Note xiii. § 53.
Metallic Veins.
212. The large specimens of native iron found in Siberia and Peru, mentioned above, § 51, are among the most curious facts in the natural history of metals. It has been doubted, however, by some, whether they really belong to natural history, or are not rather to be accounted artificial productions. If they had been found in the heart of rocks, or in the midst of metallic veins, no doubt of this sort could possibly have been entertained; but, as they lie quite on the surface, in the middle of flat countries, and at a distance from any known vein of metal, the conjecture that they may be artificial, and the remains of the iron founderies of ancient and unknown nations, is at first sight not entirely destitute of probability. This probability, however, will appear to be the less, the more carefully the specimens are examined. The metal is too perfect, and the masses too large, to have been melted in the furnaces, or to have been transported by the machinery, of a rude people. The specimen in South America weighs 300 quintals, or about 15 tons, and is soft and malleable.[109] The Siberian specimen, described by Pallas, is also very large; it is soft and malleable, and full of round cavities, containing a substance, which, on examination, has been found to be chrysolite.[110] Now, it is certainly quite impossible, that, in an artificial fusion, so much chrysolite could have come by any means to be involved in the iron; but, if the fusion was natural, and happened in a mineral vein, the iron and the chrysolite were both in their native place, and their meeting together has nothing in it that is inexplicable.
[109] Phil. Trans. 1788, p. 37. also p. 183, &c.
[110] Kirwan's Mineralogy, vol. ii. art. Native Iron.
213. Some circumstances in the description of the specimen in South America, such as the impressions of the feet of men and of birds on its surface, are not to be accounted for on any hypothesis, and certainly require more careful investigation. It is said, that this iron is very little subject to rust, and the analysis of a piece of it by Proust makes it probable, that it owes this quality to its union with nickel.[111] It appears, also, that the country of Chaco, where this specimen was found, affords many others of the same kind, one of which is mentioned in the description above referred to. That country lies on the east side of the Plata, and is a plain extremely level, and of vast extent, without any appearance of mineral veins; but such veins may nevertheless exist undiscovered, in a tract subject to periodical inundations, and where the native rock is covered with alluvial earth and gravel to a great depth. The veins maybe washed away, and the more durable substances, such as those pieces of native iron, may be left behind; and, though they must be of a formation extremely ancient, according to this hypothesis, they may not have been very long on the surface.
[111] Annales de Chimie, tom. xxxv. Messidor, p. 47.
214. Specimens of native iron have been found, less remarkable than the preceding for their size, but in circumstances that excluded all idea of artificial fusion. Of this sort was Margraaf's specimen of native iron, the first of the kind that was known; it consisted of small bits of soft and malleable iron, found in the heart of a brown iron-stone.[112] This makes it certain, that native iron is a natural production, and the mere circumstance of great magnitude, in the specimens before mentioned, does not entitle us to doubt of their having that same origin. It is a circumstance, besides, not in the least material to this argument; the smallest piece of native iron being as much a proof of fusion as the greatest; and the specimen of Margraaf being just as conclusive in favour of the Huttonian Theory, as those of Pallas or De Celis, supposing their reality in mineral productions to be completely established. À metal malleable and ductile, in ever so small a quantity, cannot be the result of precipitation from a menstruum, without a very particular combination of circumstances. Such a metal, can the other hand, can be readily produced by igneous fusion; so that here the negative and affirmative parts of the inductive argument may both be regarded as complete.
[112] Kirwan's Mineralogy, vol. ii. p. 156.
215. Mr Kirwan, in order to account for the magnitude of the two large specimens mentioned above, supposes, that small pieces of native iron (about the formation of which he appears to have no difficulty) have been originally agglutinated by petroleum, and left bare, when the surrounding stony or earthy masses either withered or were washed off.[113] This is no doubt the most singular of all the opinions which have been advanced on the subject; and, as it borrows nothing from analogy, it admits of no proof, and requires no refutation. None but a chemist of eminence could have ventured with impunity on an assertion so inconsistent with all the phenomena and principles of his science.
[113] Geol. Essays, p. 405.
