Part 74 (1/2)
But if we investigate different mountain chains, we find gneiss, mica-schist, hornblende-schist, chlorite-schist, hypogene, limestone, and other rocks, succeeding each other, and alternating with each other, in every possible order. It is, indeed, more common to meet with some variety of clay-slate forming the uppermost member of a metamorphic series than any other rock; but this fact by no means implies, as some have imagined, that all clay-slates were formed at the close of an imaginary period, when the deposition of the crystalline strata gave way to that of ordinary sedimentary deposits. Such clay-slates, in fact, are variable in composition, and sometimes alternate with fossiliferous strata, so that they may be said to belong almost equally to the sedimentary and metamorphic order of rocks. It is probable that had they been subjected to more intense plutonic action, they would have been transformed into hornblende-schist, foliated chlorite-schist, scaly talcose-schist, mica-schist, or other more perfectly crystalline rocks, such as are usually a.s.sociated with gneiss.
_Uniformity of mineral character in Hypogene rocks._--Humboldt has emphatically remarked, that when we pa.s.s to another hemisphere, we see new forms of animals and plants, and even new constellations in the heavens; but in the rocks we still recognize our old acquaintances,--the same granite, the same gneiss, the same micaceous schist, quartz-rock, and the rest. It is certainly true that there is a great and striking general resemblance in the princ.i.p.al kinds of hypogene rocks, although of very different ages and countries; but it has been shown that each of these are, in fact, geological families of rocks, and not definite mineral compounds. They are much more uniform in aspect than sedimentary strata, because these last are often composed of fragments varying greatly in form, size, and colour, and contain fossils of different shapes and mineral composition, and acquire a variety of tints from the mixture of various kinds of sediment. The materials of such strata, if melted and made to crystallize, would be subject to chemical laws, simple and uniform in their action, the same in every climate, and wholly undisturbed by mechanical and organic causes.
Nevertheless, it would be a great error to a.s.sume that the hypogene rocks, considered as aggregates of simple minerals, are really more h.o.m.ogeneous in their composition than the several members of the sedimentary series. In the first place, different a.s.semblages of hypogene rocks occur in different countries; and, secondly, in any one district, the rocks which pa.s.s under the same name are often extremely variable in their component ingredients, or at least in the proportions in which each of these are present. Thus, for example, gneiss and mica-schist, so abundant in the Grampians, are wanting in c.u.mberland, Wales, and Cornwall; in parts of the Swiss and Italian Alps, the gneiss and granite are talcose, and not micaceous, as in Scotland; hornblende prevails in the granite of Scotland--schorl in that of Cornwall--albite in the plutonic rocks of the Andes--common felspar in those of Europe. In one part of Scotland, the mica-schist is full of garnets; in another it is wholly devoid of them: while in South America, according to Mr. Darwin, it is the gneiss, and not the mica-schist, which is most commonly garnetiferous. And not only do the proportional quant.i.ties of felspar, quartz, mica, hornblende, and other minerals, vary in hypogene rocks bearing the same name; but what is still more important, the ingredients, as we have seen, of the same simple mineral are not always constant (p. 369., and table, p. 377.).
_The Metamorphic strata, why less calcareous than the fossiliferous._--It has been remarked, that the quant.i.ty of calcareous matter in metamorphic strata, or, indeed, in the hypogene formations generally, is far less than in fossiliferous deposits. Thus the crystalline schists of the Grampians in Scotland, consisting of gneiss, mica-schist, hornblende-schist, and other rocks, many thousands of yards in thickness, contain an exceedingly small proportion of interstratified calcareous beds, although these have been the objects of careful search for economical purposes. Yet limestone is not wanting in the Grampians, and it is a.s.sociated sometimes with gneiss, sometimes with mica-schist, and in other places with other members of the metamorphic series. But where limestone occurs abundantly, as at Carrara, and in parts of the Alps, in connection with hypogene rocks, it usually forms one of the superior members of the crystalline group.
