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Show 4 INTER-MOUNTAI- MINING N REIEW. k upon the deposition of the Chile, South America, as described by Moricke. gold. It would be extremely instructive It is only within a few years that if we could find an instance of a vein passing from one formathe instances of the occurrence of gold In the crystalline rocks have multiplied tion into another, as for example, from until we cannot longer regard them as slates into compact granite, so as to exexceptional, or hold that gold, as a hibit the effects of change of country-rocor of the walls upon the mineralrule, is confined to the magnesian and ization of the vein effects so strikingargillaceous slates. One of the earliest cited and best ex- ly shown in the lodes of Cornwall, in the kil-lamples of the occurrence of croase, free which are copper-bearin- g in the granite. It and gold in the midst of granite, seemingly without any extraneous origin by im- should be mentioned that the tin lodes pregnation accompanied by quartz, was are not without some gold, being thus at the Armagosa gold mines at the sink indicative of its presence in the granite. of the Mojave river, San Bernardino The granite rock of Butte and Walkerville, Mont., affords concounty, California. and The native gold in visible masses was spicuous examples of silver-bearing there disseminated in the midst of the veins, originating aptry-roc- gold-beari- ng k, as tin-beari- ng well-defin- ed gold-beari- aggregation of soda feldspar (albite) and the quartz, as if an original constituent of the mass and indigeneous to the rock, which it may have been. Pyrites were not present in the specimens obtained; but some fragments of white arsenic in granite from the same locality may be regarded as an indication of the former presence of an ar- senide in association, and as possibly an impregnation subsequent to the formation of the rock. The specimens unfortunately have been packed for years and are not now accessible. In all cases of this kind it is important ta study the rock in situ and in mass, and to note carefully all the surrounding conditions. It is not safe to make deductions from the phenomena exhibited by isolated specimens. The influence of dikes of plutonic rock upon the mineralization of veins with the precious metals, has long been known and recognized by miners. If wre accept the theory of lateral secretion for the formation of mineral veins, such veins bearing the precious metals are perhaps as good evidence as we need of the diffusion of gold and silver in the mass of the adjoining country-rocor, at least, that solutions bearing these metals may traverse the rocks by osmosis or otherwise. If such evidence may be admitted, the range of the phenomena and of the evidence is widely extended. Some of the most notable districts in veins California where traverse crystalline rocks of the granite family are at Grass Valley and Nevada, Nevada county; also at Forbestown, Butte county; Ophir, Placer county, and West Point, Calaveras county. In these localities the veins are numerous, and appear to have been formed from the In Masubstance of the country-rocriposa county, at the southern end of the Mariposas estate, where the slates k, gold-beari- ng k. age, with their accompanying large Mother Vein,,, give place to granite, we find a vein of auof the Jura-Tria- s riferous quartz which may not have any relation in origin to the vein in the slates, with which the vein in the granite contrasts strongly in its format tion and in the distribution of the gold. It is known as a pocket vein yielding the gold in isolated but rich bunches, and of a higher grade of fineness and less crystalline than the gold of the Princeton vein, which traverses the secondary slates. It would be instructive to determine the relative ages of these adjacent veins and the influence of the two different kinds of coun ng parently by lateral secretion from the body of the granite on each side. The Rainbow and Blue Bird lodes especially appear to have derived their mineral and metallic contents from the granite in much the same way as the lodes are formed in Cornwall, according to the investigations of Le Neve Faster. The occurrence of gold in thin flakes upon the surfaces and in crevices of the porphyry at the famous Contention mine at Tombstone, Ariz., has been shown; but it is not yet known with tin-beari- ng certainty whether this gold, either in its free state or combined with disseminated pyrite, was an original constituent of the dike, or whether it was derived from the diffused pyrites of the stratified beds traversed by the dike. So far as regards the plainly visible gold, it appears to be confined to the partly decomposed portions of the porphyry dike, at or near the contact with the other rocks, and to be a secondary or late deposition and not indigenous. The mines of the Homestake group in the Black Hills of Dakota afford a good example of the occurrence of gold in ancient crystalline gneiss or of schists granite These schists age. are much plicated and are traversed by felsitic dikes, distinctly intrusive. Whether these are auriferous or not remains to be ascertained, and without a careful examination it is not possible to state the source of the gold, whether indigenous to the schists or introduced from the dikes or with quaTtz veins from remote sources. The rock for the stamp mills is quarried, rather than mined in the manner usual with veins, and the rock is milled together with any veins traversing it. The ore of the Treadwell mine, Alaska, is described by Adams and Dawson as a hornblende-granitmuch crushed, altered, and impregnated with secondary quartz, calcite and pyrite. The pyrites contains the gold. In t.he region of northern Sonora, Mexico, the gold veins are chiefly in or closely associated with granitic and plutonic rocks. The veins of El Grupo concession, about 100 miles pre-Cambri- an e, gold-beari- ng southeast of Tucson, Ariz., traverse a granite, and bear both gold and silver. A dioritic rock at El Plomo, in the same State, appears to be especially favorable to the occurfine-grain- ed rence of gold. At the San Francisco mines, 100 miles Bend, Ariz., gold-beari- ng traverse Huronian southwest of Gila quartz veins or pre-Cambri- an gneis near or at the contact with a dike of diorite on one side and of granite on the other. El Cam-pana large quartz vein, also in Sonora, is at the contact between an obscurely defined felsitic rock, like granite, on one side, and a heavily-bedde- d quartzite (probably Cambrian) on the por-phyri- tic a, other. g In central and northern Arizona veins are found in granite. The famous Congress vein, now worked to a depth of some hundreds of feet, traverses granitic rock in close association with a dyke of plutonic rock. North of Indio on the Colorado desert, California, in the second range of mountains a ridge of granite contains irregularly spread patches or bunches of pyrite which, by decomposition, liberate a small amount of gold. This pyrite does not appear to be connected with any vein, but seems to be one of the original constituents of the rock. Somewhat similar pyritic impregnations in granite were worked a few years since in Arizona, near Peeples Valley and Rich Hill, north of Stanton, and yielded free gold. The close association of gold and granite at Cripple Creek must not be overlooked. Some specimens of porphy-riti- c granite with cavernous spaces partially filled with purple or amethystine fluorite are very rich in gold. Penrose, in summarizing chapter IV. of his monograph of the mining geology of the Cripple Creek district, says: The gold and associated vein minerals were probably derived from the volcanic rocks and, to a less extent, from the adjacent granite, at great or less but not extremely profound depths. Again (p. 150), The most profitable mines yet discovered, in the Cripple Creek district are in the eruptive rocks or in the granite Immediately adjacent to the main volcanic vent or vents. On Battle mountain, Cripple Creek, the veins are described as In some bodies of quartz, in cases. other cases they are impregnations and partial replacements of the country-roc- k with mineral matter along fissures. The prominent veins are both in the breccia and in the granite immediately This adjoining the breccia area. breccia, as found at the Independence No. 4 mine, is described as containing, besides volcanic rocks, large quantities of granite fragments, some of them several feet in diameter. The ore of the Independence mine consists of granite from which the mica and quartz have been partly or wholly removed, leaving a honej'-combevesicular main of partially kaolinized feldspar. (P. 201.) The phenomena of the occurrence of the precious metals in the celebrated Mercur mining district, Utah, furnish evidence which may be used In support of either view of the primal source of these metals. Both sedimentary and plutonic rocks are there found in conformable layers, and gold has been found in both, but the commercially available deposits are along and ner the plane of contact of porphyry and limestone, but below the porphyry, the deposits thus being what is ordinarily gold-bearin- well-defin- ed d, |