216. A remark of the same author, on the subject of the native gold found in the county of Wicklow in Ireland, is entitled to more attention. "That these lumps of native gold," he says, "were never in fusion, is evident from their low specific gravity, and the grains of sand found in the midst of them. I found the specific gravity of a lump of the size of a nutmeg to be only 12800, whereas, after fusion, it became 18700."[114]
[114] Ibid. p. 402.
This argument is plausible; but, I think, nevertheless inconclusive. The sand found in the gold, accounts, at least in part, for its lightness. It is only by repeated fusions that any of the metals is brought to its utmost purity and highest specific gravity; and on no supposition can the melting of gold in the mineral regions, be very likely to separate it from heterogeneous substances. That quartzy sand should be found in it, after such a process, is naturally to be expected. The impressions which the quartz crystals have left on the Wicklow gold, would be received as a full proof of the fusion of that metal, if geologists always regulated their theories by the principles which determine the belief of ordinary men.
217. Don Rubin de Celis, in the paper referred to above, mentions some masses of silver found at Quantajaia, and also some dust of platina, in terms that excite a strong desire to have more information concerning them. They are considered by him as effects of volcanic fire; so we may conclude, that they contain evident marks of fusion, and would in this system be ascribed to that heat, from which volcanic fire is but a partial and accidental derivation.
218. The state also in which gold and silver are often found pervading masses of quartz, and shooting across them in every direction, furnishes a strong argument for the igneous origin, both of the metal and the stone. From such specimens, it is evident, that the quartz and the metal crystallized, or passed from a fluid to a solid state, at the same time; and it is hardly less clear, that this fluidity did not proceed from solution in any menstruum: For the menstruum, whether water or the chaotic fluid, to enable it to dissolve the quartz, must have had an alkaline impregnation; and, to enable it to dissolve the metal, it must have had, at the same time, an acid impregnation. But these two opposite qualities could not reside in the same subject; the add and alkali would unite together, and, if equally powerful, form a neutral salt, (like sea-salt,) incapable of acting either on the metallic or the siliceous body. If the acid was most powerful, the compound salt might act on the metal, but not at all upon the quartz; and if the alkali was most powerful, the compound might act on the quartz, but not at all on the metal. In no case, therefore, could it act on both at the same time. Fire or heat, if sufficiently intense, is not subject to this difficulty, as it could exercise its force with equal effect on both bodies.
219. The simultaneous consolidation of the quartz and the metal is indeed so highly improbable, that the Neptunists rather suppose, that the ramifications in such specimens as are here alluded to, have been produced by the metal defusing itself through rifts already formed in the stone.[115] But it may be answered, that between the channels in which the metal pervades the quartz, and the ordinary cracks or fissures in stones, there is no resemblance whatever: That a system of hollow tubes, winding through a stone, (as the tubes in question, must have been, according to this hypothesis, before they were filled by the metal,) is itself far more inconceivable than the thing which it is intended to explain; and lastly, that if the stone was perforated by such tubes, it would still be infinite to one that they did not all exactly join, or inosculate with one another.
[115] Geol. Essays, p. 401.
220. The compenetration, as it may be called, of two heterogeneous substances, has here furnished a proof of their having been melted by fire. The inclusion of one heterogeneous substance within another, as happens among the spars and drusens, found so commonly in mineral veins, often leads to a similar conclusion. Thus, from a specimen of chalcedony, including in it a piece of calcareous spar, Dr Hutton has derived a very ingenious and satisfactory proof, that these two substances were perfectly soft at the same time, and mutually affected each other at the moment of their concretion.[116]
[116] Theory of the Earth, vol. i. p. 93.
Each of these substances has its peculiar form, which, when left to itself, it naturally assumes; the spar taking the form of rhombic crystals, and the chalcedony affecting a mammalated structure, or a superficies composed of spherical segments, contiguous to one another. Now, in the specimen under consideration, the spar is included in the chalcedony, and the peculiar figure of each is impressed on the other; the angles and planes of the spar are indented into the chalcedony, and the spherical segments of the chalcedony are imprinted on the planes of the spar. These appearances are consistent with no notion of consolidation that does not involve in it the simultaneous concretion of the whole mass; and such concretion cannot arise from precipitation from a solvent, but only from the congelation of a melted body. This argument, it must be remarked, is not grounded on a solitary specimen, (though if it were it might still be perfectly conclusive,) but on a phenomenon of which there are innumerable instances.
