Old Mines of Southern California / Desert-Mountain-Coastal Areas Including the Calico-Salton Sea Colorado River Districts and Southern Counties

by William H. Storms; Harold W. Fairbanks

THE BORAX MINE.

In the upturned sedimentary beds which flank the Calico Hills, dipping outward toward the desert plain on all sides of the uplift, except where purely local disturbances have caused a reverse condition, are bedded deposits of calcium borate and gypsum (calcium sulphate). Five miles east of the town of Calico is the largest known deposit of calcium borate in the district. The bed, or vein, as it is called, was discovered some years since, and finally passed into the hands of the present owners, the Pacific Coast Borax Company, whose extensive works are located at Alameda, near San Francisco.

GEOLOGY OF THE BORAX DISTRICT.

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.