VEGETATION OF DESERT MOUNTAINS 295 



deltas over which this vacillation has taken place are only very 

 slightly elevated above the adjacent plain. 



The plain at Bonita is a portion of the Sulphur Spring Valley, 

 which has a total length of about 100 miles (in the United States), 

 a width of 10 to 25 miles, and lies wholly above 4000 feet. The 

 central part of this valley is a bolson, or enclosed basin, draining 

 into an extensive alkali flat, which is crossed by the Southern 

 Pacific Railroad between Willcox and Cochise, Arizona. The 

 southern portion of the valley is drained by the Whitewater 

 Draw and the northern portion by Arivaipa Creek, a tributary 

 of the San Pedro, which is in turn a tributary of the Gila. The 

 Arivaipa is a small stream fed only by storm waters, and its 

 upper course lies through an alluvial valley from one to two 

 miles in width. This valley is sharply separated from the older 

 and higher level of the Sulphur Spring Valley by a low scarp. 

 Although the Arivaipa is thus seen to be working in the direction 

 of the ultimate tapping of the Willcox bolson, it is nevertheless 

 doing so at an extremely slow rate because of the small volume 

 of its flood waters. The great difference which has been de- 

 scribed as existing between the basal topography of the two 

 sides of the Pinaleno Mountains is undoubtedly due in great 

 measure to the presence of the strong drainage of the Gila on 

 the one side and the weak drainage of the Arivaipa on the other, 

 if we disregard the original structural characteristics of the 



region. 



VEGETATION 



The larger features of the vertical distribution of vegetation 

 in the Pinaleno Mountains are similar to those that have been 

 described by the writer for the Santa Catalina Mountains, 5 

 The outstanding features of dissimilarity between the two 

 mountains are due to the greater altitude of the former (by 

 1400 feet), and to the fact that the canons of the former are 

 more sharply cut and better watered. The higher elevation 

 results in extended areas of forest of a type which is only spar- 



5 Shreve, Forrest. The vegetation of a Desert Mountain Range as Condi- 

 tioned by Climatic Factors. Carnegie Inst. Wash. Pub. 217, 1915. 



