GEOLOGICAL SKETCH OF THE REGION OF TUCSON. 63 
the hill when under water. It much resembles the widely distributed 
red clay of the older slopes, but is darker in color. 
Some soils or loamy deposits of the mesa have noxious qualities. A 
red-colored soil found in rather limited areas near to Tucson, for, example, 
in a strip a few rods long and wide on the grounds of the university, is 
noted for its infertility and the death of trees planted in it, although they 
are as well watered as trees a short distance beyond, which flourish luxuri- 
antly. For convenience, it is designated as “‘red adobe.” It is charac- 
terized by a large amount of sesquioxide of iron and by its compactness. 
A mechanical analysis reported by Professor Forbes* of a similar soil in 
which plants died, showed that it was more than three-fourths composed 
of coarse material, with one-fifth of silt and fine clay and very little organic 
matter. The same authority notes that an abundance of lime is charac- 
teristic of soils of arid regions and is one reason for the fertility of these 
soils under irrigation. It is also noted that such soils are deficient in 
nitrogen. The chemical investigations of soils by the Experiment Station 
have been mostly upon the soils of the Salt River Valley. 
The general resemblance of the red clays of the slopes to the red argil- 
laceous lacustrine beds of the ancient Lake Quiburis in the San Pedro 
Valley should be noted. 
ALLUVIAL SOILS. 
Class No. 3 includes the alluvions of the streams—the Santa Cruz, 
the Rillito, and the Pantano Wash. All are noted for their fertility and 
depth, requiring only water to produce abundantly. 
The soils of the Rillito are more silicious, sandy, and feldspathic than 
those of the Santa Cruz, as would be expected from their source in the 
granite rocks of the Catalinas and the Rincons. The soils of the Santa 
Cruz are finer and darker in color and have their source, to a great degree, 
in the volcanic tuffs along the river from Nogales to Tucson, receiving 
additions from the granite and limestones of the Santa Rita and the 
Sierritas. 
A deposit of calcareous clay already mentioned, in the lower part of 
the valley at Tucson, is indicative of a former lake-bed, or a formerly 
submerged area, at the lower intersection of the slopes; or, possibly, it 
may represent conditions analogous to those under which the playas 
of Nevada and of Arizona were formed, i. e., sediment from clay-laden 
meteoric flood-waters. 
Several miles south of the Tucson region we find along the upper 
Sopori an extensive area of rich, dark-colored loam, evidently a deposit 
from a former fresh-water lake. The eastern barrier of the water has 
been lowered or swept away by erosion, permitting the drainage of the 
area. 

* Arizona Agricultural Experiment Station Bulletin No. 28, March, 1898, p. 91. 
