[1] See notices of Savi, Hoffman, and others, referred to by Boué, Bull. de la Soc. Géol. de France, tome v, p. 317 and tome iii, p. 44; also Pilla, cited by Murchison, Quart. Geol. Journ., vol. v, p. 266.
CHAPTER XXXVI.
MINERAL VEINS.
Different Kinds of mineral Veins. — 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 or “Slicken sides.” Shells and Pebbles in Lodes. — Evidence of the successive Enlargement and Reopening of veins. — Examples in Cornwall and in Auvergne. — Dimensions of Veins. — Why some alternately swell out and contract. — Filling of Lodes by Sublimation from below. — Supposed relative Age of the precious Metals. — Copper and lead Veins in Ireland older than Cornish Tin. — Lead Vein in Lias, Glamorganshire. — Gold in Russia, California, and Australia. — Connection of hot Springs and mineral Veins.
The manner in which metallic substances are distributed through the earth’s crust, and more especially the phenomena of those more or less connected masses of ore called mineral veins, from which the larger part of the precious metals used by man are obtained, are subjects of the highest practical importance to the miner, and of no less theoretical interest to the geologist.
On different Kinds of Mineral Veins.—The mineral veins with which we are most familiarly acquainted are those of quartz and carbonate of lime, which are often observed to form lenticular masses of limited extent traversing both hypogene strata and fossiliferous rocks. Such veins appear to have once been chinks or small cavities, caused, like cracks in clay, by the shrinking of the mass, during desiccation, or 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. Mixed with hot water and steam, metallic ores may have permeated the mass 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 downward 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 three or four feet. They hold their course continuously in a certain prevailing direction for miles or leagues, passing through rocks varying in mineral composition.
That Metalliferous Veins were Fissures.—As some intelligent miners, after an attentive study of metalliferous veins, have been unable to reconcile many of their characteristics with the hypothesis of fissures, I shall begin by stating the evidence in its favour. The most striking fact, perhaps, which can be adduced in its support is, the coincidence of a considerable proportion of mineral veins with faults, or those dislocations of rocks which are indisputably due to mechanical force, as above explained ([p. 87]). There are even proofs in almost every mining district of a succession of faults, by which the opposite walls of rents, now the receptacles of metallic substances, have suffered displacement. Thus, for example, suppose a a, Fig. 629, to be a tin lode in Cornwall, the term lode being applied to veins containing metallic ores. This lode, running east and west, is a yard wide, and is shifted by a copper lode (b b) of similar width. The first fissure (a a) has been filled with various materials, partly of chemical origin, such as quartz, fluor-spar, peroxide of tin, sulphuret of copper, arsenical pyrites, bismuth, and sulphuret of nickel, and partly of mechanical origin, comprising clay and angular fragments or detritus of the intersected rocks. The plates of quartz and the ores are, in some places, parallel to the vertical sides or walls of the vein, being divided from each other by alternating layers of clay or other earthy matter. Occasionally the metallic ores are disseminated in detached masses among the vein-stones.
It is clear that, after the gradual introduction of the tin and other substances, the second rent (b b) was produced by another fracture accompanied by a displacement of the rocks along the plane of b b. This new opening was then filled with minerals, some of them resembling those in a a, as fluor-spar (or fluate of lime) and quartz; others different, the copper being plentiful and the tin wanting or very scarce. We must next suppose a third movement to occur, breaking asunder all the rocks along the line c c, Fig. 630; the fissure, in this instance, being only six inches wide, and simply filled with clay, derived, probably, from the friction of the walls of the rent, or partly, perhaps, washed in from above. This new movement has displaced the rock in such a manner as to interrupt the continuity of the copper vein (b b), and, at the same time, to shift or heave laterally in the same direction a portion of the tin vein which had not previously been broken.
Again, in Fig. 631 we see evidence of a fourth fissure (d d), also filled with clay, which has cut through the tin vein (a a), and has lifted it slightly upward towards the south. The various changes here represented are not ideal, but are exhibited in a section obtained in working an old Cornish mine, long since abandoned, in the parish of Redruth, called Huel Peever, and described both by Mr. Williams and Mr. Carne.[[1]] The principal movement here referred to, or that of c c, Fig. 631, extends through a space of no less than 84 feet; but in this, as in the case of the other three, it will be seen that the outline of the country above, d, c, b, a, etc., or the geographical features of Cornwall, are not affected by any of the dislocations, a powerful denuding force having clearly been exerted subsequently to all the faults. (See [p. 93].) It is commonly said in Cornwall, that there are eight distinct systems of veins, which can in like manner be referred to as many successive movements or fractures; and the German miners of the Hartz Mountains speak also of eight systems of veins, referable to as many periods.