Page:The American Cyclopædia (1879) Volume XI.djvu/599

MINERAL DEPOSITS 581 of them contain minerals which could never have been formed in the presence of great heat. The veins containing great masses of copper on the south shore of Lake Superior, when first described, were considered as shining ex- amples of the truth of the igneous theory ; but the frequent occurrence of masses of native silver in the copper, both metals being distinct and almost chemically pure, prove that these metallic masses could never have been fused to- gether, as in that case they would have formed an alloy. Other evidence has been cited by Prof. Pumpelly which demonstrates that none of the copper veins have been filled by igne- ous action, but that the materials they contain have been deposited from solution. Trap dikes, which are fissures filled by injected volcanic material, have doubtless suggested the igne- ous theory of mineral veins ; but when they are carefully examined the materials which compose them are found to be quite different in their nature and arrangement from those which form mineral veins. In fact, dikes and veins have only this in common, tbat they fill similar fissures produced by subterranean violence. &. The Theory of Aqueous Deposi- tion. This theory apparently emanated from the Wernerian school, who regarded water as the great, if not the sole cause of geological phenomena. The advocates of this theory have suggested that fissures have been opened up into seas or other water basins, and that the vein material has been deposited from water, as limestone and other sedimentary rocks are laid down. A fatal objection to this theory is that we never find the materials composing true fissure veins horizontally strati- fied as aqueous sediments are, but on the con- trary these materials are often deposited ver- tically against the walls of the fissures. Again, if the vein materials were deposited from res- ervoirs into which they opened, the bottoms of these reservoirs ought to show similar sheets of matter, whereas nothing of the kind has ever been found, c. Lateral Secretion. Ac- cording to this theory, the materials of mine- ral veins have been derived from the adjacent rocks by percolation through the vein walls. If this were true, we should find the contents of veins changing with every stratum through which they pass, whereas in fact the composi- tion of a mineral vein is often nearly identical in all parts of its course, notwithstanding it may pass through a variety of strata. Again, where two systems of veins cut through the same stratum, according to this theory, in that stratum their contents should be similar, where- as we often find them totally diverse. Where two veins cross each other, they are often seen to be of different ages, and to be composed of materials so different that they must have been derived from different sources. The banded structure of fissure veins seems also quite incompatible with this theory, for it is scarcely possible to conceive of the formation of the different layers which compose these veins on the supposition that they have been deposited by exudation from the walls of the fissure, and that the totally distinct minerals composing the inner and newer layers have been transmitted through those first formed. As has been mentioned, in gash veins the cavities are filled or lined with materials de- rived from the adjacent rocks, but these cases afford us no satisfactory explanation of the filling of fissure veins, the only ones about which there is any question, d. Sublimation. Most of the minerals, and perhaps all of the metals, can be sublimed at a very high tem- perature ; and some of them, as zinc, arsenic, and mercury, are vaporized at a comparatively low temperature. The fissures about a vol- canic crater are frequently lined, sometimes filled, with minerals, some of them ores, which have plainly been driven out from the volcano in a state of vapor. Such cases have led some theorists to suppose that sublimation played an important part in the filling of mineral veins. As has been said, the deposits of mercury have often the character of impregnations, and in some instances at least we have good evidence that mercury is diffused in the form of v&por ; but these deposits have certainly very little in common with the distinctly limited, often banded and crystallized matter filling mineral veins, properly so called. Hence this theory is in the main rejected by modern mineral- ogists, e. Chemical Precipitation. This the- ory attributes the deposition of mineral matter in veins mainly to precipitation from solution, and this is the view now generally taken by those best informed on the subject. According to this theory, fissures destined to become fis- sure veins are first filled with water, usually flowing from sources deep in the earth, where, highly heated and under great pressure, it be- comes charged with mineral substances. As it approaches the surface and the temperature and pressure are reduced, its powers of solu- tion are diminished, and a large part of the materials it has carried are precipitated on the sides of the channel through which it flows. The abundant and varied deposits made by thermal springs illustrate the sufficiency of this cause. In this view, the banded structure which is exhibited by mineral veins is attrib- uted to changes during the lapse of ages in the nature of the solution, dependent upon some deep-seated cause, such as successive convul- sions opening new sources for the supply of material. Sulphur we know is one of the most common constituents of volcanic emana- tions, and the normal condition of most ores found in veins is that of the sulphide ; and we have reason to believe that they are mainly de- posited from a hot solution in which sulphur was the most conspicuous ingredient. Highly heated water or steam, containing sulphur, fluorine, and chlorine, would be capable of dis- solving most of the minerals with which it came in contact. It would certainly be charged with silica, and if flowing or driven through