Geology of Ft. Apache Region, Ariz. — Reagan. 301 
The Rhyolyte of the Pinal mountains. — This rhyolyte bord- 
ers the mountain range on both the east and west sides, but 
does not cap the summits, except in the region south of Globe. 
It may be recognized and separated from the rhyolyte of the 
Apache mountains by its color, the former being distinguished 
by its gray color, the latter by its pinkish hue. 
Air. Marvine, in his report on the trip "From Camp Apache 
to the Gila Valley" makes the following statements concern- 
ing this rhyolyte : 
"To the southeast" [of Camp Pinal] "the topography changes, the 
range rising in long even slopes to three flatly conical, massive 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 rhy- 
olitic lava is encountered bordering the range, and apparently derived 
from the higher points to the left" [to the south], "though now sep- 
arated from them, I understand, by Mineral creek. This bordering rhy- 
olyte stands nearly as high as the main granite ridge where crossed, and, 
though having a rather undulating surface, its sides break down in most 
rugged palisades, whose precipitous faces overhang the canyons and rim 
the borders of the range with almost impassable cliffs." 4 
These lavas underlie the Quaternary at all points and were 
much eroded before the Quaternary was deposited. They there- 
fore date back to the Tertiary and are most likely Pre-Pliocene. 
Other Patches of Rhyolyte. — A rhyolyte not unlike that in 
the Apache Mountain district was encountered in Pleasant val- 
ley on Cherry creek. The Catholic buttes west of Ellison are 
built up of this lava and from these buttes it extends north and 
west across the above valley to the Tonto basin, but in the val- 
ley proper it is covered with the Tertiary and Quaternary sedi- 
ments. The age of this lava undoubtedly antedates the Plio- 
cene for the reason that these deposits are superimposed. 
The Basaltic Lavas, including the Sanidin-Doleryte Type. 
— Lavas of the true basaltic type are the last lavas erupted 
in the region. They are found only in the Sierra Blanco coun- 
try, but lavas of the sanidin-doleryte type preceded them and 
flowed over the entire part of the plateau region visited, from 
the White mountains to the Canyon creek fault, the greater 
part of which still covers the country, capping continuous or 
detached mesas. There' were at least three flows of this type 
of lava separated from each other by long periods of erosion. 
A dark, compact to fine grained basalt, not unlike that de- 
scribed by Marvine along the Mongollon mesa near the San 
