Salt Spring Valley and the adjacent region in Calaveras County.
BY W. A. GOODYEAR.
Having spent some time during the past summer in Copperopolis, and the region lying west and northwest from it, I offer the following observations respecting its topography and geology. I will first notice the
TOPOGRAPHY.
For a general description of the topography, etc., of Calaveras County, including the main features of the region in question, reference may be made to Prof. J. D. Whitney’s Report upon the Geology of California, Vol. I, p. 253. In addition, however, to what is there stated, I will say that Copperopolis lies at the southwestern base of Bear Mountain, the summits of which rise to an altitude of something more than 2,000 feet above the sea. The Gopher Hills, also mentioned in the report, form a well defined and connected, though subordinate range, lying to the southwest of, and nearly parallel with the general course of Bear Mountain. This range forms a prominent feature in the topography of the region for a distance of at least fifteen or eighteen miles southeasterly from the Calaveras river. Its summits are probably 1,400 feet above the sea, and the lowest break or gap within the distance named is that through which Rock Creek finds its way to the plains below. The valley or depression between the Gopher Hills and Bear Mountain, whose average width is four to six miles, has received the name of Salt Spring Valley. Its general altitude is little less than 1,000 feet above the sea, that of the town of Copperopolis being nine hundred feet according to H. P. Handy’s survey of a railroad route from Copperopolis to Stockton. I should mention that for several miles northwesterly from Copperopolis, Bear Mountain has an outlier along its southwestern base, in the form of a low but tolerably well marked hilly ridge, between which and the base of the mountain is a narrow but continuous valley; and it is in this valley that the copper-bearing belt of Copperopolis is found. Southwest of this outlier, and for a distance of three or four miles northwesterly from Copperopolis, Salt Spring Valley consists mainly of a region of low hills, traversed by a net-work of steep and narrow gulches. Farther northwest the surface of the valley for three or four miles is more uniform, and here we find the nearly level area of “Tower’s Ranch,” and the gently sloping basin of the “Salt Spring Valley reservoir.” Beyond this, the country is again hilly to the Calaveras river. Southeast and south of Copperopolis, the surface is everywhere hilly. The slope of the Gopher Hills towards the southwest is rapid until we reach the low rolling country which forms the border of the San Joaquin Valley.
Black Creek debouches from Bear Mountain a mile or so southeast of Copperopolis, and flows to the Stanislaus. Littlejohn’s Creek takes its rise in the hilly regions of the valley west of Copperopolis, and flowing southwesterly, finds its way through the hills into Rock Creek. The latter rises in Bear Mountain, five or six miles northwesterly from Copperopolis, and flowing southwest across Salt Spring Valley, breaks through the Gopher Hills, and continues its course through the lower country to French Camp Slough, a branch of the San Joaquin. All these creeks become dry in the summer, though in winter they often carry very large volumes of water. At the point where Rock Creek breaks through the Gopher Hills is the substantial dam of the Salt Spring Valley Reservoir.
GEOLOGY.
The strike and dip of the rocks are more or less variable; but, so far as my observations extend in the region described, they have everywhere the same general northwesterly trend and high northeasterly dip which characterize so large a portion of the gold-bearing slates of central California. The strike is usually from N. 50° W. to N. 70° W., (magnetic) and the dip from 50° northeast to vertical. I have seen no case here of a decided southwesterly dip, nor of a low one to the northeast. It is somewhat remarkable, by the way, that this high northeasterly dip should be so general as it is in the great mass of auriferous slates which forms the southwestern flank of the Sierra Nevada. It is towards the granite axis of the chain, instead of from it, as would seem more natural. The causes of this are by no means as yet fully explained. It is a circumstance, however, which would lose none of its interest in the future, if, as certain facts mentioned in the Geological Report, Vol. I, p. 286, might possibly seem to indicate, further explorations should prove it to be in general a great inversion of the strata—their upper portions having been “forced back by immense pressure from above, producing a condition of things similar to that so often observed in the Alps, which is known as the ‘fan structure,’ and has so much perplexed geologists.” When we take into account the enormous denudation, amounting to thousands of feet in perpendicular depth, which is known to have taken place in the Sierras within the most recent geological periods, and the whole of which, in this case, must also have belonged to the inverted portion of the strata—unless indeed the inversion were produced by a peculiar sliding and bending of the strata by their own weight, the upper flexure having been since entirely removed—and when, in addition to this we consider the hundreds of miles in length, and the great thickness of the strata in question, we can perhaps begin to appreciate the magnitude of the movements and forces which would be involved in producing such an effect. It would indeed, if true, be a striking illustration of the grandeur of the scale upon which many of the physical features of this country have been cast, as compared with those of other and better known regions. But it is hardly worth while to speculate further upon probabilities like this in the present state of our knowledge, and I return to my subject.