221. According to this theory, veins were filled by the injection of fluid matter from below; and this account of them, which agrees so well with the phenomena already described, is confirmed by this, that nothing of the substances which fill the veins is to be found any where at the surface. It is not with the veins as with the strata, where, in the loose sand on the shore, and in the shells and corals accumulated at the bottom of the sea, we perceive the same materials of which these strata are composed. The same does not equally hold of metallic veins: "Look," says Dr Huston, "into the sources of our mineral treasures? Ask the miner from whence has come the metal in his veins? Not from the earth or air above, not from the strata which the vein traverses: these do not contain an atom of the minerals now considered. There is but one place from whence these minerals may have come; this is the bowels of the earth; the place of power and expansion; the place from whence has proceeded that intense heat, by which loose materials have been consolidated into rocks, as well as that enormous force, by which the regular strata have been broken and displaced."[117]
[117] Theory of the Earth, vol. i. p. 130.
222. The above is a very just and natural reflection; but if, instead of interrogating the miner; we consult the Neptunist, we will receive a very different reply. As this philosopher never embarrasses himself about preserving a uniformity in the course of nature, he will tell us, that though it may be true, that neither the air, the upper part of the earth's surface, nor even the sea, contain at present any thing like the materials of the veins, yet the time was when these materials were all mingled together in the chaotic mass, and constituted one vast fluid, encompassing the earth; from which fluid it was, that the minerals were precipitated and deposited in the clefts and fissures of the strata.
223. It is alleged, in proof of this hypothesis, that mineral veins are found to be less rich as they go farther down, whereas they ought to be richer if they were filled by the projection of melted matter from below. But the fact, that mines are less rich as they descend farther, though it may hold in some instances, is not general, and may therefore be supposed to arise from local causes, such as are, in respect of us, accidental, and beyond the limits to which our theories can be expected to reach. Thus the mines of Mexico and Peru are said to be subject to the preceding rule; but in the mines of Derbyshire and Cornwall, the very contrary is understood to take place. Besides, what we are pleased to call the riches of a mine, are riches relatively to us, and relatively to a distinction which nature does not recognise. The spars and veinstones which are thrown out in the rubbish of our mines, may be as precious in the eyes of nature, as conducive to the great objects of her economy, and are certainly as characteristic of mineral veins, as the ores of silver or gold, to which we attach so great a value. Unless the former are in smaller quantity, or less highly crystallized at great than at small depths, which I believe is not alleged, no conclusion can be drawn from substances, which occupy in general but a small proportion of any vein, and, in their dissemination through it, do not seem to be always guided by the same law.
224. Again, if the veins were filled by deposition from above, we ought to discover in them such horizontal stratification as is the effect of deposition from water, and we should perceive no marks of the materials having been introduced with violence into their place. The Neptunists cannot object to the trial of their theory by these two frets.
As to the first, it is acknowledged, that there is a certain regular disposition of the substances in mineral veins, as stated [§ 59], but it is one which has hardly any thing in common with the real phenomena of stratification. It consists in the distribution of the principal substances in coats parallel to the sides of the vein, each substance forming a separate coat. In a vein, for instance, containing quartz, fluor, calcareous spar, lead, &c. we might expect to find a lining of quartz crystals, applied immediately to the walls of the mine, and following exactly the irregularities of their surface; next, perhaps, a coat of fluor, then of calcareous spar, and last of lead ore in the centre of the vein, the same order being observed on the opposite side. These successive coats, it is material to remark, are not in planes, but in uneven surfaces, of which the inequalities are evidently determined by those of the walls, that is, of the rock which forms the sides of the vein; neither are they horizontal, but are parallel to the walls, whether these be perpendicular or inclined. Here, therefore, there is no appearance of the action of that statical law which has directed the arrangement of the other strata, and which tends to make the plane of every stratum deposited by water perpendicular to the direction of gravity. The coating of the veins has therefore been performed under the conduct of some other power than that which presides over aqueous deposition. If, as the Neptunists maintain, the materials in the veins were deposited by water, in the most perfect tranquillity, it is wonderful that we do not find those materials disposed in horizontal layers, across the vein, instead of being parallel to its sides; and it seems very unaccountable, that the common strata, deposited as we are told while the water was in a state of great agitation, have so rigorously obeyed the laws of hydrostatics, ([§ 38].) and acquired a parallelism in the planes of their stratification, which approaches so often to geometrical precision; while the materials of the veins, in circumstances so much more favourable for doing the same, have done nearly the reverse, and taken a position, often at right angles to that which hydrostatical principles require. This is a paradox which the Neptunian system has created, and which therefore it is not very likely to resolve.
