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The volcanic deposits occupy a large area in the lower part of this county, rendering the working of quartz subordinate to hydraulic and tunnel mining.
The metamorphic belt is in great part covered by volcanic materials. From near Auburn to the Sacramento plain, granite is the underlying rock. In this several quarries have been opened, furnishing a superior building material. Iron ore (hematite) occurs in considerable quantity a few miles from Auburn, and under as favorable conditions, as regards extent and location, as at any other point in the auriferous slate series. The north and Middle Forks of the American river flow through deep gorges or narrow canons, which they have eroded in the volcanic outflows, cutting deep below them into the slates, of which they afford fine exposures.
The towns of Iowa Hill, Wisconsin Hill, and Todd's Valley, mark an important line of hydraulic mines, extending across this county. In places the detrital beds have a thickness of more than five hundred feet, the "cement," or coarse compacted gravel below, often being one hundred feet in depth. On the Middle Fork of the American, the detrital beds reach to the summits bordering the canon; at Sarahsville, near which place is an immense mass of serpentine, they recede towards the north. These deposits are covered by beds of sedimentary volcanic materials capped by basaltic lava, which forms the summit of the ridge between the North and Middle Forks of the American river. This ridge is cut by deep canyons or gorges, in one instance two thousand feet in depth, with sides sloping at as high an angle as forty-five degrees. The auriferous slates beneath are sometimes eroded to a depth of fifteen hundred feet, and peculiar facilities are thus afforded for the study of their structure.
It was in this vicinity that Prof. Whitney observed the very interesting fact, illustrative of the probable fan-like structure of the strata flanking the central portion of the Sierra. These usually show an easterly dip, towards the chain; in these deep vertical sections, it was noticed that the upper one thousand or twelve hundred feet had the normal dip to the east, but below this there was a gradual curve, and at the bottom the dip was to the west, as if the upper portion of the strata had been forced back by immense pressure from above—a variety of structure, of which there are many examples in the Alps, and which, for a long time, perplexed European geologists.
The auriferous belt in this county is wide, and includes extended areas of granite, one of which passes but a little to the east of Grass Valley. The limestone belt may also be traced through the southwestern part of the county. It is exposed at a place called Lime-kiln, ten miles south of Grass Valley, and is in line with the fossiliferous limestone at Pence's ranch, known to be of carboniferous age.
As we proceed northward on the auriferous belt, the strike of the strata becomes more nearly north and south, the system of northwest and southeast trends gives out, and we find a preparation for the north and south lines of upheaval, which characterize the mountain chains of western Nevada and northeastern California. The rocks retain, however, the same marked easterly dip, and toward the lower side of the belt the inclination seems to be greater than it is further east.
Grass Valley is justly celebrated as being the principal quartz mining center of California, the business having been commenced here at an early day, since which it has been prosecuted with many vicissitudes, but generally with marked success. The veins here, though numerous, are not generally large; their richness, however, compensates for their want of size. Their average width is perhaps two feet, while some, that have proved extremely productive, have not averaged above a foot or eighteen inches. They are for the most part highly mineralized, and have evidently been formed by aqueous action. They abound in the sulphurets of iron, copper and lead, and occasionally zinc; arsenical pyrites also sometimes occur, as for instance in the Norambagua mine, and on Heuston Hill. The gold, is generally associated with the sulphurets, though it is found sometimes in beautifully crystalline masses in pure quartz; it is irregularly distributed throughout the veinstone, which is often barren, but frequently very rich. The rocks in the vicinity of Grass Valley are so highly metamorphosed as to obliterate all traces of stratification; and it is, therefore, impossible to state the true position of the veins with reference to them. The most productive vein has been that upon Massachusetts and Gold hills. In working seventy thousand tons of rock from this mine, the average yield of gold was over eighty dollars per ton. The sulphurets occurring in the Grass Valley mining district are generally rich in gold. In quantity they usually do not exceed more than one or two per cent, of the mass of ore; though in some mines they are more abundant. They are now carefully collected and worked by Planner's chlorination process, by which over ninety per cent. of their entire contents in gold is saved. The experience gained at this place, as well as in working other quartz lodes elsewhere in California, some of which have been developed to great depths, tends to disprove the theory that the yield of gold diminishes in the ratio of the depth attained.
Prof. B. Silliman, in speaking of the Eureka mine, near Grass Valley, observes that from the date of its location, February 7th, 1851, to the close of 1858, it proved only a source of expense; and its history is instructive, as suggesting that shallow surface exploration, in gold mining may be as unsatisfactory as they are known to be in other mining enterprises. So late as 1858 five thousand tons of quartz, taken above the drain level, or thirty feet from the surface, yielded in the mill less than ten dollars per ton gold—not returning expenses. A shaft sunk to a depth of about fifty feet afforded quartz, however, which yielded fifteen dollars per ton, and the amount of gold rapidly increased to twenty-eight dollars per ton at one hundred feet. Between the one hundred and the two hundred feet levels the average yield was about thirty-seven dollars per ton, and between the two hundred and three hundred feet levels the average has been about fifty dollars per ton, rising to sixty-four dollars in the last months of 1866.
