SOUTHERN PACIFIC LINES 193 
and consists mostly of volcanic rocks in widely extended sheets and 
several stocks, erupted from craters or possibly from some cracklike 
vents in early Tertiary time. On the east slope of the mountains is 
an old quarry in light-colored volcanic tuff which has been used for 
building one of the university buildings and many houses in Tucson. 
The Tumamoc Hills, an outlying knoll of the Tucson Mountains 
just west of the city, consist of a succession of lava flows (andesite, 
rhyolite, tuff, and basalt) and several intrusive masses that are pro 
ably of Pleistocene age. 
From Tucson the railroad follows the wide flat adjoining the 
Santa Cruz River, which has a sandy bed of many braided channels, 
usually dry. At times of rain the Santa Cruz carries 
considerable water. According to records of the 
United States Geological Survey the flow at Tucson 
woe 1,511 ageregated 57,200 acre-feet in 1914 and 24,700 acre- 
feet in 1915. The Santa Cruz is an affluent of the 
Cortaro. 
Elevation 2,156 feet. 
ation 80.* 
Gila, which its channel reaches in the neighborhood of Phoenix, but 
even in Garcés’ time it sank into the sands near Picacho Peak, andl at 
present it rarely ‘ai i 
flows even that far. 
However, there is 
considerable under- 
flow in the sand and 
gravel of the valley 
fill, especially below 
the mouths of Rillito FicGure 49.—Secti fth t side of the T M ins, Ariz., about 
Creek and Cafiada 3 miles south of the Ajon 2milessouth of Amole Peak. a, Agglom- 
. erate; rs, red sandy shale 
del Oro, and this 
water is pumped for irrigation. The irrigated area is entered near 
Jaynes, a short distance out of Tucson, where there is a State experi- 
mental farm; it continues with some interruptions nearly to Naviska. 
* Rhyolites and andesites, in part 
porphyritic, are the principal rocks, 
with some tuff and basalt. Amole 
Peak (ah-mo’lay), the highest summit, 
and some other knobs consist of in- 
usive, occur on 
e west side of the north end of the 
range. Picacho de la Calera, an out- 
lying butte to the northwest, consists 
of limestones of Carboniferous and 
Devonian age (Escabrosa and Martin) 
with abundant fossils. These lime- 
stones are underlain by 300 feet of 
typical Abrigo limestone with trilobites 
and other fossils of Upper Cambrian 
age, and at the base, lying on pre- 
| shows the relations 
| Cambrian schist, is 200 feet of Bolsa 
quartzite. 
e section in Figure 48 
at this place. 
Along the foot of the southwest side 
of the Tucson Mountains are extensive 
exposures of sandstone and sour foot 
lieved to be of Lower 
They lie nearly level and « are overan 
by the Tertiary volcanic 
the east and by a sheet of rhyolite * 
the west. Farther north the 
Figure 49 shows the relations in this 
part of the area, 
