216 GUIDEBOOK OF THE WESTERN UNITED STATES 
diabase that doubtless were feeders of sills in the Apache group, 
which probably formerly overlay the granite in this area also. The 
sahuaro, or giant cactus, is conspicuous here and in the country to the 
west. (See p.179.) The narrow depths of the canyon in this vicinity 
are occupied by Apache Lake, a long picturesque reservoir held by the 
Horse Mesa Dam, built in 1926 in a tight canyon cut by the Salt 
River through the rhyolite at the west end of Horse Mesa, 17 miles 
below the Roosevelt Dam. According to the Bureau of Reclamation 
the Horse Mesa Dam is 305 feet high (bedrock to top of coping) and 
540 feet long. The head of 264 feet gives about 43,000 horsepower. 
The lake has a storage capacity of 245,000 acre-feet. The highway 
skirts the lake but does not reach the dam. 
Six miles below the Roosevelt Dam, where the road climbs onto a 
high spur, and at various other points in the next few miles there are 
fine views of Apache Lake and its high encompassing cliffs of volcanic 
rocks. These rocks belong to the succession that lies in a syncline 
constituting the southwest flank of the Mazatzal Mountains, Horse 
Mesa, and the highlands south of Apache Lake. (See fig. 58.) 
In this region there are many fine views of Four Peaks (elevation 
7,645 feet), in the Mazatzal Mountains to the north, and of the 
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Pp EER EES SU SERALTE TSUN a a PAE en Ie Cer 
Oe Cr Poor okie sec SMe at oe a Seat \ 
tal scale : ; Vertical scale 
4 Miles ° 500 4500 Feet 
wide, 
FicURE 58.—Section showing relations of Tertiary voleanic succession 15 miles southwest of 
Roosevelt Dam, Ariz. Tr, Rhyolite tuff; Ta, andesite; Agr, granite 
ridges capped by Apache or voleanic rocks to the south. The Four 
Peaks are also visible from many points westward to Phoenix. The 
Mazatzal Mountains contain deposits of quicksilver ore of low quality 
but of considerable extent which may prove to be of economic impor- 
tance. They are in schists of pre-Cambrian age.” 
About 14 miles below the Roosevelt Dam the highway crosses & 
low divide, leaving the Salt River Valley, and passes into the valley 
of a branch of Fish Creek. Here in a short distance the granite is 
hidden by the volcanic succession just mentioned, of which the lower 
_ members (andesite or latite) are dark gray to bright red. These are 
_ overlain by a 2,000-foot succession of light-colored tuffs, agglomerates, 
and lava flows (largely rhyolite), most of which are so hard and mas- 
sive that they present huge cliffs. These are especially prominent on 
h Creek, as shown in Plate 30, and in the canyon of the Salt River, 
, F.L., U. 8. Geol. Survey Bull. 620, pp. 111-128, 1916. 
