The Student's Elements of Geology Part 75

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.

It is true, as Humboldt has happily 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 recognise our old acquaintances-- the same granite, the same gneiss, the same micaceous schist, quartz-rock, and the rest. There is certainly a great and striking general resemblance in the princ.i.p.al kinds of hypogene rocks in all countries, however different their ages; but each of them, as we have seen, must be regarded as geological families of rocks, and not as definite mineral compounds. They are 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 they underwent metamorphism, 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. It would, however, be a great error to a.s.sume, as some have done, 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. Not only do the proportional quant.i.ties of feldspar, 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 (Chapter 28 and Table 28.1).

SUPPOSED AZOIC PERIOD.

The total absence of any trace of fossils has inclined many geologists to attribute the origin of the most ancient strata to an azoic period, or one antecedent to the existence of organic beings. Admitting, they say, the obliteration, in some cases, of fossils by Plutonic action, we might still expect that traces of them would oftener be found in certain ancient systems of slate which can scarcely be said to have a.s.sumed a crystalline structure. But in urging this argument it seems to have been forgotten that there are stratified formations of enormous thickness, and of various ages, some of them even of Tertiary date, and which we know were formed after the earth had become the abode of living creatures, which are, nevertheless, in some districts, entirely dest.i.tute of all vestiges of organic bodies. In some, the traces of fossils may have been effaced by water and acids, at many successive periods; indeed the removal of the calcareous matter of fossil sh.e.l.ls 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.

Those who believed the hypogene rocks to have originated antecedently to the creation of organic beings, imputed the absence of lime, so remarkable in metamorphic strata, 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. 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 volcanoes, 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. Although we can not descend into the subterranean regions where volcanic heat is developed, we can observe in regions of extinct volcanoes, 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.

If all the calcareous matter transferred in the course of ages by these and thousands of other springs from the lower part of the earth's crust to the atmosphere could be presented to us in a solid form, we should find that its volume was comparable to that of many a chain of hills. Calcareous matter is poured into 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.

The scarcity of limestone in many extensive regions of metamorphic rocks, as in the Eastern and Southern Grampians of Scotland, may have been the result of some action of this kind; and if the limestones of the Lower Laurentian in Canada afford a remarkable exception to the general rule, we must not forget that it is precisely in this most ancient formation that the Eozoon Canadense has been found. The fact that some distinct bands of limestone from 700 to 1500 feet thick occur here, may be connected with the escape from destruction of some few traces of organic life, even in a rock in which metamorphic action has gone so far as to produce serpentine, augite, and other minerals found largely intermixed with the carbonate of lime.

CHAPTER x.x.xVI.

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."

Sh.e.l.ls 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 ma.s.ses 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 ma.s.ses of limited extent traversing both hypogene strata and fossiliferous rocks. 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, during desiccation, or 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. Mixed with hot water and steam, metallic ores may have permeated the ma.s.s 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 downward 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 three or four feet. They hold their course continuously in a certain prevailing direction for miles or leagues, pa.s.sing through rocks varying in mineral composition.

THAT METALLIFEROUS VEINS WERE FISSURES.

(FIGURES 629, 630 and 631. Vertical sections of the mine of Huel Peever, Redruth, Cornwall.

(Figure 629. Vertical section of the mine of Huel Peever, Redruth, Cornwall.

Tin.)

(FIGURE 630. Vertical section of the mine of Huel Peever, Redruth, Cornwall.

Copper.)

(FIGURE 631. Vertical section of the mine of Huel Peever, Redruth, Cornwall.

Clay and copper.))

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 (Chapter 5). 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, Figure 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, 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 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, Figure 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 Figure 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. (Geological Transactions volume 4 page 139; Transactions of the Royal Geological Society Cornwall volume 2 page 90.) The princ.i.p.al movement here referred to, or that of c-c, Figure 631, extends through a s.p.a.ce 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 Chapter 5.) 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.

