Geology of Ft. Apache Region, Ariz. — Reagan. 291 
an angle of about io°, the last recognized southwestern remnant of 
the Plateau beds. The northeastern flanks of the valley, where ex- 
posed, consisted of red granite and allied rocks. 
In crossing the range the granite axis was found to extend con- 
tinuously to camp Pinal and semed to form nearly all its northern and 
lower parts. To the southeast, however, the topography changes, the 
range rising in long, even slopes ot three flatly conical masive moun- 
tains which form its highest points, and have the contour of rhyolyte, 
of which they are probably composed. At camp Pinal a light pink 
rhyolyte lava is encountered bordering the range, and apparently de- 
rived from the higher points to the left, though now separated from 
them by the canyon of Mineral creek. This bordering rhyolyte stands 
nearly as high as the main ridge where crossed, and, though having 
a rather undulating surface, its sides break down in most rugged pal- 
isades, whose precipitous faces overhang the canyons and rim the bor- 
ders of the range with almost impassable cliffs. These form an abrupt 
termination to that mountainous country bordering the plateau region 
through which we had passed, and extending a long distance either 
way, look off at once upon the mountain studded deserts of the 
great Gila valley." 9 
Buttes. 
The buttes of the region are of two kinds : Volcanic necks 
and buttes of the Monadnock type according to professor 
Davis' definition of monadnocks. 
Buttes of the nwnadnock type. — These buttes are all includ- 
ed within the "rim" of the Ellison dome and are bits of relief, 
usually composed of Tonto sandstone, which have withstood 
the processes of baselevelling. Among them are : Chiddesche, 
Twin buttes, John Dazen, Sombrero butte, Juniper butte ( De- 
vonian) and Catholic buttes (Archaean schists and lavas). The 
volcanic necks are considered under volcanoes. 
Drainage. 
The drainage of the Fort Apache region affords an excel- 
lent illustration of a partially diverted and inverted system, 
effected by complicated faulting and tilting. 
The master stream is Salt-Black river. Its location is de- 
termined at least within the Apache area by a fault-scar]), ex- 
tending in an east and west direction. The lower portions of 
the tributaries on the north have also been determined in their 
positions by a series of north and south faults. The heads of 
the tributaries do not appear to have been disturbed in con- 
sequence of the tilting. In other words, in this portion of the 
area the streams are in fact antecedent. The lower courses 