In Salt Spring Valley, the rocks consist almost entirely of slates, with little variety of character, generally thin-bedded, fine-grained and argillaceous, sometimes magnesian or chloritic, and often splitting with facility into very thin sheets. The thinnest bedded varieties are usually fragile, and the structure is often wavy; but sometimes the cleavage is regular and thin enough, and the rock possesses sufficient strength to furnish a tolerable material for roofing purposes; although no attempts have been made, so far as I know, to thus apply it;—and, in fact, the expense attendant upon its excavation and transportation would preclude any extensive use of it, even if its quality were unsurpassed, which it is not.
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.
A partial description of the copper mines of Copperopolis will be found in the “Geology of California,” Vol. I, pp. 254-257. The depth of the main shaft in the “Union” is now stated to be a little over five hundred feet, and the greatest depth reached in the “Keystone,” is said to be five hundred and sixty feet. All the deposits of ore here worked lie parallel with the strike and dip of the inclosing strata. The great ore mass of the “Union” Mine forks or divides into two branches towards the northwest; and at the lowest depth now reached, its width or thickness, after having reached a maximum, is again diminishing. In the “Keystone” Mine there have been two separate and nearly parallel bodies of ore worked to a considerable extent, and a third one was struck last spring previous to the suspension of work in the mine. The two main bodies of ore in this mine have “pinched out” or disappeared in various directions in their lines of strike and dip. They seem to have an irregular lenticular form, and together with the great mass of the “Union” appear to lie in what are called “shoots,” which pitch at an angle of 50° or 60° in the direction of the strike towards the northwest. The northwesterly prolongation of the strike of the great “Union” deposit does not coincide with either of the “Keystone” deposits, but passes east of them. There have been other and smaller deposits in the “Union” ground, more or less worked, lying west of the main body, some of which may possibly connect with the “Keystone” shoots, though the best information I could obtain leads me to think otherwise, and that they were probably isolated lenticular masses. The mass of the great deposit in the “Union” Mine consists of an intimate mixture of chalcopyrite and iron pyrites, containing on an average sixteen to seventeen per cent. of copper. Well defined selvages are not to be seen at Copperopolis, and the country rock is impregnated in all directions, sometimes to a considerable distance from the purer ore, with more or less finely disseminated copper and iron pyrites. In Europe it would pay to crush and work much of the wall rock itself for the copper which it contains; but here it is entirely worthless, as even ten to twelve per cent. ore is not worth mining and shipping at present prices.
It will be seen that the more recent and deeper developements in the Copperopolis mines have only served to confirm the opinion expressed two years ago by the State Geologist (Geol. Vol. I, p. 225) that “the deposits of copper ore in this region, like nearly all the others in California, do not appear to be included in regular fissure veins, but rather to form independent masses [the italics are mine] lying in the direction of the strike of the inclosing rocks, and dipping with them.” It seems, further, that they are here arranged in some sort en échelon. There is no evidence whatever of the existence here of a regular and continuous vein of copper ore, stretching for miles through the country, as some have supposed. (See Ross Browne’s Report, p. 144.)