Besides the proofs of mechanical action already explained, the opposite walls of veins are often beautifully polished, as if glazed, and are not unfrequently striated or scored with parallel furrows and ridges, such as would be produced by the continued rubbing together of surfaces of unequal hardness. These smoothed surfaces resemble the rocky floor over which a glacier has pa.s.sed (see Figure 106). They are common even in cases where there has been no shift, and occur equally in non-metalliferous fissures. They are called by miners "slicken- sides," from the German schlichten, to plane, and seite, side. It is supposed that the lines of the striae indicate the direction in which the rocks were moved.

In some of the veins in the mountain limestone of Derbyshire, containing lead, the vein-stuff, which is nearly compact, is occasionally traversed by what may be called a vertical crack pa.s.sing down the middle of the vein. The two faces in contact are slicken-sides, well polished and fluted, and sometimes covered by a thin coating of lead-ore. When one side of the vein-stuff is removed, the other side cracks, especially if small holes be made in it, and fragments fly off with loud explosions, and continue to do so for some days. The miner, availing himself of this circ.u.mstance, makes with his pick small holes about six inches apart, and four inches deep, and on his return in a few hours finds every part ready broken to his hand. (Conybeare and Phil. Geol. page 401 and Farey's Derbyshire page 243.)

That a great many veins communicated originally with the surface of the country above, or with the bed of the sea, is proved by the occurrence in them of well- rounded pebbles, agreeing with those in superficial alluviums, as in Auvergne and Saxony. Marine fossil sh.e.l.ls, also, have been found at great depths, having probably been ingulfed during submarine earthquakes. Thus, a gryphaea is stated by M. Virlet to have been met with in a lead-mine near Semur, in France, and a madrepore in a compact vein of cinnabar in Hungary. (Fournet Etudes sur les Depots Metalliferes.) In Bohemia, similar pebbles have been met with at the depth of 180 fathoms; and in Cornwall, Mr. Carne mentions true pebbles of quartz and slate in a tin lode of the Relistran Mine, at the depth of 600 feet below the surface. They were cemented by oxide of tin and bisulphuret of copper, and were traced over a s.p.a.ce more than twelve feet long and as many wide. (carne Transactions of the Geological Society Cornwall volume 3 page 238.) When different sets or systems of veins occur in the same country, those which are supposed to be of contemporaneous origin, and which are filled with the same kind of metals, often maintain a general parallelism of direction. Thus, for example, both the tin and copper veins in Cornwall run nearly east and west, while the lead veins run north and south; but there is no general law of direction common to different mining districts. The parallelism of the veins is another reason for regarding them as ordinary fissures, for we observe that faults and trap dikes, admitted by all to be ma.s.ses of melted matter which have filled rents, are often parallel.

FRACTURE, RE-OPENING AND SUCCESSIVE FORMATION OF VEINS.

a.s.suming, then, that veins are simply fissures in which chemical and mechanical deposits have acc.u.mulated, we may next consider the proofs of their having been filled gradually and often during successive enlargements.

Werner observed, in a vein near Gersdorff, in Saxony, no less than thirteen beds of different minerals, arranged with the utmost regularity on each side of the central layer. This layer was formed of two plates of calcareous spar, which had evidently lined the opposite walls of a vertical cavity. The thirteen beds followed each other in corresponding order, consisting of fluor-spar, heavy spar, galena, etc. In these cases the central ma.s.s has been last formed, and the two plates which coat the walls of the rent on each side are the oldest of all.

If they consist of crystalline precipitates, they may be explained by supposing the fissure to have remained unaltered in its dimensions, while a series of changes occurred in the nature of the solutions which rose up from below: but such a mode of deposition, in the case of many successive and parallel layers, appears to be exceptional.

(FIGURE 632. Copper lode, near Redruth, enlarged at six successive periods.)

If a vein-stone consist of crystalline matter, the points of the crystals are always turned inward, or towards the centre of the vein; in other words, they point in the direction where there was s.p.a.ce for the development of the crystals. Thus each new layer receives the impression of the crystals of the preceding layer, and imprints its crystals on the one which follows, until at length the whole of the vein is filled: the two layers which meet dovetail the points of their crystals the one into the other. But in Cornwall, some lodes occur where the vertical plates, or COMBS, as they are there called, exhibit crystals so dovetailed as to prove that the same fissure has been often enlarged. Sir H. De la Beche gives the following curious and instructive example (Figure 632), from a copper-mine in granite, near Redruth. (Geological Report on Cornwall page 340.) Each of the plates or combs (a, b, c, d, e, f) is double, having the points of their crystals turned inward along the axis of the comb.