225. Mere words should have little power to mislead, in a science which treats of sensible objects, such as are always easily subjected to the examination of sight or of touch; yet there is some appearance as if the Neptunists were misled in this, and other instances, by the term stratification. Though an incrustation on the perpendicular face of a rock has very little affinity to a stratum, such as we are accustomed to see deposited by water, yet the same name being once imposed on both, mineralogists have proceeded to reason concerning them, as if they were precisely the same thing, and were both to be ascribed to the same cause. Indeed every perpendicular or highly inclined bed of stone, is inexplicable as an effect of aqueous deposition, in a system, unprovided, as the Neptunian is,[118] with the means of raising up such beds from a horizontal into a vertical position. This observation may also be extended to all cases of vertical stratification. Water cannot directly arrange its deposits in planes highly inclined, and therefore I have often wondered to see the Neptunists contending so eagerly for the stratification of certain rocks, such as granite, which, being vertical, or highly inclined, was much less friendly to their system than the entire absence of all stratification would have been. I was disposed to admire their candour, when the use which they made of the fact convinced me, that I ought only to wonder at their inconsequential reasoning. The Huttonian Theory is, indeed, the only one which possesses the means of reconciling the elevation of the strata with their horizontal deposition, and which is entitled to consider stratification, in whatever plane it may be, as originally the work of the ocean. The geologists who attach themselves exclusively to the action of water, will never be able to extend the dominion of that element so far as Dr Hutton has done, by combining it with fire.
226. But, though the Neptunian system were provided with engines, powerful enough to raise up strata from a level to a vertical plane, this would avail nothing in the present instance; since, on no supposition, can the incrustations on the perpendicular sides of a vein have ever been horizontal. On no supposition, therefore, can these incrustations be received as a proof of aqueous deposition: it may indeed be certainly inferred from them, that the matter which they consist of was fluid at the time of their formation; but the absence of all appearance of a horizontal disposition, in any part of the vein, amounts nearly to a demonstration, that this fluidity did not proceed from solution in a menstruum. We must therefore conceive the coats to have been formed during the refrigeration of the melted matter injected from the mineral regions into the clefts and fissures of the strata. ([§ 59.])
227. Mineral veins, particularly at their intersections with one another, contain abundant marks of the most violent and repeated disturbance ([§ 56.]). Not to mention that they owe their first formation to the fracture and displacing of rocks already consolidated, it appears, that they have originated at very different periods, and that the birth of each has been accompanied with convulsions, which shook the foundations of the earth. In Cornwall, for instance, the principal veins, and those which they distinguish particularly by the name of Lodes, have nearly the same direction with the strata or vertical schistus, extending from about E. N. E. to W. S. W. These, however, are often intersected nearly at right angles by other mineral veins, called Cross Courses, and this hardly ever happens without the latter moving, or, as it is called, heaving the former out of their direction. This plainly indicates, that the cross courses are of later origin than the others, and that their formation was accompanied with such a force, as must, in many substances, have moved the whole body of rock which constitutes the promontory of Cornwall, and probably much more, for several yards, in a horizontal direction. Sometimes, also, both the longitudinal and the cross vein are forced out of their place by a third. These disturbances arise not only from mineral veins, but from veins of porphyry and granite, the production of which has been attended with no less violence than of the others.
228. What is here said of Cornwall, is the history, in some degree, of all mineral countries whatever. The great horizontal translation which has thus accompanied the formation of veins; the movement impressed on such vast bodies of rock, and the frequent renewal of these immense convulsions; are not to be explained by the mild and tranquil dominion of the watery element. They require the utmost power that is known any where to exist, and were it not for the admonitions of the volcano and the earthquake, we might doubt if even subterraneous heat itself possessed an energy adequate to these astonishing effects.