The finding of “copper indications,” i.e., of small and isolated bodies of ore, distributed with some constancy through a narrow belt of country, for no matter how many miles in length, is anything but conclusive evidence of the existence beneath of a regular vein of corresponding length (which, by the way, if it existed, would be an anomaly in the mining world)—especially when all the developments of the most extensive workings hitherto made point so decidedly and strongly to the opinion that there is no true vein at all. Such “indications” are however evidences, so far as they go (and they go a good way in this direction) of the probable existence of other large bodies of ore distributed here and there along the belt in question. It is not improbable that such may be found in the future, and it would not be strange even if some of them should surpass in magnitude and value the great deposit of the “Union,” which has already yielded such enormous quantities of copper, and is yet far from being worked out.
A description of the auriferous deposit of Quail Hill, in the Gopher Range, together with a similar one at Whisky Hill (called also the “Harpending Mine”) in Placer County, by Prof. B. Silliman, was read before the California Academy of Natural Sciences, at their meeting of April 15th, 1867, and will be found in their published “Proceedings,” Vol. III, pp. 349-351. This paper describes well the particular deposits in question, as well as the general appearance and character of the formation in which they occur. Such deposits, however, are not confined to one or two localities; but there are other points in Calaveras County at which gold is known to exist in considerable quantity, and with similar mode of occurrence. Among these I may mention Quail Hill No. 2, near the Napoleon Copper Mine, two or three miles southeast of Quail Hill No. 1, and the “Plymouth Rock,” or “Austin and Hathaway” claim, at Rich Gulch, near the Calaveras River. Moreover, the geological causes and the peculiar chemical decomposition of the rock, which have been involved in the formation of the deposits in question, are by no means confined to the localities where gold is known to occur. On the contrary, they may be traced with considerable constancy through a narrow belt of country along the southwest flank of the Gopher Hills, and stretching from the Calaveras River southeast for a distance of at least fifteen miles, and perhaps farther. Towards the northwest, the same belt crosses the Calaveras; but how much farther it extends in this direction I have no present means of knowing. It is not unlikely that a similar formation may be found to exist, here and there at least, in the same general line of strike, nearly parallel with the stratification of the country, through Amador and El Dorado Counties to Placer, and perhaps beyond. The possibility of this at least is worth remembering. Throughout this belt, in the Gopher Range, surface cuts and shafts, of greater or less depth, made and sunk in prospecting for copper, are of frequent occurrence. In fact, this is the same belt that has been so often mentioned as “the second important copper-bearing belt of Calaveras County,” and located some six or seven miles southwest of the main copper belt of Copperopolis. The “importance” of this belt, on account of the copper ores which it contains, has been most grossly exaggerated. An amusing illustration of this fact is to be seen in a “map of the copper mines in Calaveras County,” published a few years since, which represents the whole region in question as literally covered for miles with highly colored “locations” or “copper claims,” the whole of which, with few exceptions—and these due not to copper but to gold—have served no further end than that of rendering their locators and owners sadder and wiser men. At one locality, indeed, viz, the “Napoleon Mine,” a body of copper ore was found which in many countries would have been remunerative, and was worked to a considerable extent; but the working here was attended only with loss, and was some time since entirely discontinued. It should be remembered, however, in speaking of the copper mines of California, that not only have they had to contend with the general ignorance of copper mining, and especially of copper metallurgy which has existed throughout the State, and with extremely high prices for labor and transportation; but also that, for a year or two past, the largely increased supply of ore from the mines of Chili in South America, and elsewhere—together with the diminished demand and consequent low price for metallic copper, reacting with increased effect upon the value of the ore—have told with crushing weight even upon the best mines. There are certainly not more than one or two, perhaps not even a single deposit of copper ore in the known world, which surpasses or equals, in magnitude and intrinsic richness combined, that of the “Union” Mine of Copperopolis; and yet it is said that even the “Union” itself, which is the only mine now active at Copperopolis, is hardly more than paying expenses at present rates. So far then as my observations extend, there is simply nothing whatever in this “second copper belt” which can for some time to come justify the expenditure of money in searching for copper here; though it is not impossible that, besides the “Napoleon” Mine, other deposits of ore may exist within the belt, which at some future time, and under more favorable circumstances of labor, fuel, and transportation, may become of value for the copper which they contain.
