ENVIRONMENT OF THE ROOTS. 



LEADING CHARACTERISTICS OF THE SOIL. 



In the vicinity of Tucson there are several sharply distinguishable phys- 

 iographic areas which have well-defined characteristics as to soils, water 

 and temperature relations, exposure and plant covering. For the present 

 ])urpose these areas may be described as follows: (i) Tumanioc Hill, upon 

 which the Desert Laboratory is situated; (2) the flood plain of the Santa 

 Cruz river; (3) the bajadaor mesa*; (4) West Wash. In addition to these 

 areas a portion of the high bajada which lies about 15 miles east of Tucson 

 and the upper bajada slopes at the western base of the Rincon mountains, 

 an altitude about 1,300 feet above Tucson and 20 miles distant, were 

 observed. These areas are among the most sharply defined of all of those 

 of low altitude in the vicinity of Tucson. 



TUMAMOC HILL. 



Tumamoc Hill is a low mountain rising about 800 feet above the sur- 

 rounding bajada, and is an isolated member of the Tucson range. The 

 northern slope is fairly gentle ; the other sides are more or less precipitous. 

 . A wash or arroyo, which heads in the southeastern part of the mountain, 

 runs along the eastern base, and another wash, called here West Wash, is 

 along the western side. Bold outcrops of volcanic rock occur on the west, 

 south, and east sides particularly, and irregular masses of rock form descend- 

 ing steps along the northeastern face. In crevices of the rock and incrust- 

 ing thinly the surface of the upper rocks which are not exposed is a hardpan, 

 the caliche, which is practically impervious to water. 



The soil of Tumamoc Hill is an adobe clay, malpais, which is derived 

 from the lava rock. It contains small particles of volcanic rock and of 

 caliche, about equaling the malpais in volume. The soil as thus constituted 

 varies greatlv in depth. In places it merely covers the rocks, while at 

 others it lies in pockets and may be 50 cm. or more deep. 



FLOOD-PLAIN OF THE SANTA CRUZ RIVER. 



The flood-plain of the Santa Cruz river at Tucson is about a mile in width. 

 Toward the eastern side of the plain the river has in recent years cut a 

 channel 3 to 5 meters deep ; formerly the water which washed down from 

 the mountains to the south, from the Tucson mountains, or from the bajada 

 to the east and north, spread over the entire floor of the flood plain. This 

 alteration of the river course has probably affected the water table of the 

 plain in a marked degree, causing it to sink to a level lower than that for- 

 merly occupied. 



The soils of the plain have been exposed along the banks of the river so 

 that examination of them to the depth attained by the river is an easy 



*The term bajada is used to designate the gently sloping masses of detrital material, 

 which depend from the bases of mountain masses, and form such a prominent feature 

 of deserts. 



