The borax mine occurs as a bedded vein in the sedimentary strata, which
in Tertiary times were uplifted in the Calico range. The sediments are
composed of sandstones, sandy clays, and clayey sands, comprising a
succession of heavy-bedded, deep-water deposits, and shallow-water,
thin-bedded shales and sands. These variations in the character of the
strata are numerous, and mark the many oscillations of the region, whose
rising or sinking either submerged the strata beneath the waters of a
deep lake, or lifted them until the water flowed over the mud flats only
in thin sheets, which, exposed to the rays of the sun, sometimes
evaporated entirely. Climatic conditions doubtless also were an
important factor in the history of these strata, which are upwards of
1,000 feet in thickness.
Underlying the sediments are the tufas of the Calico region, and beneath
them is found the mass of liparite which underlies this entire region.
The sediments are not materially different from those in the immediate
vicinity of Calico. The rocks have not suffered in the region about the
borax deposit the slightest metamorphism.
The borax vein is traceable for several thousand feet, striking along
the western and northern side of the largest sedimentary hill in the
range, and finally passing down a cañon to the eastward, where it
becomes a well-defined vein. Toward the western end the borate of lime
appears to be much mixed with the sandy sediments, gypsum, and clays,
giving the appearance of having been formed near the shore line of the
basin in which this great mass of material has been left as a residuary
deposit, due to the evaporation of the water containing the calcium
borate.
To me it seems that what is now one of the most valuable deposits of
mineral in the State was at one time the site of a Tertiary lake of
considerable but as yet undetermined size. That although subjected to
the same oscillations as the remainder of the region a basin formed, in
which the waters collected, carrying with them the mineral salts derived
from the rocks of the neighboring country. That finally the climatic
conditions became such that the supply of water was less than the loss
by absorption and evaporation, and the waters of the lake slowly
diminished, it finally disappearing entirely, leaving on the floor of
the lake a thick deposit of calcium borate of snowy whiteness.
After the deposition of the borax bed a general subsidence of the region
occurred, the waters of the great Tertiary lake once more covering the
whole country. Again the sands and finer sedimentary material—the
erosion of the mountains—were carried down and found a resting place on
the floor of the lake, the borax bed being finally covered with several
hundred feet of this detritus. Now, as the same formation in which the
borax mine is found, and even the lower members of the rocks of that
age, are seen resting upon the high ridges and on some of the peaks of
the Calico hills, it would seem highly improbable, to say the least,
that these sediments were built up from the ruins of the Calico
Mountains themselves, but their source was in more distant ranges.
Besides the regular vein-like deposit of calcium borate found at the
borax mine, there are numerous small veinlets in other parts of the
district in which calcium borate and gypsum are found filling cracks and
cavities, probably as the result of infiltration. So common are these
small fissures and beds of borax and gypsum that that portion of the
sedimentary strata lying east of the town of Calico is usually spoken of
as the borax formation by the miners of the district. To thoroughly
investigate all the phenomena connected with these wonderful deposits
and their mode of formation would require more time than was at my
disposal.
As has been previously stated, remnants of the sediments are still found
lying high up on the flanks of the mountains, and even far into the
interior of the hills, and there is every probability that the entire
region included in the Calico District, as well as the country for many
miles around, was at one time buried a thousand feet beneath these
stratified rocks.
With the uplift, the strata inclosing the borax mine were tilted and
folded, and now the sheet of white calcium borate which once lay
glistening in the sun on the bed of a desert dry lake stands like a
great vein traversing the country. There are apparently two of these
veins in close proximity to each other, but I believe them to be one and
the same, being repeated as the result of an anticlinal fold. An ideal
cross-section of the borax mine is here given (p. 347), showing the
nature of the folding at that point; it is not drawn to a scale, being a
sketch only.
To the southward of the mine is seen a large mass of liparite, which has
been pushed up from below. I had not the time necessary to trace out the
line of fracture, but I am of the opinion that it occurs on the line of
the great fault shown on the map of the fault system of the region.
The borax vein is from 7 to 10 feet in thickness Where it has been
exposed in the underground workings. The mineral is the variety of
calcium borate called colemanite, named in honor of Wm. T. Coleman. It
occurs in glassy crystals, some of them having large faces. Many
handsome specimens of this mineral are on exhibition in the Mining
Bureau museum. The mineral is mined in the same manner as ores of gold
or silver. Inclined shafts are sunk on the vein, drifts and levels run,
and the stopes carried up as in any other mine.
[Illustration: _LONGITUDINAL SECTION OF BORAX MINE
CALICO DISTRICT SAN BERNARDINO CO._]
The material, when hoisted to the surface, is loaded in great wagons
hauled by twenty animals and taken to Daggett, where it is shipped to
the works in Alameda. The process of extracting the boracic acid from
the rock as practiced in these works is not given to the public. It is
known that the mineral is crushed and bolted like flour, after grinding
with burrs, but the subsequent treatment is not known outside the works.
To the Superintendent, J. W. S. Perry, I am indebted for a sketch of the
underground working of this remarkable mine, which is reproduced above.