It has been already remarked that the zone or belt of surface decomposition in which the “Quail Hill” and other similar mines occur, may be traced with considerable constancy for at least fifteen or eighteen miles, and that it is not improbable that it is much longer than this. We cannot, however, infer from our present knowledge that the decomposed or “calico” rock is continuous throughout the belt, or even for any considerable portion of its length. On the contrary, its distribution within the belt appears capricious and local, i.e., it seems to occur in more or less detached and isolated masses, which vary largely in form and size, and are irregular and indefinite in outline; so that little more can be predicated of their occurrence in general, than that they are mostly confined within a comparatively narrow belt, and that their longest dimension exhibits a general tendency to approximate parallelism with the axis of the belt, and the stratification of the inclosing country. Sometimes, as for instance, along the northeastern side of the Quail Hill formation, this tendency is so strongly developed, and the passage from the decomposed to the undecomposed rock is so rapid, as to form for some little distance a tolerably straight and well defined “wall” or line of demarcation, parallel, or nearly so, with the stratification of the country. But the change or passage from the decomposed or “calico” rock to the surrounding undecomposed country, though sometimes rapid is always gradual, so far as I have seen; and though we cannot yet speak much from underground explorations, the surface appearances throughout the country would indicate decidedly that so regular a line of demarcation as this at Quail Hill is the exception, and not the rule. The southwestern limit of the decomposed mass of Quail Hill has been found at several points; but here the change from the decomposed to the undecomposed rock is not so rapid; and though the explorations here, being shallow and limited, are insufficient to determine this point with certainty, it is not probable that any such regularity of demarcation exists here as upon the opposite side. Most of the “calico rock” of this belt still retains distinctly the structure of the undecomposed rock from which it was formed. The crystalline hornblendic rock is thus seen to have been largely altered by the decomposing agency, and even the hornstone, which lay in its track, seems to have been more or less affected by it. The decomposition has been purely an oxidation, accompanied by such mechanical and chemical changes as filtering mineral waters might produce. It is probably superficial, both in origin and character, extending to no great depth, although the main level at Quail Hill is nearly one hundred and twenty feet beneath the summit of the hill, and the decomposition of most of the rock at this depth, so far as exploration has gone, is as perfect as at any higher level. It is certainly long subsequent in date to the metamorphism of the surrounding country, and is unquestionably largely due to the action of the products of the oxidation of metallic sulphurets (chiefly those of iron and copper) which were originally distributed through the rock. At the same time it is not easy to account for the whole of it in this way alone, since at certain localities undecomposed sulphurets are seen near the surface, and in rock which is apparently much more permeable to atmospheric influences than was much of that which has been more deeply decomposed; and again, much of the decomposed rock, though retaining well its original structure, shows far too little traces of sulphurets to readily account for so general and thorough a decomposition as has taken place. It is all indeed more or less colored by oxide of iron, but much of it is not deeply colored, and the undecomposed hornblendic rock itself, in the absence of all sulphurets, contains sufficient iron in the state of protoxide to impart a strong coloring when the rock is decomposed and the iron passes to the state of sesquioxide. Much of the iron originally present has undoubtedly been removed in a soluble form, as sulphate, etc. But in rock which preserves its original structure, as well as most of this does, pyrites, if originally present, would have left traces of its existence in the form of casts or cavities in the decomposed mass, which might or might not have been filled with ferric oxide or other matter. In certain localities the decomposed rock is in fact filled with such cavities, often cubical in form, attesting the former presence of large quantities of disseminated sulphurets. But in other localities they are few and far between, and here accordingly the decomposition can hardly be supposed to have been due to the local presence of sulphurets alone.
The exact methods by which the general and local decomposition has been effected, and those by which the rock was originally impregnated with metallic ores—as well as the manner in which certain substances, as barytes, now found as sulphate, and true porphyry, now found as kaoline or lithomarge, have found their present situation in the belt in question—all these would possess both interest and importance in a high degree, could they be more definitely known. Such questions, however, cannot be answered with certainty, and their discussion here would lead us too far into the doubtful realm of chemical geology.
