The Oro Grande Mining District is located immediately east of the
Southern California Railroad, at the town of Oro Grande. It is commonly
supposed to embrace all the mines for some miles around, though, in fact
each group of mines or hills has been given a separate name, but as
these so-called districts are mostly without organization, all the
claims and mines will be considered under one head.
The geology of the district about Oro Grande is complex, the formations
being uplifted, greatly faulted, and broken, besides the intrusion of
dikes of felsitic rock, diorite, and quartz porphyry. I had not
sufficient time at my disposal when at Oro Grande to study out the
somewhat intricate geological problem, and will describe the region in
general terms.
Commencing at the town of Oro Grande, which stands on the bank of the
Mojave River, the country rises in a gentle slope toward the hill half a
mile distant; gently rolling hills are reached, which in turn give place
to more rugged masses, and finally to a rough mountainous area, the
hillsides being almost precipitous. The lowlying country about the base
of the hills is mostly made up of schistose micaceous rocks, a quartzose
mica schist predominating. The first hills of any consequence are
eruptive, mostly a light-greenish felsite and a coarse-grained
porphyritic rock. Beyond are prominent hills of a dense, hard quartzite
resting upon a crystalline limestone, the highest hills being made up of
practically the same materials (quartzite and limestone), in part
schistose with some mica schist, jasper, and many intrusive dikes of all
the previously mentioned eruptives, prominent among them being diorite
of dark-green color.