12 DROUGHT RESISTANCE OF OLIVE IN SOUTHWESTERN STATES. 



I 



' 1 



nil 



Fig. 2.— Diagram showing the mean monthly rainfall 

 at Casa Grande, Maricopa, Phoenix, and Mesa, Ariz., 

 as presented in Table I. 



The country around Casa Grande is a wide plain, through the 

 level of which the mountains appear to be thrust up, so abruptly 

 do the scattering groups and single low peaks break the surface. 



These mountains are largely 

 composed of a soft, rapidly dis- 

 integrating granite, with much 

 feldspar in its composition, and 

 their decay determines the char- 

 acter of the soil, which is coarse 

 and gravelly around the moun- 

 tain base, sandy with more of 

 clay a little farther awa}^, and 

 of a stiff clay nature mingled 

 with bars of sand and gravel 

 along the drainage courses, 

 scarcely as 3^et marked as stream 

 channels, which serve to carry 

 away the run-off from the occa- 

 sional torrential rains so char- 

 acteristic of the region. 



The most important of these 

 water courses, sometimes digni- 

 fied on the maps by being called the Santa Cruz River, is locally given 

 the more appropriate name "Santa Cruz Wash." While along its 

 upper course, from the Mexican boundary down to Tucson, there is 

 a pretty well-marked channel and a 

 more or less continuous flow of water, 

 in the neighborhood of Casa Grande a 

 slightly cut channel, a broad, well- 

 marked flood area, and a still broader 

 belt of mesquite growth mark the course 

 of the so-called river. 



The popular idea that there is a 

 strong underflow of water the entire 

 length of this valley is given support by 

 the heavy belt of mesquite which occurs 

 with more or less regularity along the 

 course. This tree is well known through- 

 out the desert regions of the South- 

 west as possessing a remarkable root 

 system, able to penetrate to water- 

 bearing strata at depths of 30 to 50 

 feet. The further fact that the railroad wells along the line of 

 the Southern Pacific Company, particularly those at ]\Iaricopa 

 and Casa Grande, 2 or 3 miles away from the main channel, afford 



192 



Fig. 3.— Diagram showing the mean 

 monthly relative humidity at Phoenix, 

 Ariz., as presented in Table II. 