But whatever may have been the agencies at work, it is evident that there is nothing in all this to remind us of a true vein formation. It appears that the zone in question is neither a vein, nor generally speaking a system of veins. On the other hand, it possesses emphatically in general the characteristics of what the Germans style an impregnation—an impregnation indeed which exhibits a certain regularity as being mostly confined within a narrow zone, and stretching through a considerable extent of country, but which within these limits shows the greatest irregularity of form, and much variety of character. Veins of quartz occur here and there within the belt; but they are not more frequent here than elsewhere, and their occurrence has probably little or no direct connection with the peculiar character of the belt itself. There is very little that deserves the name of quartz at Quail Hill, though much of the surface rock is pretty highly silicious in character.
The impregnation of the rock with metallic sulphurets, particularly with sulphurets containing copper, has in certain localities been sufficiently powerful and concentrated to assume, in greater or less degree, the characteristics of segregated veins of limited extent. This has been the case at the Napoleon mine, and also at Quail Hill, where there is, or was, a band of oxidized ores of copper traversing the decomposed rock in a direction parallel with the general stratification. This band consisted chiefly of the green and blue carbonates of copper, mingled with ferruginous and earthy matter, and accompanied by barytes. The last named mineral, so common a veinstone in other parts of the world, but hitherto so rare in California, occurs here in considerable quantity. Its form is granular compact, sometimes quite pure, but usually contaminated and intermingled with other matters. Crystallized specimens of it have not been found here to my knowledge. It is hardly probable that the barytes itself contains either gold or silver; yet it certainly occurs here in the most intimate contact with both, as I have seen respectable particles of gold in place upon the immediate surface of compact specimens of barytes—and a sample of heavy concentrated barytic sand from the tailings of the mill, of sufficient fineness to pass through a sieve of one hundred holes to the linear inch, yielded to the assay over eleven dollars per ton in gold and silver.
The thickness of the copper band varied from one to three or four feet. Its outlines were indefinite, and its original characteristics of form, etc., much obscured by the complete decomposition both of itself and the surrounding rock. It was without doubt originally a segregated mass of sulphurets; and though it seems now to have nearly or quite run out and disappeared, it may be found to come in again as such, in depth, unaltered below the line of surface decomposition.
Other bands of similar character may perhaps exist in the yet undeveloped portions of the mine. But the great mass of decomposed material which forms Quail Hill as a whole, retaining as it does to so great an extent the original structure of the country rock from which it was formed, can in no proper sense be called a vein; although its extent, when considered as a repository of the precious metals, is something far transcending the size of ordinary veins.
The gold and silver of these formations which have recently attracted so much attention, and have become the object of extensive mining operations at Quail Hill, seem to be distributed at the latter place, to a greater or less extent, throughout the whole mass of the decomposed rock. The surface earth of the hill, also, everywhere contains gold, which may be discovered by washing it in the pan; but this ceases to be the case on the hillsides as soon as the limits of the decomposed rock are passed. Some of the gold, as stated by Prof. Silliman in his communication to the California Academy, already referred to, is quite coarse; but much of it is exceedingly fine and difficult to save in the mill. It is a noticeable fact in the distribution of the precious metals at Quail Hill, that the cupreous ores and the material in their vicinity have hitherto been found to be always rich in gold and silver, and to contain chiefly, if not exclusively, the coarsest gold.
