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.