Four miles northeast of the Black Hawk Mines, lying down on the desert,
is a formation of limestone and quartzite, resting on a massive
crystalline rock, containing quartz, feldspar, biotite-mica, and
hornblende. This reef extends from the mouth of Texas Cañon out upon the
plain, sloping downward at an angle of approximately 5° for a distance
of 4 miles, where it terminates in a bluff 40 to 100 feet in height.
Along the entire eastern edge of this deposit it drops off abruptly as
though sharply eroded. At the northern end the reef is faced by a low
range of hills composed of the above-mentioned hornblende rock. From
this point it swings west and with irregular outline extends for 5 or 6
miles toward Rabbit Springs. The entire area, fully 25 square miles, is
cut by gulches varying from 20 to 150 feet or more in depth, that have
been eroded through the strata and down into the underlying crystalline
rocks. These cañons seem to have resulted from natural drainage, being
started by slight depressions in the rolling plateau of limestone. On
the extreme northern edge at one point, hills of considerable size have
been formed by the folding and tilting of the strata. The limestone has
been subjected to violent compression, as the whole area is faulted and
broken into millions of fragments.
I have examined not less than twenty mining claims on the reef and
traveled over the greater part of its area, and am sure I never saw a
single piece of limestone that would weigh 300 pounds, most of the
pieces measuring under 6 or 8 inches. Considerable masses along certain
zones have been granulated, and even pulverized. This fractured rock has
all since been loosely cemented by the infiltration of carbonate of lime
into the seams. Geologists who have examined this peculiar deposit do
not agree entirely upon its mode of formation. Some believe it to be the
result of chemical precipitation of carbonate of lime from calcareous
springs, similar to the Formation Springs in the Yellowstone Park. There
are many things about Silver Reef which would at once suggest the
probability of this mode of deposit, but I am very doubtful that such a
theory will stand a thorough investigation.
It was asserted that at one time a shaft sunk in the reef passed through
the lime deposit and into the “wash” of the desert beneath. On
investigation I found that the lime had indeed been cut through, but the
underlying rock proved to be crystalline hornblende rock in place,
though somewhat decomposed. At any rate it was not desert “wash.” Over
considerable areas the lime is underlaid by a stratum of quartzite of
variable thickness, less than a foot in some places, and again in others
10 or 12 feet. Over certain limited areas quartzite is wanting
altogether. The lime is mostly crystalline, varying in color. A small
portion is as white as snow, the greater part is gray or bluish, and
some of it black.