The geology of the Calico Mountains at first sight looks simple enough,
but a more thorough investigation quickly convinced me that there were
structural problems to be studied of more than passing importance, as
they seemed to have a bearing upon the extent of the ore deposits. The
most complex region is that immediately about the town of Calico, in the
vicinity of the mines. The balance of the mountain area is more simple.
In a general way the Calico uplift consists of a core of massive
rhyolite, overlying which are heavy deposits of light-colored breccia
and tufa. Along the flanks of the range, and in some places extending
well up into the mountains, are accumulations of undoubted sedimentary
origin, sandstone, sandy shales, and argillaceous rocks, which, with
some local exceptions, dip away from the central mass on all sides
toward the desert plain. While in the district I made some notes on the
general geological features, but not having sufficient time at my
disposal to complete these investigations, I have determined not to
present my views until I have had an opportunity to investigate the
region more carefully.
Subsequent to the uplift of these mountains, erosion has carved deep
cañons and removed great mountain masses. The central area is now
entirely denuded, whereas it was at one time covered with from 100 to
200 feet of tufa and upward of 1,000 feet of sedimentary strata. Not
only have these more recent accumulations been removed, but a large
amount of the hard, dense liparite has also been disintegrated and
carried away by the violent storms which are characteristic of the
desert. Faults are very numerous throughout that portion of the
mountains lying along the south side of the range. They extend for at
least 10 miles in an easterly and westerly direction. The mines occur
along this faulted zone.
The rocks of the region are a violet to brown rhyolite, often
porphyritic; green, yellow, and white tufa; yellowish and greenish
breccia; a greenish gray, fine-grained rock, which has been called
hornblende andesite by Mr. Lindgren, and a yellowish or buff to light
gray felsitic rock, which may be either rhyolite or an older felsite. It
is extremely difficult to distinguish between these rocks, even with the
aid of thin sections under the microscope. I think, however, upon
structural grounds, that I may call the rock felsite. As this is one of
the important questions upon which I have not thoroughly satisfied
myself, it will be left until such time as I have opportunity to make
the necessary investigation.