The scarcity, then, of carbonate of lime in the plutonic and metamorphic rocks generally, seems to be the result of some general cause. So long as the hypogene rocks were believed to have originated antecedently to the creation of organic beings, it was easy to impute the absence of lime to the non-existence of those mollusca and zoophytes by which sh.e.l.ls and corals are secreted; but when we ascribe the crystalline formations to plutonic action, it is natural to inquire whether this action itself may not tend to expel carbonic acid and lime from the materials which it reduces to fusion or semi-fusion. Although we cannot descend into the subterranean regions where volcanic heat is developed, we can observe in regions of spent volcanos, such as Auvergne and Tuscany, hundreds of springs, both cold and thermal, flowing out from granite and other rocks, and having their waters plentifully charged with carbonate of lime. The quant.i.ty of calcareous matter which these springs transfer, in the course of ages, from the lower parts of the earth's crust to the superior or newly formed parts of the same, must be considerable.[487-A]
If the quant.i.ty of siliceous and aluminous ingredients brought up by such springs were great, instead of being utterly insignificant, it might be contended that the mineral matter thus expelled implies simply the decomposition of ordinary subterranean rocks; but the prodigious excess of carbonate of lime over every other element must, in the course of time, cause the crust of the earth below to be almost entirely deprived of its calcareous const.i.tuents, while we know that the same action imparts to newer deposits, ever forming in seas and lakes, an excess of carbonate of lime. Calcareous matter is poured into these lakes, and the ocean, by a thousand springs and rivers; so that part of almost every new calcareous rock chemically precipitated, and of many reefs of sh.e.l.ly and coralline stone, must be derived from mineral matter subtracted by plutonic agency, and driven up by gas and steam from fused and heated rocks in the bowels of the earth.
Not only carbonate of lime, but also free carbonic acid gas is given off plentifully from the soil and crevices of rocks in regions of active and spent volcanos, as near Naples, and in Auvergne. By this process, fossil sh.e.l.ls or corals may often lose their carbonic acid, and the residual lime may enter into the composition of augite, hornblende, garnet, and other hypogene minerals. That the removal of the calcareous matter of fossil sh.e.l.ls is of frequent occurrence, is proved by the fact of such organic remains being often replaced by silex or other minerals, and sometimes by the s.p.a.ce once occupied by the fossil being left empty, or only marked by a faint impression. We ought not indeed to marvel at the general absence of organic remains from the crystalline strata, when we bear in mind how often fossils are obliterated, wholly or in part, even in tertiary formations--how often vast ma.s.ses of sandstone and shale, of different ages, and thousands of feet thick, are devoid of fossils--how certain strata may first have been deprived of a portion of their fossils when they became semi-crystalline, or a.s.sumed the _transition_ state of Werner--and how the remaining organic remains have been effaced when they were rendered metamorphic. Some rocks of the last-mentioned cla.s.s, moreover, must have been exposed again and again to renewed plutonic action.
FOOTNOTES:
[483-A] See notices of Savi, Hoffmann, and others, referred to by Boue, Bull. de la Soc. Geol. de France, tom. v. p. 317.; and tom. iii. p. xliv.; also Pilla, cited by Murchison, Quart. Geol. Journ., vol. v. p. 266.
[487-A] See Principles, _Index_, ”Calcareous Springs.”
CHAPTER x.x.xVIII.
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 s.h.i.+fting other veins--Polis.h.i.+ng of their walls--Sh.e.l.ls 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, Glamorgans.h.i.+re--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 ma.s.ses 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 const.i.tuted 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 inc.u.mbent 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 ma.s.ses 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 c.h.i.n.ks or small cavities, caused, like cracks in clay, by the shrinking of the ma.s.s, which has consolidated from a fluid state, or has simply contracted its dimensions in pa.s.sing from a higher to a lower temperature. Siliceous, calcareous, and occasionally metallic matters, have sometimes found their way simultaneously into such empty s.p.a.ces, 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 a.s.semblage 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 a.s.sume 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, pa.s.sing through rocks varying in mineral composition.
[3 Ill.u.s.trations: Fig. 513. Fig. 514. Fig. 515. Vertical sections of the mine of Huel Peever, Redruth, Cornwall.]
_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. 62.). 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. 513., 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 s.h.i.+fted 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, a.r.s.enical pyrites, bis.m.u.th, 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 ma.s.ses among the veinstones.
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 the shock of a third earthquake to occur, breaking asunder all the rocks along the line c _c_, fig. 514.; the fissure in this instance, being only 6 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 heaved the rock in such a manner as to interrupt the continuity of the copper vein (_b b_), and, at the same time, to s.h.i.+ft or heave laterally in the same direction a portion of the tin vein which had not previously been broken.