Geology of Ft. Apache Region, Ariz. — Reagan. 299 
■siderable distance from the crater affected the surrounding strata 
in a similar way. 
As to how these buttes were formed since they are pos- 
sessed of neither the form nor the structure of a surface erup- 
tion, the writer conceives them to be vestiges of flues though 
which an eruption reached the surface at the time of the flood- 
ing- of the region with lava. The last contents of the flues, 
congealing within them, formed pillars of trap that opposed a 
stubborn resistance to the atmospheric agents which have de- 
stroyed the surrounding strata. They are casts, in lava, of 
which the mould was the conduit of a volcano, now not only 
extinct, but destroyed by erosion. In short, they are volcanic 
necks. 
Lava Flows. 
The erupted rocks are: Diorytes (and Diabases) ; Trachytes 
and Rhyolytes ; Basalts, (including the Sanidin-doleryte lav- 
as). These were erupted in the order given. 
Diorytes (and Diabases). — In the Canyon creek region the 
great dike and the lavas interstratified with the Algonkian 
series are dark to dark olive green in color, rarely form cliffs, 
and on weathering crumble into a rather light olive green, 
coarse to fine sand. This lava was also noticed at several 
places in the Tonto and Cherry creek basins. They are un- 
doubtedly of Algonkian age. 
Trachytes and Rhyolytes. — The whole region, with a few 
exceptional patches, seems to have been covered with lavas of 
one or the other of these classes or with both. As has already 
been stated. Sierra Blanco is made up of trachyte, veneered 
with a sheet of sanidin-doleryte. From these mountains the 
two sheets extend over the entire eastern part of the region 
here described, reaching to the southwest more than forty 
miles to the Nantan escarpment, along which the lava series, 
according to G. K. Gilbert, 10 consists of: 
feet 
"1. Typical sanidin doleryte ; in beds 20 to 50 feet thick and re- 
markably continuous IOOO 
2. Silicic trachyte, (rhyolyte :) feet 
a. Pale pink to white, lithoid, light, somewhat brec- 
ciated ' IIO 
c. Purple, hard, fine grained, brecciated; base nol seen. .320 [• 460 
b. Dark brown, brecciated and amygdaloid^] 30 
Total [f4 6o< 
