San Benito County History

This section is excerpted from A Memorial and Biographical History of the Coast Counties of Central California by Henry D. Barrows and Luther A. Ingersoll, and published by The Lewis Publishing Company in 1893.

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Geology.

The following account of the geological formation of San Benito county is by Prof. F. B. Abbe:

"The origin of the Mount Diablo and Gabilan ranges of mountains, which belong to the Mesozoic and Cenozoic periods just subsequent to the great carboniferous age, is due to the contraction of the crust of the earth in cooling. The ocean swarmed with shell-fish at this time, and with the upheaval vast quantities of their remains were exposed to view, and to-day, wherever we may wander, we constantly meet with some fossil form of this ancient life, from the minute foraminifera of marble and limestone to the immense oyster shell weighing fifty pounds. There are but two points where any presence of igneous action is manifest; one at Frémont's Peak, the culminating point of the Gabilan range the other further south near the old Santa Bonita quicksilver mine. At the former point the range terminates abruptly in a series of jagged ridges, pitching into the plains below at sharp angles. These ridges are composed mainly of calcareous rocks, having been previously in a state of fusion, succeeded by aqueous action. On the fused walls and roof of a cave in the peak are stalactites and other curious aqueous formations.

Unlike the structure of the Sierras, these ranges had a more gradual growth, there being an entire absence of lava. Occasionally, a great convulsion would take place, disrupting huge masses of the crust, but not destroying its structure. This is clearly shown at several places in the southern part of the county. For many miles the course of the San Benito river cuts through or skirts these mountains. The strata exposed thus have many twists and curves.

In following the course of these strata, the general direction for some distance is horizontal, when it is suddenly broken off, just beyond, strata having the same markings have a vertical direction, and still farther on the direction is again horizontal, showing that during one of those violent disturbances, huge masses of the crust were forced bodily upward, dropping back into the same place again endwise. These are termed "faults" in mining parlance, and are commonly met with in mountains of this nature.

The formation of our valleys and cañons, geologically, is claimed by scientists to be due to the action of glaciers and floods. During the ~on known as the glacial age, the northern half of the United States was buried under an ice cap, the rim extending as far south as the latitude of San Benito county. Following this age of ice was a period of great floods and tempests. The disintegrating mountain walls were carried into the valleys below, covering their floors successively with layers of clay, boulders, gravel, sand, and the various barns, thus burying the original crust hundreds of feet. The boring of artesian wells has made this clear; fossil shells have been raised in the San Juan valley from the depth of 120 feet. From wells in San Felipe have been taken, at various depths, animal and even human remains, besides fragments of vegetation of both modem and extinct species. The exposed face of any high bluff showing the variety and thickness of the stratum, is an open page in the history of this period.

In discussing the composition of the rocks of our mountains and valleys, which more properly belong to the science of mineralogy? it is our aim to merely classify and describe the different mineral found here. The development of the mineral resources, excepting a few cases, is yet in its infancy. More attention, however, is being given to this important branch of our industries, so that ledges, and deposits of ores and minerals of many varieties and of great industrial value, are being constantly located.

The three metals, silicon, calcium and aluminum, form the base of the three most prominent classes of minerals found in the country, and which are known as the silicate, calcites and clays. The silicates are more abundant, but the calcites represent nearly every form known to mineralogy, ranging from the primitive chalk rock to the highly crystalline forms of spar and selenite. Other forms common here are marble, limestone, alabaster, gypsum, satin-spar, Iceland spar and dolomite.

The limestone is of the finest quality, and the deposits of great extent. The products from the marble, lime, and cienega kilns meet with favor with builders.

The gypsum and marble deposits are yet undeveloped, waiting better transportation facilities. The latter has been pronounced of fine quality. It has dark-colored streaks running through it, the whole when polished presents a beautiful appearance. Were it a little closer grained, it would fully equal the famous Italian product in durableness. The oldest of the three classes, the silicites, is not so fully represented as to variety, the lower grades only being common. The sandstone of the Bromas district possesses valuable features as a building stone, many of the oldest structures in San Juan being built of it.

Other forms of this class scattered in more or less quantities throughout the country, and valuable to the lapidarian for inlaid and ornamental work, are agate, chalcedony, milky and rose-tinted quartz, jasper and flints, all of which are susceptible of a high polish.

Granite of this class, the oldest of all metamorphic rocks, is here found side by side with marble, an occurrence rarely met with, owing to the vast periods of time elapsing between the origin of the two rocks.

Soapstone, another silicate, is met with in several localities, the product from one of the deposits being shipped to San Francisco and used in the manufacture of a variety of articles.

The alurninites are not well represented in the county, although clay banks, shales and slates are met with everywhere. A fact not generally known is that under every man's home in the county lies a "gold vault" in the form of that most useful, as well as beautiful metal, aluminum, which awaits only the magic key of chemistry to deliver it to the fortunate possessor.

The other minerals and metals found here, each of which forms a base of a class, are sulphur, asbestos, coal, petroleum, iron, copper, antimony and cinnabar, from which comes mercury.

Coal is found in extensive deposits, but like all the coal on the Pacific slope, is of a soft, lignitic variety, owing to the recent period of its origin. It is well adapted for the manufacture of gas and household purposes, but where an intense heat is required, as in blast furnaces, etc., it is found wanting.

The two metals that have made San Benito county famous as a mining county, and of which fact it may well be proud, are antimony and mercury, commonly called "quicksilver." These metals are met with in but few localities throughout the world, but here. they appear to exist in inexhaustible quantities in the Mount Diablo range. The New Idria mine is ranked as the third largest quicksilver mine in the world.

From the Shriver antimony mines have been taken huge masses of the metal in needle-like crystals, much sought after for cabinet uses. The metal is used most extensively for the manufacture of type, possessing the property of contracting when in a state of fusion, and expanding on resuming a solid form.

Much has been said and written about the presence of gold and silver within the borders of San Benito county. Many twice-told tales of discovery of ledges of fabulous wealth have oft thrilled groups of listeners to such an extent that they caught the mining fever and started in search of these hidden riches; but fate had decreed otherwise, and the ledges remained to be rediscovered.

From almost every cañon of the encompassing mountains "color" may be washed, but that either gold or silver, in paying quantities, is to be found in San Benito county, or even in the coast ranges, the science of geology denies. The precipitation of these metals occurred before these mountains rose from the bosom of the Pacific, with the exception of a few isolated peaks. Another law of geology is, that where there is an absence of lava, there will be like absence of these metals.

In conclusion it is safe to say that probably in no spot of like extent within our country is there a greater variety of minerals and a richer field of geological research than in San Benito county.

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This section is excerpted from A Memorial and Biographical History of the Coast Counties of Central California by Henry D. Barrows and Luther A. Ingersoll, and published by The Lewis Publishing Company in 1893.