200 GUIDEBOOK OF THE WESTERN UNITED STATES 
After crossing the Gila River to Poston siding, near which a branch 
line leads to Florence, 6 miles east, and thence to the mining town of 
Christmas, in eastern Pinal County, the railroad deflects northwest- 
ward and follows near the north bank of the river through Blackwater 
and Olberg sidings. To the east and north of the railroad are many 
buttes of granite, the highest of which, Walker Butte, is capped by 
lava. At Olberg is a quarry in lava and scoria, which are used 
extensively for road making. 
North of Olberg is Malpais Mountain (mal-pah-ees’), which con- 
sists of lavas and tuff * capping granite which appears also in ridges 
and detached buttes to the east; it also constitutes Santan and Gold- 
mine Mountains, farther north. Yellow Peak and Rock Peak, a few 
miles north of Olberg, are capped by conglomerate of Tertiary age. 
South of Olberg are the prominent granite ridges of the Sacaton 
Mountains, with various outlying buttes. These are all typical desert 
mountain ridges, with steep rocky surfaces rising abruptly from the 
long, gentle slopes of wash and valley fill, which is very thick in the 
adjoining valleys. At most places large parts of the flanks of these 
mountains are buried by detritus and only the tops protrude, and 
doubtless there are many others that are entirely buried. If this 
valley fill were removed the Salt River-Gila plain would present a very 
rugged topography, with ridges and buttes 1,000 to 2,000 feet high. 
The filling has progressed for centuries, is still actively going on, and 
will continue until the present ridges and buttes are worn very low 
and the smaller ones buried entirely. A view of a typical desert valley 
in this region is given in Plate 25, A. 
Just south of Olberg is a dam that diverts water from the Gila River 
into canals to supply the lower portion of the Gila River Indian Reser- 
vation. This reservation occupies a wide area in the Gila Valley and 
according to the report of the United States Commissioner of Indian 
Affairs for 1932 contains about 4,000 Pima Indians, 500 Maricopa 
Indians, and a few others. Many of these Indians irrigate farms, 
using the water provided for them by the Government and raising 
alfalfa and other crops which under irrigation flourish in the rich soil 
of the valley lowlands. In Padre Garcés’ time (1775) the largest 
Pima settlement was located in this immediate neighborhood, with a 
population of about 5,000. He called it Sutaquison, but Padre Kino 
80 years earlier had named it Encarnaci6n. 
*$ These volcanic rocks cover an area 
of about 9 square miles and consist of 
several flows, in all several hundred feet 
thick, dipping gently south-southwest. 
At one locality a sheet of olivine basalt 
is exposed lying on a 200-foot sheet of 
latite, in part tuffaceous, which in turn 
lies on the old granite. Under the 
microscope the latite is seen to consist 
mostly of volcanic glass crowded with 
microliths; it contains some orthoclase, 
albite, biotite, and olivine. 
