Page:The American Cyclopædia (1879) Volume XI.djvu/600

 582 MINERAL DEPOSITS rocks containing even minute quantities of sil- ver, gold, lead, iron, copper, or other metal, would gather these materials, and coining to- ward the surface would precipitate them in the form of sulphides. The replacement of animal and vegetable tissues by mineral mat- ter, which often occurs in their fossilization, affords proof that chemical solution is entirely adequate to produce all the phenomena exhib- ited in the filling of mineral veins. In petri- fied wood the vegetable tissue is replaced, par- ticle by particle, by silica, evidently deposited from solution. The sulphides of copper and iron often replace wood in the same way, and this could only take place on a great scale, as it often does, when the rocks were saturated with a solution containing these metals. The formation of geodes, the filling of the cavities of amygdaloids with agate, chalcedony, and zeolites, the sheets and stalactites of iron and lead in the Galena mines, and the stalactites of lime in caves, prove that such solutions are constantly flowing through the rocks beneath us. In some formations and localities mollus- cous fossils are found completely replaced by galena, pyrites, and blende, the lime of their shells having been carried away and the differ- ent ores deposited in its place. In the cavities left by some shells, successive layers of sul- phides of lead, iron, and zinc are deposited, showing in miniature almost precisely the phenomena observed in mineral veins. A few general features in mineral veins remain to be noticed. Oftener than otherwise, the mineral veins of any district are seen to belong to one or more systems in which the individual veins have nearly a common bearing and a general similarity of composition. In the mining dis- trict of Cornwall, England, there are two principal systems of veins, one running nearly north and south, the other approximately east and west. The latter carry copper and tin, the former chiefly lead and iron. In. the lead region of the upper Mississippi there are also two principal systems of veins, which vary somewhat in their bearing, but are generally known as the north and south and east and west courses. In the mining district of Frei- berg, Saxony, nine systems of veins are said to have been identified ; and in the silver belt extending parallel with the Pacific coast, from Idaho and Nevada to Chili, which has been almost constantly shaken and shattered by earthquakes, the systems of veins are almost innumerable. In less disturbed regions, like the Mississippi lead district, the courses of the veins coincide with the jointing of the rocks, and thus in many instances exhibit a kind of polarity; that is, one set of joints coincides with or approaches in direction the meridian, while the other is nearly at right angles to this. There is little doubt that the system in the jointing of rocks, and hence in the bearings of mineral veins, often determined by the joint- ing, has been considerably affected by terres- trial magnetism. It is also probable that this cause has operated to control or influence the deposition of mineral matter in veins. Mr. Fox of England found that the water in the copper mines was a weak solution of salts of copper, and that the galleries filled with this solution were in fact cells of galvanic bat- teries, from which well marked currents were produced. Solutions similar to these found in old copper mines, but hotter and stronger, have undoubtedly filled most of the fissures now occupied by metalliferous veins. It is easy to see that such cells might generate pow- erful magneto-electric currents, by which the metals, especially silver and copper, might be precipitated in great quantity, just as they are now precipitated in the electro-plating process. Gossans. Nearly all mineral veins are found to be very much weathered and decomposed along their line of outcrop. -The decomposi- tion generally extends down to the permanent water level, below which the ore is in its nor- mal state, and this for the most part is sul- phide. When exposed to the action of atmos- pheric water and air, the sulphides are oxi- dized, and the whole mass of the veinstone is frequently rendered soft and spongy, and high- ly colored in various ways. When the vein contains much iron pyrites, this is converted into the hydrated sesquioxide of iron, coloring all the decomposed mass brownish red. From this fact the changed portion of the vein is called in Germany the Eisenhut, iron hat. In Corn- wall the decomposed portion of a mineral vein is called a gossan ; and this term has been uni- versally adopted in all mining districts where English is spoken. In the gossans of veins we usually find the sulphides of silver converted into chloride, bromide, iodide, &c., with many sprigs and masses of native silver. Copper ore, generally the sulphide of copper and iron in its normal state, is converted first into red or black oxide, and then into malachite, azu- rite, and chrysocolla, the carbonates and sili- cate of copper. Locally, when effected by saline solutions, as in South America, ataca- mite, the chloride, is produced. All these secondary forms of ores are more easily treated than the sulphide, and the gossan which con- tains them is usually loose and easily excavated. This portion of a mineral vein is therefore much more easily and cheaply worked than that which lies below the permanent water level. Hence the first workings of mineral veins are frequently highly remunerative, while the cost of deeper excavations in harder rock, and the expense of treating the more intractable sul- phides, cause subsequent operations below the water level to result in disappointment. In many mining districts, like those of the south- ern Alleghanies in the United States and of Sonora in Mexico, the first comers, by work- ing the gossans, were able practically to skim the cream of the mineral veins, carrying off great profits, and leaving to the second gener- ation an inheritance of which the value is often worse than doubtful.