanic ash was thrown out. In eastern Oregon extensive lake de- 

 posits, the Mascall beds, were formed at that time. 



The climate of the region in the period succeeding the lava flows 

 must have been more moist than at present. Before the middle of the 

 Pliocene, however, the Cascade Mountains had been elevated and these 

 mountains robbed the winds from the ocean of their moisture long 

 before they reached eastern Washington. A dry period necessarily 

 ensued. 



The time of the uplifting of the Blue Mountains is not certainly 

 known, but their present elevation had probably been reached before 

 the beginning of the Pliocene. 



In Pleistocene time the presence of glaciers in northern Washington 

 seems to have caused a return of moist conditions over the region. At 

 that time also the gorge of the Columbia River through the Cascade 

 Mountains seems to have been blocked, resulting in the formation of 

 a large lake. Lake Lewis, which covered a large part of the Columbia 

 Basin. In Walla Walla County this lake probably extended eastward 

 as far as Eureka, but no evidences of it east of this point have been 

 found. 



METHODS OF STUDY EMPLOYED 

 The principal aim in the present study has been to determine the 

 vei-tebrate associations of the region. The wide range of climatic and 

 vegetational conditions which occurs gives a very good opportunity to 