The sides or walls (2, 3, 4, 5 and 6) are parted by a thin covering of ochreous clay, so that each comb is readily separable from another by a moderate blow of the hammer. The breadth of each represents the whole width of the fissure at six successive periods, and the outer walls of the vein, where the first narrow rent was formed, consisted of the granitic surfaces 1 and 7.

A somewhat a.n.a.logous interpretation is applicable to many other cases, where clay, sand, or angular detritus, alternate with ores and vein-stones. Thus, we may imagine the sides of a fissure to be incrusted with siliceous matter, as Von Buch observed, in Lancerote, the walls of a volcanic crater formed in 1731 to be traversed by an open rent in which hot vapours had deposited hydrate of silica, the incrustation nearly extending to the middle. (Principles chapter 27 8th edition page 422.) Such a vein may then be filled with clay or sand, and afterwards re-opened, the new rent dividing the argillaceous deposit, and allowing a quant.i.ty of rubbish to fall down. Various metals and spars may then be precipitated from aqueous solutions among the interstices of this heterogeneous ma.s.s.

That such changes have repeatedly occurred, is demonstrated by occasional cross- veins, implying the oblique fracture of previously formed chemical and mechanical deposits. Thus, for example, M. Fournet, in his description of some mines in Auvergne worked under his superintendence, observes that the granite of that country was first penetrated by veins of granite, and then dislocated, so that open rents crossed both the granite and the granitic veins. Into such openings, quartz, accompanied by sulphurets of iron and a.r.s.enical pyrites, was introduced. Another convulsion then burst open the rocks along the old line of fracture, and the first set of deposits were cracked and often shattered, so that the new rent was filled, not only with angular fragments of the adjoining rocks, but with pieces of the older vein-stones. Polished and striated surfaces on the sides or in the contents of the vein also attest the reality of these movements. A new period of repose then ensued, during which various sulphurets were introduced, together with hornstone quartz, by which angular fragments of the older quartz before mentioned were cemented into a breccia. This period was followed by other dilatations of the same veins, and the introduction of other sets of mineral deposits, as well as of pebbles of the basaltic lavas of Auvergne, derived from superficial alluviums, probably of Miocene or even Older Pliocene date. Such repeated enlargement and re-opening of veins might have been antic.i.p.ated, if we adopt the theory of fissures, and reflect how few of them have ever been sealed up entirely, and that a country with fissures only partially filled must naturally offer much feebler resistance along the old lines of fracture than anywhere else.

CAUSE OF ALTERNATE CONTRACTION AND SWELLING OF VEINS.

(FIGURES 633 to 635. Irregular fissures.

(FIGURE 633.)

(FIGURE 634.)

(FIGURE 635.))

A large proportion of metalliferous veins have their opposite walls nearly parallel, and sometimes over a wide extent of country. There is a fine example of this in the celebrated vein of Andreasburg in the Hartz, which has been worked for a depth of 500 yards perpendicularly, and 200 horizontally, retaining almost everywhere a width of three feet. But many lodes in Cornwall and elsewhere are extremely variable in size, being one or two inches in one part, and then eight or ten feet in another, at the distance of a few fathoms, and then again narrowing as before. Such alternate swelling and contraction is so often characteristic as to require explanation. The walls of fissures in general, observes Sir H. De la Beche, are rarely perfect planes throughout their entire course, nor could we well expect them to be so, since they commonly pa.s.s through rocks of unequal hardness and different mineral composition. If, therefore, the opposite sides of such irregular fissures slide upon each other, that is to say, if there be a fault, as in the case of so many mineral veins, the parallelism of the opposite walls is at once entirely destroyed, as will be readily seen by studying Figures 633 to 635.