50 BOTANICAL FEATURES OF NORTH AMERICAN DESERTS. 
The extensive strata of diatomite and volcanic ash, upward of 100 
feet thick, are cut through by the San Pedro, exposing a succession of 
snow-white cliffs. 
THE SANTA RITA RANGE. 
The elevated and picturesque range on the east and south side of the 
valley of Tucson is noted for its rugged outline and sharp summit, known 
as Wrightson’s Peak or Old Baldy, rising to an elevation of 9,432 feet, 
on which, in the rainy season, clouds are the first to gather like storm 
signals, and in the winter season its peak is the first to become whitened 
with snow. 
This range extends in a general northerly direction from Patagonia, 
on the Sonoita, in Santa Cruz County, to the Rincon Mountains, a dis- 
tance of nearly 40 miles. It is characterized by a great diversity of 
rock formations—granitic, volcanic, plutonic, and sedimentary. 
Its northern portion consists of a broad development of Paleozoic 
strata in a succession of hills and ridges eastward to the Whetstone 
Mountains, which border the valley of the San Pedro. Southward 
toward the Box Canyon the strata, consisting chiefly of red shales, lime- 
stone, and a basal quartzite, are uplifted at high angles and form the crest 
of the range, resting upon granite and facing the valley of Tucson, as 
at Helvetia, where copper ores are mined near the contact of the strata 
with the granite. The quartzites are probably Cambrian. ‘Their out- 
crops form a sharp ridge with a serrated outline, as seen from the valley 
of Tucson. 
The Box Canyon is a notable geologic feature, bisecting as it does the 
entire range from east to west like a gigantic vertical cleft, and probably 
follows an east and west faulting plane. It drains an extensive area 
on the eastern side toward Rosemont and Greaterville, but delivers its 
waters into the Santa Cruz Valley on the western side. It exposes the 
edges of inclined strata right and left, but especially on the south side, 
where we find the succession of strata to be approximately as follows: 
At the base is a massively brecciated granite, with small veins of 
gold-bearing quartz. Resting upon this granite and dipping east we find 
a conglomerate and coarse sandstone succeeded by red shales, sandstones, 
and limestones and a conglomerate of limestone pebbles resembling a 
conglomerate in the Huachuca Mountains. The limestone beds near 
Greaterville contain fossils of the Devonian age.* : 
South of the Box Canyon the structure of the Santa Rita becomes 
much more complex. The Paleozoic sandstones and limestones resting 
upon a granite foundation give place to granite hills and rocks of volcanic 
origin chiefly in the form of rhyolitic tuffs, consolidated ashes, agglomer- 

* For a list of fossils and other data concerning this section of the range reference 
is made to a paper in the American Geologist, vol. xxvi1, March, 1got. 
