58 BOTANICAL FEATURES OF NORTH AMERICAN DESERTS. 
The Southern Pacific Railroad is built upon this slope. For the upper 
part of the slope, or from Vail’s to Pantano, the grade parallelsthe Pantano 
Wash, which cuts its way through the slopes from the Rincon Mountains 
on the north and the Empire Mountains on the south, and is bordered 
by high terraced banks on its way to the junction with the Rillito at 
Old Fort Lowell. 
The Sierritas Range presents one of the best examples of long, continu- 
ous slopes flanking it on all sides, but particularly on the east, north, and 
west. The most accessible and best-known portion of the slopes is 
found in the gradual rise of the ground from near the Mission of San 
Xavier to the base of the ridges, a distance of about 1o miles and a rise 
of about 1,000 feet, or about 100 feet to the mile, but apparently a plain. 
The Sierritas slope as seen from the Tucson Mountains a few miles 
south of Tumamoc Hill is shown by plate 51. 
MATERIALS OF THE SLOPES. 
The sections of the higher detrital slopes shown by the banks of arro- 
yos all show approximately the same general composition and arrange- 
ment of the detritus in rude strata. The boulders and gravels corre- 
spond in mineral composition to the nature of the rocks in the ridges 
from which they were derived. Thus the slopes from the Catalinas and 
the Rincons are made up of granitic and gneissic detritus and those from 
the Santa Ritas and Tucson Mountains of tufaceous and porphyritic 
materials. The alternation of coarse and fine mate.ials, such as gravels 
and sandy clays, is found in all the sections and is regarded as signifi- 
cant of other conditions than those of ordinary stream deposition. It is 
also noteworthy that the clay-like beds increase in frequency and in 
thickness as depth is reached. 
Wells dug in the mesa at Tucson show a series of rudely stratified 
layers of boulders, gravel, and sandy clays with caliche near the surface 
and carbonate of lime in small disseminated crystals lower down. The 
gravels and boulders are clearly such as were washed from the Santa Cata- 
lina and the Rincons, though now separated from those sources by the 
valley of the Rillito and the Pantano Wash. 
Thin layers of fibrous gypsum are occasionally found in digging wells. 
upon the Tucson Mesa. It also occurs under the tufa of Sentinel Hill, 
near the Laboratory. 
By the courtesy of W. A. McGovern, superintendent of the Southern 
Pacific Railway at Tucson, I am able to give a copy of the record of the 
nature of the material passed through from the surface to the depth of 
1,480 feet at one of the company’s wells bored at Esmond, a station on 
the mesa 15 miles east of Tucson and about half-way between Tucson and 
Pantano and nearly on the 3,0o00-foot contour line. This section shows in 
a general way a succession of beds of gravel and clay, with the thickest bed, 
