14 NOBTH AMERICAN FAUNA [No. 55 



EXTREME ARID DIVISION OF UPPER SONORAN ZONE 



One is impressed by the apparent nonconformity of the elevations 

 of the Great Basin division of Upper Sonoran east of the Cascades 

 over different parfe of the State. These variations, however, in- 

 volve well-known and long-recognized principles of climatic con- 

 trol of distribution. The actual altitudinal limits of zone levels are 

 known to vary considerably with direction and steepness of slope 

 exposure, elevation of base level, aridity, type of prevailing winds, 

 and other less-apparent factors of modification, but the long-estab- 

 lished fauna and flora serve as the most reliable index to zone level 

 of any point as determined by average climatic conditions. For in- 

 stance in the Columbia Eiver Valley with a base level of 100 to 300 

 feet, the Sonoran Zone extends from the bottom of the valley up to 

 about 2,000 feet on cold northerly slopes, lower on steep, and higher 

 on gradual slopes, and to about 3,000 feet on warm southerly slopes. 

 With a higher base level of about 1,000 feet at the junction of the 

 Grand Ronde River with the Snake River near the northeast corner 

 of the State, the Sonoran Zone extends up to about 3,500 feet on warm 

 slopes in the Grande Ronde and Imnaha Canyons. Still farther up 

 in the Snake River Valley with a base level of 2,150 feet at Ontario, 

 the upper limits of the zone run correspondingly higher, while in the 

 extensive lake-basin valleys of the Malheur, Warner, Abert, and 

 Summer Lakes sections, with base levels of 4,100 to 4,500 feet, the 

 upper limits of the zone reach to approximately 4,500 feet on north- 

 erly and 5,000 feet on southerly slopes. The actually coldest points 

 of slope exposure are northeast and the warmest southwest, as 

 pointed out long ago by Merriam (1890, pis. /-//, following index). 



Thus the elevation of base level in eastern Oregon is shown to raise 

 the upper limits of the zone in higher valleys fully 2,000 feet by 

 simply holding up the absorbed and radiated heat of the sun's rays 

 to a fairly uniform distance above the surface where they fall upon 

 the earth. Other zone levels are modified in the same manner and 

 by many other generally recognized factors of climate and distribu- 

 tion, but the Upper Sonoran of eastern Oregon most admirably illus- 

 trates the general principles of zone modification by change of base 

 level. 



A somewhat detailed knowledge of the plant and animal life of a 

 region is necessary for the recognition and full understanding of the 

 lire zones in the field. The colored map is at best but a feeble guide 

 to the actual areas which are far more detailed and graphic when 

 spread before one in the garb of their native plant and animal life. 



MAMMALS 



In eastern Oregon the Upper Sonoran Zone is clearly indicated by 

 the presence of black-tailed jack rabbits, little speckled and gray 

 ground squirrels (CitellMs townsendii, vigilis, and canus), the ante- 

 lope squirrel, desert wood rat, Oregon grasshopper mouse, silky cliff 

 mouse, large-eared harvest mouse, several species of pocket mice 

 (Perognathus parvus, lordi, Columbian^ and mollipilosus) , five-toed 

 kangaroo rats, several species of pocket gophers (Thomomys town- 