229. From the heaving of one vein by another, it is evident, that there was a force of protrusion in the direction of one of them, that acted at the time of its formation. This force cannot be accounted for on the supposition that veins were produced by the mere shrinking of the strata; for the rocks could not, in that case, have been rent asunder, and impelled forward at the same time. It appears most likely, that fissures in the strata were made, at least in many instances, and the matter poured into them, nearly at the same time, both being effects of the same cause, the expansive force of subterraneous heat.
230. It is remarked, at [§ 56], that the shifting of the strata is best observed where the veins make a transverse section of beds of rock, considerably inclined to the horizon. It is also true, that in some cases the near approach of the strata to the level, may make the shifts produced by the veins very easy to be discovered. Thus in Derbyshire, where the mineral veins are in secondary strata, nearly horizontal, there is almost no instance in which the corresponding strata are not observed to be on different levels, on the opposite sides of the same vein.
231. The fact described by De Luc, and referred to at [§ 55], may, for what we know of it, admit of being explained in two ways. The great wedge of rock which appears to be insulated between two branches of the same vein, may either be a mass that has been broken off, and sustained by the melted matter that flowed all around it; or, it may be a mass of rock contained between two veins that are in reality distinct, and of different formation. Whether this last supposition is the truth, would probably be evident from a careful examination of both parts of the vein; as some difference of character cannot fail to be the consequence of different formation. If no such difference is observed, the two branches must be supposed to belong to the same vein, and the only probable explanation of the insulation of so large a mass of rock will be by the first mentioned supposition. This fact, therefore, notwithstanding the great attention M. De Luc has bestowed on it, still requires further examination, before it can be decided whether it inclines to the Huttonian Theory, as on the first supposition, or is, as on the latter hypothesis, equally balanced between it and the Wernerian.
232. Whatever be the case with this fact, the general one of pieces of rock being found insulated in veins, is certainly favourable to the notion of an injected and ponderous fluid having originally sustained them. Where, as happens in some instances, the stones contained in the veins have no affinity to any of the rocks above, they cannot be supposed to have come any how but from below, and to have been carried up by the matter of the vein. The instance from the slip at the Huddersfield Canal has been already mentioned.
233. The preceding observations have been principally directed against that theory of veins which supposes them to have been filled by deposition from water. There is another theory maintained by some of the Neptunists, that the metals in veins were introduced there by infiltration.[119] This opinion is sufficiently refuted by the fact, that rarely any metallic ore is found out of the vein, or in the rock on either side of it, and least of ail where the vein is richest. This is inconsistent with the notion of the ore being carried into the vein by water percolating through the adjacent rocks, unless some satisfactory reason is assigned, which determined the water to leave the ore in the vein and no where else. Besides, this hypothesis does not account for the formation of the spars and veinstones which fill the vein, and which appear clearly to have been brought there at the same time with the ore, and no doubt by the same cause.
[119] Geol. Essays, p. 401.
234. The veins, properly so called, are indefinitely extended; but there are also thin plates of spar, and of crystals of different kinds, often found included in rocks, and shut in on all sides, to which the name of veins is commonly applied. These last ought certainly to be distinguished from the former, and may not improperly be called Plate Veins or Lenticular Veins, the plate or cake of spar of which they consist having very often the form of a lens, though, as may be supposed, considerably irregular. Either of these terms being derived entirely from external characters, has the advantage of involving nothing theoretical.
The lenticular veins are certainly not formed like the usual mineral veins, by injection, since they are shut in, on all sides, by the solid rock. When they are found, therefore, in stratified rocks, such as have not themselves been melted, we must conceive them to be composed of materials more fusible than the surrounding rock, so that they have been brought into fusion by a degree of heat which the rest of the rock was able to resist, and, on cooling, have assumed a sparry structure. When they are found in rocks, of which the whole has been fluid, they must be considered as component parts of that mass, which, by an elective attraction, have united with one another, and separated themselves from the substances to which they had less affinity.
The veins of this kind seem to be connected with those called in Derbyshire Pipe Veins, in which the ores of metals are sometimes found. The pipe veins, indeed, are not in all cases completely insulated, but sometimes communicate with the veins properly called mineral. I am too little acquainted, however, with their natural history, to Be able to say with certainty to which of the two species they ought to be referred.