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Its course is marked by many flexures. The inclosing rock is a dark colored, fine grained, argillaceous shale. The mine has furnished specimens of crystaline gold, having brilliant faces of rare and unequalled beauty. Iron pyrites, blende and galena occur in greater quantities here than at the Pine Tree and Josephine mines.
Mines Elsewhere in the County.—Heavy outcrops of quartz occur near Coulterville. A few miles northwest of the town is a massive outcrop known as the Penon Blanco (white rock). Here the quartz mine, known as McAlpine's lode, has been extensively, and, it is said, profitably worked. The outcrop is generally considered to be a continuation of those which mark the position of the Pine Tree and Josephine veins. outcrops of quartz occur along a northwest and southeast line, for a distance of seventy miles from the mines on the Mariposa estate, extending as far north as Jackson, in Amador county.
It is equally certain that the principal quartz veins and the most extensive placer mines in the counties of Mariposa, Tuolumne, Calaveras, and Amador, are nearly in the line of this succession of outcrops. An interesting quartz vein a few inches thick, containing crystalline cinnabar, occurs in the metamorphic slates, on the Merced river, near Horseshoe Bend.
The eastern portion of this county lies in the high regions of the Sierra, and is underlain by granite. In the western part of the county the auriferous slate belt attains a width of about twenty-five miles. The metamorphic rocks are marked by very different lithological characters—the slates are silicious and argillaceous, rather than talcose. Sandstones are so highly metamorphosed as to have a trappean character, making it often difficult to distinguish between eruptive and metamorphic rocks.
Limestone occurs at various localities in Tuolumne county. It is generally crystalline, of a bluish gray color, though where most highly altered it is white. It is quarried extensively near Columbia, and affords a good material for building purposes, monuments, etc. The mining region in this county is very extensive, and contains not only numerous quartz mines, but large areas of deep deposits of auriferous gravel, covered by sheets of basaltic lava, which have flown down the western slopes of the Sierra, filling and closing the channels of former rivers, directing their courses, and remodeling the topography of the entire region. The detrital deposits of this county have furnished more fossil remains of large animals than the same formations in any other part of the State.
Table Mountain.-—In this county is, perhaps, the most striking example of the flat, table-like masses of basaltic lava capping the auriferous detrital deposits, and brought out into bold relief by the erosion of the softer materials on both sides of them. The well known Table mountain of Tuolumne county is a vast lava flow from the lofty volcanic region beyond the Big Trees of Calaveras. It forms a nearly unbroken ridge on the north side of the Stanislaus, two thousand feet or more above the river. Its upper surface is nearly level, but the edges and the surrounding country have been denuded to an enormous depth by forces which its superior hardness enabled it to resist. The Stanislaus river now runs at a depth of two thousand feet below, and could not have existed at the time of the volcanic outflow, which must have sought the lowest channels. That this was the case, and that where the Stanislaus now runs there was a mass of mountains, is not a mere matter of speculation, for this lava flow is seen to have crossed the present valley of the Stanislaus at Abbey's Ferry, and must have followed the course of an ancient channel. It follows, that since the ancient valley was thus filled with the volcanic mass, that an amount of denudation, not less than three or four thousand feet, has taken place within the most recent geological epoch.
This is one of the many examples supplied along this belt of the results of extensive lava outflows from the higher portions of the Sierra. They are not confined to this county, being a marked feature in tho mining counties north of Tuolumne, particularly Nevada and Sierra. This whole region has been remodeled, and where are now deep canyons and gorges there were formerly hills, which determined the course of the streams of molten lava. We thus have, on the western flank of the Sierra, an ancient as well as a present river system. If further evidence of this fact were wanting, it is furnished in the character of the detrital deposits, and the surfaces of the rocks, in the ancient channels, which, lying beneath the lava, and the accumulations of volcanic material, have been largely developed in the system of tunnel mining now extensively prosecuted in all the leading mining districts of the State.
Fossil Remains —As before stated, these ancient deposits are of tertiary age—they have been referred to the pliocene epoch. Since the time of their deposition, and the period of that intense activity that followed, enormous denudation has taken place and continued to the present time, resulting in the formation of new and shallower deposits from the disintegration of the old. In this superficial detritus the works of man are found so closely associated with the bones of the mastodon and elephant, that the conviction necessarily follows that he existed previous to the disappearance of these animals from a region in which they were no doubt numerous. These, as well as discoveries of like nature made in Europe, prove the human race to be of much greater antiquity than is generally supposed. The remains of the mastodon and elephant have not been found in the deposits beneath the lava, but the bones and teeth of animals, and pieces of silicified wood, are common in these older auriferous gravels; impressions of leaves in the clay beneath the gravel are also found. Of the animals peculiar to the deposits beneath the lava there are the rhinoceros, an extinct species of horse, and also a species allied to the camel.
Six miles east of Sonora, in the neighborhood of Soulsbyville, are other volcanic deposits originating in the high Sierra. Near Soulsbyville, lava, fifty feet in thickness, rests upon a stratum of volcanic ash and pumice stone, deposited in a stratified form. These deposits contain the bones and teeth of animals similar to those found beneath the lava of Table Mountain.
