SOUTHERN PACIFIC LINES 227 
South of Bosque siding are the Sand Tank Mountains, which 
consist of a long, high ridge of schist and granite and a high, wide tab- 
ular mesa of volcanic rocks in a succession nearly 2,000 feet thick. 
This region was a center of great volcanic activity in Tertiary time, 
when widespread sheets of lava were poured out over the land. These 
have since been uplifted, tilted, faulted, and greatly eroded. 
Gila Bend is a town sustained by cattle, irrigation, and mini 
interests and is the headquarters for the Gila Bend Indian Reserva- 
tion, near by, where there is a colony of about 224 
Gila Bend. : : = . 
ppt Papago Indians. The climate is very dry, with a 
Popaktinso - Mean annual precipitation of only 6 inches. A branch 
New Orleans 1,627 railroad connects Gila Bend with Ajo (ah’ho, Spanish 
get for garlic), 30 miles to the southwest, a copper-mining 
town which has a population of 3,003. Copper has been mined at 
Ajo since 1855, mainly from the Cornelia mine. Most of the ore 
carries less than 1 per cent of copper, but it is easily worked and 
occurs in large amount. The ores are mainly disseminated in monzo- 
nite porphyry and a small amount is disseminated in veins in rhyolite 
and tuff, into which the porphyry is intruded. It is estimated that 
40,000,000 tons of ore is available. There are also dikes of diorite 
and later porphyry, all presumably of Tertiary age. In 1929 a 
total of 3,582,000 tons of ore containing from 1 to 1% per cent of 
copper was treated. 
South of Gila Bend are the Sauceda Mountains, a high range con- 
sisting mainly of a thick succession of Tertiary volcanic rocks of which 
the latest member is basalt. Hat Mountain, a prominent landmark 
25 miles south of Gila Bend, has a cap of this basalt, a remnant of a 
lava flow of Tertiary time. 
Gila Bend is in the broad valley of the Gila River, which in making 
its huge bend southward around the Gila Bend Mountains approaches 
within 4 miles of the town. In this region the river is a wide water- 
course which ordinarily carries only a small flow. It was in this 
icinity that Padre Kino found a prosperous Opa (Maricopa) Indian 
rancherfa in 1699, and it was visited in 1774 by Anza and Garcés, who 
called it the Pueblo de los Santos Apéstoles San Simén y Judas. 
There were other rancherias along the river at which the Indians were 
raising two crops of grain a year by irrigation with river water. This 
was the farthest east that the Maricopa Indians had advanced up th 
river, but they have since moved to the region southeast of Phoenix. 
® At the Sand Tanks, a watering 
place in these mountains 23 miles south- 
east of Gila Bend, the water is found in 
holes eroded in a conglomerate of Ter- 
tiary age which dips 20° N. This rock 
lies on granite gneiss and consists most- 
ly of tuffs and sandy tuffs containing 
pebbles of granite, schist, and volcanic 
rocks of various kinds. ists i 
the central ridge are mostly chloritie, 
and there are many transitions from 
schist to gneiss. Fine-grained biotite 
granite and phyllite also occur. 