The distribution of the gold at Quail Hill is not uniform, the more slaty and ferruginous portion of the decomposed rock being generally the richest in ore, while the compact porphyritic kaoline contains but traces of gold, if any, and some of the other and more compact rock is comparatively poor. The original distribution of the sulphurets here seems also to have followed approximately the same law—the kaoline containing in general but little trace of their existence, while the more slaty rock is often full of their cavities. Hematite, as well as the hydrated sesquioxide of iron, occurs here in small quantities; and a curious point in this connection is the fact that, while much of the best ore is very highly charged with the hydrated sesquioxide, the hematite has been found hitherto to contain little or no gold. The origin of the decomposed porphyry at Quail Hill is a point of much interest, and it may be a question whether it is not the remnant of an intrusive igneous dyke. The arguments in favor of this supposition consist in the entire dissimilarity in character and structure between it and the surrounding material, as well as in the rarity of porphyry in the region round about. In fact, I have nowhere else in this portion of the country seen anything deserving of the name, while the whole texture and appearance of this mass at Quail Hill are precisely such as would have resulted from the decomposition in place of a true feldspathic porphyry. But however strongly these facts may seem to argue in favor of an igneous origin, it is not easy to reconcile such a supposition with its mode of occurrence here. Other masses of similar character may exist within the hill; but so far as existing developments have cut or uncovered the one of which I speak, the indications are that it is irregular in outline, quite limited in extent, and of approximate lenticular shape. Moreover, in certain places, it seems to pass gradually into the eastern country rock, without any distinct line of demarcation, the change in the texture of the rock being even more gradual than the passage from the decomposed to undecomposed material. At certain points, but a few feet from the eastern “wall,” the kaoline is as perfectly porphyritic in its texture and appearance as in any portion of the mass, while between the two is every grade of passage from the one to the other—the country rock being neither distinctly porphyritic in texture, nor chiefly feldspathic in composition. I am strongly inclined to think, therefore, in spite of its peculiar and distinctive character, that this porphyritic mass is but a local result of the metamorphism of sedimentary strata, which, in many portions of this region, seems to have been as varied in character as it has been high in degree.
The degradation of such formations as this at Quail Hill, has undoubtedly furnished some of the placer gold of the region; but the evidence does not by any means justify us in supposing that it has furnished the whole of it. Gopher Gulch, which runs at the foot of Quail Hill, and its branches, for a mile above this point, or nearly to the summit of the Gopher Range, and hundreds of feet above the level of the Quail Hill formation, were in early days rich in placer gold, much of which was very coarse. Other gulches in the vicinity have also furnished more or less gold high up towards the summit of the range. Moreover, the quartz veins, which here and there occur in the hard metamorphic rock, are known, some of them at least, to contain gold, and such have probably played their part in the formation of the placers.
I have already mentioned the fact of the prominent association of the precious metals with ores of copper at the Quail Hill mine; but this fact derives still further interest from what follows. As far as my observations have extended in Calaveras County, and also at Whisky Hill, in Placer County, wherever gold and silver have yet been found in paying quantities in the decomposed rock formation, there also, or close at hand, are found the oxidized ores of copper, carbonates and silicates; and conversely, I have nowhere seen oxidized ores of copper in this decomposed rock which were not, comparatively at least, rich in gold and silver. It is true that sufficient developments have not yet been made to enable us to state whether this is the general fact or not. It is possible that the association of these ores may be to a certain extent accidental; but it is not unlikely that it may be otherwise;—and at all events this is a point well worthy of attention and further investigation.
As this finishes my remarks upon the “calico rock” formation. I will close by simply mentioning a point relating to the lower country of Calaveras County, that I have not yet seen publicly noticed elsewhere. The low, rolling hills which form the eastern border of the San Joaquin plain between the Stanislaus and Calaveras Rivers, contain extensive beds of horizontally stratified material, which is probably sedimentary-volcanic in origin. The color of these beds is usually varying shades of gray. They contain no pebbles, so far as I have seen; they generally crumble easily, and resemble in appearance a friable sandstone. But their grain or grit, which is pretty fine, is also quite clean and sharp as well as hard, and rough-polishes rapidly the hardest steel when rubbed upon it.
These beds are of considerable thickness, and cover many square miles of country. Their stratification has evidently not been disturbed since they were deposited, though they have been largely eroded. The frequent flat tops of the hills, and the level benches, which these beds have produced along their sides, by irregularities of wear, impart a peculiar aspect to the scenery.
Professor Silliman read the following: