236 GUIDEBOOK OF THE WESTERN UNITED STATES 
separated from the schist by beds of arkose and shales, which crop 
out on the north side of the river and are also well exposed in slopes 
and cuts 2 to 3 miles north of Blaisdell. The beds dip 20° SE. and 
are overlain by terrace gravel of Quaternary age. The arkose is yellow 
and reddish and made up of granite detritus, in part thin bedded and 
showing mud cracks on some of its surfaces. Some of the pebbles 
are half an inch in diameter. A few interbedded strata of shale are 
of yellowish tint. These beds are probably of Tertiary age. A few 
sahuaros are present, this vicinity being about the western margin of | 
their wide zone of distribution. 
Southwest of Blaisdell siding the railroad leaves the wide trench 
excavated by the Gila River and ascends about 100 feet to the terrace 
plain of the Yuma Desert, which extends far to the south and south- 
west. It continues on this plain to Yuma. From the Vicinity of 
Fortuna and Araby sidings Pinnacle Rock, far to the west in California, 
is in sight. A large gold mine was formerly operated near this peak, 
The vegetation on the Yuma Desert is very scant; on the alluvial flat 
along the river, however, there is considerable irrigation by water 
pumped from wells of moderate depth in the gravel and sand deposits. 
(Turn to sheet 26.) 
Yuma, one of the oldest towns in the Southwest, long the com- 
mercial center for a large surrounding area, and now the headquarters 
of a productive irrigation project, is situated on the 
east bank of the Colorado River just below the mouth 
Population 4892, Of the Gila River. The Gila here is in a broad alluvial 
Nees 4 terraced valley, from which a few low granite knobs 
: protrude, and this rock is reported in deep borings. 
The bridge abutment at Yuma is on very coarse granite conglomerate 
of Tertiary age, which also forms the knoll on which the ruins of the 
old Territorial prison remain. Its components probably were derived 
from granite knobs in the center of town and to the southeast, This 
same formation underlies the basalt that caps Black Mesa, west of 
a Dam, on the Colorado River 10 miles above Yuma. 
Yuma is famous for its high summer temperature and large per- 
centage of sunshine, but with the low annual precipitation of 3.1 
inches “ (40-year average), the humidity is so slight that during the 
greater part of the year the heat is not oppressive. Relying on the 
almost perpetual sunshine, a hotel near the railroad station formerly 
racing Pe sign ‘‘Free board every day the sun doesn’t shine” 
Ppl. 04, 4). 
Yuma. 
“ The precipitation varies greatly danger is greatly exaggerated. The 
from year to year, having been 11.4 average temperature for 29 years is 
inches in 1905 and 0.6 inch in 1899. 72°, with extreme from 20° to 
The greatest amount of rain usually | 117° in the bottom lands and 29° to 
; falls in midsummer. Sandstorms occur | 117° on the mesa. 
ally, but their importance or 
