MINERAL VEINS.
Werner's doctrine that mineral veins were fissures filled from above — Veins of segregation — Ordinary metalliferous veins or lodes — Their frequent coincidence with faults — Proofs that they originated in fissures in solid rock — Veins shifting other veins — Polishing of their walls — Shells and pebbles in lodes — Evidence of the successive enlargement and re-opening of veins — Fournet's observations in Auvergne — Dimensions of veins — Why some alternately swell out and contract — Filling of lodes by sublimation from below — Chemical and electrical action — Relative age of the precious metals — Copper and lead veins in Ireland older than Cornish tin — Lead vein in lias, Glamorganshire — Gold in Russia — Connection of hot springs and mineral veins — Concluding remarks.
The manner in which metallic substances are distributed through the earth's crust, and more especially the phenomena of those nearly vertical and tabular masses of ore called mineral veins, from which the larger part of the precious metals used by man are obtained,—these are subjects of the highest practical importance to the miner, and of no less theoretical interest to the geologist.
The views entertained respecting metalliferous veins have been modified, or, rather, have undergone an almost complete revolution, since the middle of the last century, when Werner, as director of the School of Mines, at Freiberg in Saxony, first attempted to generalize the facts then known. He taught that mineral veins had originally been open fissures which were gradually filled up with crystalline and metallic matter, and that many of them, after being once filled, had been again enlarged or re-opened. He also pointed out that veins thus formed are not all referable to one era, but are of various geological dates.
Such opinions, although slightly hinted at by earlier writers, had never before been generally received, and their announcement by one of high authority and great experience constituted an era in the science. Nevertheless, I have shown, when tracing, in another work, the history and progress of geology, that Werner was far behind some of his predecessors in his theory of the volcanic rocks, and less enlightened than his contemporary, Dr. Hutton, in his speculations as to the origin of granite.[489-A] According to him, the plutonic formations, as well as the crystalline schists, were substances precipitated from a chaotic fluid in some primeval or nascent condition of the planet; and the metals, therefore, being closely connected with them, had partaken, according to him, of a like mysterious origin. He also held that the trap rocks were aqueous deposits, and that dikes of porphyry, greenstone, and basalt, were fissures filled with their several contents from above. Hence he naturally inferred that mineral veins had derived their component materials from an incumbent ocean, rather than from a subterranean source; that these materials had been first dissolved in the waters above, instead of having risen up by sublimation from lakes and seas of igneous matter below.
In proportion as the hypothesis of a primeval fluid, or "chaotic menstruum," was abandoned, in reference to the plutonic formations, and when all geologists had come to be of one mind as to the true relation of the volcanic and trappean rocks, reasonable hopes began to be entertained that the phenomena of mineral veins might be explained by known causes, or by chemical, thermal, and electrical agency still at work in the interior of the earth. The grounds of this conclusion will be better understood when the geological facts brought to light by mining operations have been described and explained.
On different kinds of mineral veins.—Every geologist is familiarly acquainted with those veins of quartz which abound in hypogene strata, forming lenticular masses of limited extent. They are sometimes observed, also, in sandstones and shales. Veins of carbonate of lime are equally common in fossiliferous rocks, especially in limestones. Such veins appear to have once been chinks or small cavities, caused, like cracks in clay, by the shrinking of the mass, which has consolidated from a fluid state, or has simply contracted its dimensions in passing from a higher to a lower temperature. Siliceous, calcareous, and occasionally metallic matters, have sometimes found their way simultaneously into such empty spaces, by infiltration from the surrounding rocks, or by segregation, as it is often termed. Mixed with hot water and steam, metallic ores may have permeated a pasty matrix until they reached those receptacles formed by shrinkage, and thus gave rise to that irregular assemblage of veins, called by the Germans a "stockwerk," in allusion to the different floors on which the mining operations are in such cases carried on.
The more ordinary or regular veins are usually worked in vertical shafts, and have evidently been fissures produced by mechanical violence. They traverse all kinds of rocks, both hypogene and fossiliferous, and extend downwards to indefinite or unknown depths. We may assume that they correspond with such rents as we see caused from time to time by the shock of an earthquake. Metalliferous veins, referable to such agency, are occasionally a few inches wide, but more commonly 3 or 4 feet. They hold their course continuously in a certain prevailing direction for miles or leagues, passing through rocks varying in mineral composition.
