DEPARTMENT OF BOTANICAL RESEARCH. 77 



The central feature of the valley is an elongated floor from 10 to 

 20 miles in width, which is level from side to side and has an average 

 fall of about 20 feet to the mile throughout its length. Surrounding 

 the central floor is a more extensive area of bajada, and beyond this 

 is an almost unbroken circle of mountains and hills. The drainage- 

 ways which cross the bajadas are found to terminate at the edge of 

 the central floor. Only in the case of the Baboquivari Valley does a 

 tributary, nearly 50 miles in length, penetrate the central floor, for a 

 distance of about 4 miles. Stream-ways are not absent in the floor of 

 the valley, but they are of ephemeral character, and rarely exceed 

 4 feet in width and half a mile in length. There are no dry lakes. 



The history of the valley is one of gradual filling and leveHng in 

 which the streams deposit their loads on reaching a gentler gradient 

 and have insufficient volume to cut through their own deposits. From 

 time to time, short temporary stream-ways are cut through these 

 areas of deposition, only to be filled again by the development of small 

 lateral drainages. The constant shifting of the regions of cutting and 

 deposition is largely determined by the sporadic occurrence of torren- 

 tial rains in different parts of the drainage basin. 



The vegetation of the bajadas appears to be determined by the 

 character of the soil. On the sands and hght loams derived from 

 granitic mountains there is a heavy cover of Parkinsonia and Acacia, 

 while Covillea and Carnegiea are absent. On the heavier soils, with 

 coarse rock fragments, derived from volcanic mountains, there is a 

 light stand of Covillea, Prosopis, Parkinsonia, and Carnegiea; or at 

 the base of such bajadas are frequently found pure stands of Covillea. 

 The floor of the valley is occupied by three types of vegetation, which 

 are not so ob\dously related to types of soil: (a) pure stands of Co- 

 villea near the edges or on very slightly elevated areas; (6) open or 

 closed parks of Prosopis with a ground-cover of Isocoma and perennial 

 or annual grasses; and (c) very open areas with isolated bushes of 

 Lycium or Atriplex. 



Covillea appears to occupy areas in which the soil-surface has long 

 been stabilized, Prosopis the areas not recently subjected to change 

 of level, and Lycium and Atriplex the areas that bear evidence of 

 recent overflow or sheet-flood deposition. 



Field and laboratory work has been inaugurated with a view to 

 investigating the role played by the soil in determining the sharp 

 differences of vegetation which characterize the Avra Valley. The 

 vegetation has been mapped in detail for a representative portion of 

 the vaUey, and the character of the soils is under investigation with 

 respect to this area in particular. An opportunity is here afforded to 

 determine whether different types of vegetation may exist under 



