3 oo The American Geologist. November, loos. 
But west of the Plateau break in the Apache and Pinal 
mountains the acidic lavas become a rhyolyte varying in color 
from white to a dark gray in the former, to a pink in the latter. 
For convenience the lavas of the two districts will be consid- 
ered separately. 
Rhyolyte in the Apache Mountains. — From Sombrero butte 
south across the Apache mountains to Globe, and in fact the 
entire area included between Pinal creek and the Plateau re- 
gion, is covered with rhyolyte where it has not been removed 
by erosion or covered with sediments. At least two flows have 
occurred in this district separated from each other by a long 
lapse of time. The last flow was a whitish colored rhyolyte 
and is found on Cherry creek interstratified with the Tertiary 
deposits of the Hinton formation. This flow either did not 
cover a large area or its extension has been almost wholly re- 
moved. The rhyolyte of the first flow is a massive, fine 
grained, heavy, dark gray to blue and light colored rock, 
weathering to a seal brown, and breaking down in palisade 
cliffs. It is the lavas of this flow that are the most extensive of 
the rhyolytes in this region and that once covered the enire 
area with the lava sheet 2500 feet in thickness, provided the 
lava sheet extended to the level of the top of Chromo butte, as 
there are reasons to believe it did. (See discussion on Volcanic 
Buttes), 1500 feet of which still remains on the northeast flank 
of the Apache mountains. 
That the date of the lava flows was subsequent to the fault- 
ing is shown by the fact that they extend to the very edge of 
the Plateau escarpment but do not cross over it. Furthermore, 
their ejection occurred a long time before the deposition of the 
Hinton Tertiary formation, so long before that erosion had 
removed most of the lava from the region. 
The age of these lavas seems to be Tertiary and pre-Plio- 
cene. 
With reference to the vents from which the lavas were 
emitted, it is the opinion of the writer that the rhyolytes repre- 
sent fissure eruptions and were associated with the Plateau- 
Canyon creek dike south of Salt river, and that the craters and 
buttes which are now in the region were only active in the last 
eruptions, the lavas then ejected being wholly removed, except 
where interstratified with the Tertiary. 