Gold Mining.—Nearly the whole region between Kincaid Flat and as far north as the Stanislaus river has been worked, proving one of the most productive placer mining districts in the State. The surface of the limestone, with its deep crevices, has acted favorably in the retention of the gold.
Many quartz veins have been and are still being extensively and profitably developed in Tuolumne county; several of those heretofore worked having yielded very large returns. At the present time the business is being prosecuted in a number of districts with satisfactory results. The great "mother vein," so termed, appears in an outcrop near Jamestown, forming the eminences known as Whisky Hill, Poverty Hill, and Quartz Hill. It is of very large, though of variable dimensions, and, while barren in many places, has paid at least moderately well in others, the above localities having been the scenes of extended and tolerably successful mining and milling operations.
The belt of auriferous metamorphic rocks continues on through the central portion of Calaveras county, its width remaining about the same as in Tuolumne. The southwestern portion of the former is rarely covered except by superficial detritus; but the northeastern, in the neighborhood of the junction of the slates and the granite, is marked by the occurrence of gravel deposits, covered by volcanic outflows, similar to those in Tuolumne county.
Union Copper Mine.—The western portion of the belt includes the celebrated Union Copper Mine, a few years since largely and profitably worked, though but little has been done upon it for the past two years, owing to the low price of copper ore, and to lawsuits pending against the present owners. The ore is not found in a regular fissure vein, but lies apparently in independent lenticular masses. Large shipments were made from this mine for several years after it was first opened. The ore is the yellow sulphuret, (chalcopyrite), with a mixture of iron pyrites. The inclosing rocks of this deposit are chiefly chlorite and chloritic slates. Serpentine, presenting indications of copper, occur west of Copperopolis, apparently trending with the formation.
Gold Mining.—The great quartz vein of California passes to the east of these copper deposits. It appears at Carson Hill, at Albany Hill, at Angels, and both south and north of San Andreas. It has been extensively worked at various points, the mines of Carson Hill alone having furnished four million dollars of gold. From the Morgan claim over two million dollars are said to have been taken from a small space. The slates adjoining the vein have proved very rich, paying as much as eighty dollars to the ton. The placers in this vicinity were also formerly very prolific. The gold, however, is here so irregularly distributed in the quartz veins as to have rendered the business of mining for it very fluctuating and hazardous. The Stanislaus mine, near Santa Cruz Hill, in the vicinity of Robinson's Ferry, has furnished remarkable specimens of auriferous rock, in which, associated with the gold, are the rare tellurides of silver and gold, in larger quantity than they have been found elsewhere in the State.
The placer and hydraulic mines of Calaveras county are extensive, and have generally proved fairly and often highly remunerative. Volcanic deposits are widely diffused over the northeastern section of the county. Limestone, deeply eroded on the surface, occurs towards the eastern portion of the gold bearing belt.
An exposure on the road from the Stanislaus river to Murphy's shows a thickness of five hundred feet of volcanic and sedimentary material resting on the limestone. The upper portion, over one hundred feet in thickness, is basaltic lava, resting upon a series of beds of sand, clay and volcanic ashes containing boulders of quartz. The surface of this limestone, in the vicinity of Murphy's, has been considerably worked for placer gold; imbedded in it are veins of quartz, some of which have also been mined. A vein occurring in it contains not only gold, but cinnabar in small quantity, together with vitreous copper, and some blue and green carbonate of copper. A second instance of the occurrence of cinnabar in the rocks of the Sierra.
The main gold bearing belt passes through the central portion of Amador county, but is much narrower here than in Calaveras, being only about twelve miles wide. Towards the eastern border of the states we have a continuation of the limestone of Tuolumne and Calaveras. In placer mining, once active here, but little is now being done. Along the line of the main belt there are a number of prominent mines, foremost amongst which is the Hayward or Amador claim, consisting of the Eureka and Badger lodes. The cost of stamping and milling quartz at one of the mills of this company, where water is plenty, is stated to be sixty-six cents per ton; less, perhaps, than at any other mill in the State.
At Volcano we have the same limestone formation, with small veins of quartz imbedded in it. Here, also, the detrital mass is thick, and has been profitably washed in many places. In one of the beds in this vicinity, a distinctly marked quartz vein occurs in the gravel, showing how recently veins have been formed.
On the Cosumnes and_ Mokelumne rivers fine sections of the sedimentary, with superimposed lava deposits, are exhibited.
In the tertiary foot-hills bordering the Sacramento valley, coal has been found, but too limited in quantity and of too poor a quality to be of any other than mere local value.
EL DOBADO COUNTY.
The geological features of this county are similar to those of Amador, but the volcanic formations are not so extensively developed as in the latter. There are some detrital deposits here still worked by the process of hydraulic washing. The belt of auriferous rocks occupies a great breadth here, it being nearly thirty miles broad, in a direction at right angles to the trend of the slates, which largely predominate. Some portions of these are of Triassic age, a determination based, in part, upon the resemblance of the impressions found in the slates to the fossils from known triassic rocks occuring at Washoe, and in the Humboldt mining region, in the State of Nevada. Quite a number of fossils of unquestionably Triassic age have been found by members of the State Geological Survey, in Plumas county, farther north.