Geology of Ft. Apache Region, Ariz. — Reagan. 279 
like the Silurian, is exposed as a narrow strip encircling the 
Ellison dome from the Nantan mountains on the southeast 
around by the northeast to the Tonto basin where it forms one 
of the "rim" series of that basin. The strip thus formed is 
sometimes more than three miles wide, though it is usually 
much less. Besides this belted strip several of the buttes of 
the Tonto region belong to this formation, one of which is 
situated three miles northeast of Ellison. This butte is called 
Juniper butte on the government maps ; but among the settlers 
it is called Shell mound, because of its numerous shell fossils. 
The formation is Devonian as is indicated by the fossils ob- 
tained, Oriliis livia, Spirifer fornacula and Acervularia david- 
soni, and corresponds very much to the Devonian formation 
at the falls of the Ohio. 
The Lower Red Wall Group. — The first series above the 
Devonian is from 400 to 1000 feet of alternating gray limestone 
and shale. It terminates at the top in a massive to shaly coarse 
>and rock, often of the mill-stone grit type. The lime rocks are 
very fossiliferous from the middle of the series to the top and 
seem to belong to the lower Carboniferous group ; but as yet 
the writer has not identified fossils enough from the group to 
verify such a conclusion. 
This group of rocks is exposed along the canyon walls of 
Salt river from its confluence with Cibicu creek east farther 
than was visited, in the canyon of White river from Fort 
Apache west to Salt river and in the canyons of Cedar, Carrixo 
and Cibicu creeks in their lower courses. It is the surface rock 
for many miles on either side of Salt river east of that river's 
confluence with Cibicu creek from which place it extends in a 
wide belt around the Ellison dome to the Tonto basin. The ex- 
posures of this series make good soil on which grow the best 
grass and timber of the western part of the Fort Apache Indian 
reservation, and the Grasshopper Spring division of it is said to 
be one of the leading grass producing regions in Arizona. 
The Upper Red Wall Group. — The writer has included in 
this group all of the strata superimposed on the mill-stone grit 
rock of the group just described, to and including the dark 
gray limestone series about 1000 feet above its base. The ser- 
ies here is very variable, indicating a rapid transformation from 
west to east. Ripple marks and old shore lines are noticeable 
