Proceedings of the California Academy of Sciences, Volume III, 1863-1867

by Various

The earthy covering of the rocks throughout the valley is usually very shallow and the soil poor, (Tower’s ranch is, however, an exception) and in many places the thin sharp edges of the slates project in such a way as to form an exceedingly jagged surface, though the projections are low, generally not exceeding two or three feet in height. Much of the surface is strewn with float quartz, usually in the shape of small but partially rounded pebbles. Quartz veins of small or moderate size, parallel with the stratification, are not uncommon. Iron pyrites is of frequent occurrence, with a little gold in the quartz. Some of the veins have been more or less worked, but none of them to any great extent. About three or four miles westerly from Copperopolis, in the hilly portion of the valley, is a ten-stamp quartz mill, and a short distance from this, on Littlejohn’s Creek, is the site of an older one, which was burned down. Neither of these mills ever yielded much profit, so far as I can learn, nor does the present one seem likely to do so.

Several of the gulches in this vicinity are said to have yielded gold enough in the past to pay for working, although the diggings were not rich or extensive. It is stated also that some years since, in one of these gulches, a quartz boulder was found, weighing about one hundred pounds, which yielded between two and three thousand dollars’ worth of gold. There are three or four quartz veins near here, from which more or less rock has been crushed. Portions of the rock from one of these veins, the Winnemucca, a prettily-shaped vein of three to four feet in thickness, are very cellular in structure, and some of it shows fine gold quite freely to the naked eye. The metal however, must be very irregular in its distribution, or the ore would have paid better in the mill than the three or four dollars per ton which I am told it yielded; and in fact, the general character of the float quartz of the region, when taken in connection with the probable origin of the valley itself, and the fact that no important placer “diggings” have been found here, does not seem to favor the probability that these quartz veins will ever prove of much value. Between the present mill and the site of the old one, as well as certain other localities in the valley, are springs containing various alkaline salts, from which the name “Salt Spring Valley” is derived.

Accompanying the copper formation of Copperopolis, and just west of it, is an immense body of serpentine, lying parallel with the general stratification of the slates, and traceable for miles along the valley by the openings made in it in the workings for copper. Opposite a point 1,000 or 1,200 feet northwest of the upper shaft of the Keystone claim, but on the southwest flank of the outlier of Bear Mountain, already noticed, is another heavy mass of serpentine. How far this extends in a northwest and a southeast direction I do not know, as I have not followed its line of outcrop, but it is certainly not less than 1,000 feet in length.

The lithological character of the Gopher Hills is entirely different from that of Salt Spring Valley. They consist mainly of a pretty hard and tough, more or less coarsely crystalline, and dark-colored hornblendic or pyroxenic rock, which is evidently metamorphic, probably of a grit or sandstone. Epidote is not uncommon in this rock, and calcite is occasionally found, though rare. Through most of this region the original stratification has been largely obscured, or nearly obliterated. Its general course, however, can still be traced without difficulty in the more or less elongated and flattened form, and the general trend which the rocky outcrops frequently assume when viewed from a little distance.

The texture of the rock varies considerably. In general it is rather coarsely crystalline; but not unfrequently it is much finer, or even compact; sometimes it is jointed. At one locality, in particular, (“Goodwin’s,” or “Sheep Ranch” Gulch) I noticed this jointed structure so well developed that a compact and very tough, almost imperishable rock could be quarried with facility, if desired, in nearly rectangular blocks and slabs.

It is not uncommon to find among these hills those peculiar holes in the rock which were hollowed out and used by the Indians as mortars in which to grind their food. I observed a number of similar holes in the hard rock, precisely in the bed of Rock Creek, in the ravine a short distance below the dam of the Salt Spring Valley Reservoir. It may be a question here, whether they owe their origin to the Indians or to the action of the stream, though from the peculiar deep and narrow form, I am inclined to ascribe them to the former. Heavy masses of flinty rock or hornstone also occur, particularly upon the southwest flanks of the range. This rock usually exhibits a much more distinct bedding than the ordinary mass of the hills. Its stratification is often perfectly regular, and sometimes the layers are beautifully thin and delicate. There is a very heavy outcrop of this finely banded rock in the ravine a short distance below the dam at Rock Creek. Higher up the hill, upon the road known as “Black’s Grade,” another outcrop of the same formation has been cut across in building the road, and here a portion of the same flinty rock is thickly filled with fossils, which appear to belong either to some species of crinoids or fucoids, though the structure is too much obliterated, and the specimens too much distorted to admit of definite recognition. They are apparently flattened in a direction parallel with the banding of the rock. From the general mode of occurrence of this hornstone, and from the frequent sharp and distinct lines of demarcation between it and the adjacent hornblendic rock, it might be inferred that the former traversed the latter as veins, and the delicate banding of the rock, although parallel to the general stratification of the country, would not preclude such an assumption. But the fossils speak decidedly against it, and it is probable that the hornstone is a metamorphic form of fine sedimentary deposits, and that the banding is the result of the original stratification. Quartz veins occur here occasionally, and some of them at least are auriferous, though I know of none having been worked with profit hitherto. It is not improbable, however, that some of them may be found remunerative in the future, since many of the gulches among the hills here, in the early days of mining, were rich in placer gold. The degree of metamorphism throughout these hills has been very high; but I have seen no evidence of any direct igneous action—at least no rock that I could identify as eruptive, with the single exception, perhaps, of a small and apparently completely isolated body of well characterized granite, which occurs near the base of the Gopher Range, and between its highly metamorphosed rocks and the San Joaquin Valley, which is overlaid with tertiary and other recent formations. The occurrence of this patch of granite here, isolated as it seems from any other similar rock, is certainly a point of much interest; but I have not been able to study its relations. Its stratigraphical and topographical position is similar to that of the Folsom granite, and it may be connected with it in origin. If it should hereafter appear that there is a well characterized, though more or less interrupted line of granitic outcrops traceable throughout central California, along the lower foothills of the mountains, and west of the great belt of auriferous slates, it would have a most important bearing upon the theory of the general structure of the Sierra Nevada. The existence of such a line, indeed, might point to a very different, and perhaps more probable, modus operandi than that already suggested, by which the auriferous slates themselves may have reached their present position, and received their easterly dip.

One of the most interesting points connected with the geology of the Gopher Hills, is the auriferous belt in which occurs the “Quail Hill” Mine, and of which I shall speak further presently.

Of the geology of Bear Mountain I know but little, having crossed it by but a single route. Where I have seen it, however, it consists largely of a similar rock to that which forms the mass of the Gopher Hills. Chromic iron is said to occur in considerable quantity at a certain locality in Bear Mountain, the exact whereabouts of which I could not learn. The slates of the valley extend, in general, completely up to the base of the Gopher and Bear Mountain ranges on either side, and sometimes a short distance up their flanks; but here the transition to the harder crystalline rock is usually quick and well marked.

Salt Spring Valley probably owes its existence, as such, entirely to inequality of denudation; the comparatively friable slates yielding much more readily to mechanical action than the harder and more highly metamorphosed rock on either side, which has thus been left in the form of mountain ridges, projecting many hundreds of feet above the adjacent region, while the intervening and surrounding rock has been swept away to the plains below.