Geology of Ft. Apache Region, Ariz. — Reagan. 289 
the former representing the oldest eruptions of the region and 
the latter the most recent. The thickness of the sanidin-dolor— 
yte often exceeds 1000 feet and that of the trachyte (rhyolyte) 
500 feet. Concerning this region G. K. Gilbert makes the fol- 
lowing statements : 
"The Nantanes plateau is the southern continuation of the Sierra 
Blanco sheet of sanidin-doleryte. It is imperfectly tabular, narrow, and 
elongated in a northwest and southeast direction. Its northeast face, 
looking toward South fork of White Mountain river, and Prieto park, 
. is abrupt in general character, but is broken by numerous canyons. 
The southwest face, overlooking the head-waters of the San Carlos 
and Benito, is an abrupt escarpment, exhibiting in section the lava 
beds that constitute the plateau. The southwestern escarpment of 
the plateau is not the work of erosion, but is unquestionably due to a 
fault — a fault of not less than two thousand feet throw and which has 
occurred since the eruption of the sanidin-doleryte. The strike of the 
fault, and of the plateau is northwest, strictly parallel to the Gila 
and Pihaleno ranges, which lie ten to twenty-five miles, respectively, 
to the southwest ; and the plateau should perhaps be regarded as a 
member of the basin range system. If it is so, then it is in longitude 
1 10 degrees, the most northerly member.'* 7 
The Plateau-Canyon Creek Escarpment. — The west line 
of the Plateau south of Salt river is a fault-scarp caused by an 
upthrust. The fault-scarp is continuous to the north of Salt 
river and is expressed in the Canyon Creek escarpment. The 
cause of the upthrust is readily determined to be an intrusion 
of trachyte (rhyolyte) at the south end, and an intrusion of 
hornblende biotite granite and rhyolyte and dioryte at the north 
end of a fissure which crosses the country in a north and south 
direction. The faulting left an escarpment of at least 4000 feet, 
2000 feet of which still remains in many places. 
The Apache Mountains. — Between Globe and the Salt 
springs on Salt river is a series of mountain ridges commonly 
called the Apache mountains. They extend in a curved line 
from the west side of the Sierra Ancha, curving around by the 
south of trie plateau and the Xantan mountains. They are com- 
posed of a series of semi-circular faulted blocks traversed by 
a series of transverse radiating faults of secondary importance. 
The mountains, as a whole, are one of a series of blocks faulted 
off of the Plateau. Their core is granite and allied rocks, as 
is also the crest where not covered with Tonto sandstone or 
volcanoes. These mountains are Hanked on their southern side 
