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:
On the Occurrence of Glauberite at Borax Lake, California.
BY B. SILLIMAN.