There are in fact two distinct veins in the Eureka mine, separated from each other by a mass of greenstone, or metamorphic sandstone, about twenty-eight or thirty feet in thickness. The smaller of these veins is on the south, and has not been explored, but is well defined at a point where the shaft and cross cuts have exposed it. The greenstone forms the hanging wall of the main vein, and is particularly regular and smooth, in some places beautifully polished. The foot wall consists in some parts of soft serpentine. It may be interesting to analyze a little more in detail the returns of this mine, as illustrating a point already alluded to, viz: its progressive increase of gold with increase of depth. From October, 1865, to December 31, 1865, the quantity of quartz crushed was twenty-four hundred and forty-five tons, yielding an average of thirty-three dollars and eighty-seven cents per ton, and costing to mine and reduce thirteen dollars and fifty-one cents. From January 1st to June 1st, 1866, the crushing was forty-seven hundred and three tons, averaging forty-six dollars and sixty-eight cents per ton, at a cost of twelve dollars and fifty-two cents per ton. From June 1st to September 30th, 1866, the amount of quartz crushed was forty-two hundred and twenty-seven and three-fourths tons, giving an average yield of sixty dollars and thirty-three cents per ton, at a cost of fifteen dollars and seventy-eight cents per ton. For the whole year ending September 30th, 1866, the total crushing was eleven thousand three hundred and seventy-five and three-fourths tons, yielding a general average per ton of forty-five dollars and eighty-three cents, at a mean cost per ton of thirteen dollars and seventy-five cents. The total product of bullion from this mine for the year ending September 30th, 1867, was $585,000—average yield of the ore having been $48 per ton.
Nevada city is another important quartz mining locality in this county. Nevada county also claims special attention, on account of some of the most extensive hydraulic washings to be found in the State. The great ancient river channel of Sierra county, known throughout California as the Blue Lead, enters this county on the north, at Snow Point, and probably continues across it, connecting with the detrital deposits at Red Dog, and thence through Placer county to Todd's Valley. Though it is impossible to reconstruct the ancient river system in the absence of more full and perfect data, enough is known to establish the fact that their course was approximately at right angles to that of the present streams.
In the hydraulic washings at Red Dog, great numbers of trunks of trees have been uncovered in the operations of mining; they are silicified, and are shown to have been subjected to the force of violent currents before they were covered by the thick detrital deposits. In the finer sedimentary layers, impressions of leaves are found, but animal remains occur less frequently than in similar deposits in the more southern counties. Auriferous gravel deposits, beneath volcanic formations, have been worked in the vicinity of both Nevada City and Grass Valley. At the former place, above the lower twenty feet constituting the pay gravel, is a bed of lignite, with much iron pyrites resulting from the reducing action of decaying vegetable matter.
Between French Corral and San Juan, along the Middle Yuba river, is a belt of hydraulic washings famous for their productiveness; this is about one thousand feet wide, and towards its eastern end the bottom of the deposits is at an elevation of at least one thousand feet above the river, which has cut its channel since their deposition. The lower portion of these detrital deposits, which consist of pebbles and boulders of quartz, granite, and the metamorphic rocks of the Sierra, firmly compacted and cemented together, is often of a bluish color, contrasting with the brownish yellow of the upper portion, due to oxidation of iron. This deposit appears to be the continuation of a known ancient river channel, traversing the entire western portion of Sierra county, and running parallel with the famous Blue Lead already mentioned.
This county lies wholly in the high portions of the Sierra north of Nevada county. The lowest point in it, where the north Yuba river cuts its western boundary, is over two thousand feet above the sea. The auriferous slates are exposed in its western portion, though they are generally covered by accumulations of volcanic origin, consisting largely of breccia, or volcanic conglomerate. Some of the summits, formed by basaltic lava capping the slates, are estimated to be over eight thousand feet in height, and in this county form the crest of the Sierra Nevada mountains. The slates exposed in the numerous deep canyons, with which the county is furrowed, are seen to inclose large masses of serpentine and talcose slate; they also include many promising quartz veins.
Within five hundred feet of the summit of the Downieville Buttes, or, as they are sometimes called, the Sierra Buttes, at an elevation of eight thousand feet, are the quartz mines belonging to the Sierra Buttes and Independence Mining companies. Here, an immense vein, from six to thirty feet in width, cuts across the ravines and gulches from east to west, dipping at an angle of forty-two degrees to the north, a more detailed description of which is given in the chapter on the subject of "Mines and Mining," to be found in another part of this volume.
Sierra county, as before remarked, is almost wholly covered by' beds of volcanic origin, cut in numerous places by the streams which have eroded their channels to an immense depth in the underlying states. The auriferous gravel deposits of this county are probably more extensive than are to be found elsewhere in the State. The famous Blue Lead, or ancient river channel, has been traced from Sebastopol, in the northern part of the county, south, crossing the course of the present streams nearly at right angles, to Snow Point, in Nevada county, its course being marked by a long line of tunnel claims and mining camps.
The phenomena exhibited here do not differ materially from those presented in Table Mountain, Toulumne county. A map of Sierra county, prepared by Messrs. Grossman & Cochran, the former of whom has had peculiar advantages in the study of the ancient river system of this county, represents four of the ancient river channels as having a generally northerly and southerly course, and crossed by the present streams, instead of running parallel to them, as is the case in Tuolumne county. The valley of Table Mountain river is shown to have been filled with one volcanic outflow or stream, but in Sierra county