62 BOTANICAL FEATURES OF NORTH AMERICAN DESERTS. 
The first class includes the detrital deposits of the mountain slopes, 
the .boulders, gravel, and sands broken from the rocks of the ridges and 
spread out in the valley forming the ‘“‘mesas.’’ The area of such deposits 
is nearly that of the area of the valley. The deposits of the two other 
classes are really comparatively insignificant. As a general rule, the 
heavier and coarser materials are found near the mountains, while the 
finer gravels and sands are distributed at a greater distance and at the 
lower levels; yet large boulders up to 12 inches in diameter are found in 
digging wells far out on the mesa east of Tucson and south of the valley of 
the Rillito. The surface is generally free from large boulders or coarse 
gravel. <A vertical section shows that the materials are rudely stratified, 
layers of coarse boulders, partly rounded, alternating with gravelly and 
sandy layers and with the red clays of class No. 2. There are also the 
included calcareous beds of caliche, and in places thin layers of gypsum. 
At a depth of go to 100 feet permanent water is found under the mesa 
slope for a few miles east and north of the city. 
The surface earth of the mesa is essentially arenaceous and affords 
good natural roads. The prevailing color is white or gray, rather than 
red, though in some low places there are local deposits of red claylike 
loam, derived probably from the interstratified red clays of class No. 2. 
There is a notable absence of humus in all the soils of this class. The 
conditions do not appear to have been favorable to the presence of organic 
matter in any form. Decay of vegetation, promoted by moisture, has 
not been possible. The nearest semblance to a soil favorable to vegeta- 
tion upon the mesa lands about Tucson is found around and under the 
low-growing shrubs, which are important factors in the distribution and 
conservation of the sandy loams. This growth of plants, and especially 
that of the creosote bush, Covzllea tridentata, not only protects the surface 
of the desert, so called, from the sweeping action of furious winds, but 
by checking the velocity of the wind causes the deposition of the wind- 
driven earth and sand about the roots. 
RED CLAY SOILS. 
The soils of class No. 2 are derived largely from the red argillaceous 
deposits interstratified with the gray gravels of the slopes. These red 
beds are most developed and visible in the section of the detrital slopes 
of the western end of the Catalina Range and on the road northward to 
Oracle. They are found at other localities where the detrital slopes have 
been cut through and exposed by erosion. 
On Tumamoc Hill, although little can be seen beside the surface of 
black rock, the hill is not wholly without what may be called a soil—a 
soft brown-colored or red clay accumulation largely hidden under the 
loose rocks or in clefts and hollows protected from the washing action of 
rain. This red-brown clay does not appear to be the result of the decay 
of the rocks, but rather a residue of a once more extended deposit upon 
