53. Again, pieces of native manganese have been found possessing so exactly the characters peculiar to that metal when reduced in our furnaces, that it is impossible to consider them as deriving their figure and solidity from any cause but fusion. The ingenious author who describes these specimens, La Peyrouse, was so forcibly struck with this resemblance, that he immediately drew the same conclusion from it which is drawn here, attributing the only difference, which he remarked between the native and the artificial regulus, to the different energy with which the same agent works when employed by nature and by art.[17]
[17] Theory of the Earth, vol. i. p. 68. Journal de Phys. Janvier, 1786.
54. All these appearances conspire to prove, that the materials which fill the mineral veins were melted by heat, and forcibly injected, in that state, into the clefts and fissures of the strata. These fissures we must conceive to have arisen, not merely from the shrinking of the strata while they acquired hardness and solidity, but from the violence done to them, when they were heaved up and elevated in the manner which has already been explained.[18]
55. When these suppositions are once admitted, the other leading facts in the history of metallic veins will be readily accounted for. Thus, for instance, it is evident to what we must ascribe the fragments of the surrounding rock that are often found immersed in the veins, and encompassed on all sides by crystallized substances. These fragments being no doubt detached by the concussion, which at once tore asunder and elevated the strata, were sustained by the melted matter that flowed at the same time upward through the vein. Large masses of rock are often found in this manner completely insulated; one of these, which M. De Luc has described with great accuracy, is no less than a vast segment of a mountain.[19]
[19] Lettres Physiques, &c. tom. iii. P. 361.
56. The immense violence which has accompanied the formation of mineral veins, is particularly marked by the slips and shifts of the strata on each side of them all tending to show what mighty changes have taken place in those regions, which our imagination erroneously paints as the abode of everlasting silence and rest. This shifting of the strata is best observed, where the veins make a transverse section of beds of rock, considerably inclined to the horizon. There it is common to see the beds on one side of the vein slipped along from the corresponding beds on the other side, and removed sometimes in a horizontal, sometimes in an oblique direction. In this way, not only the strata are shifted, but veins, which intersect one another, are also shifted themselves. They are heaved, as it is called in the significant language of the miners, and forced out of their direction. It is impossible, in such a case, but to connect in the mind the formation of the vein, and the production of the slips which accompany it, and to regard them as parts of the same phenomenon.
57. Where these slips are horizontal, and exhibit great bodies of strata carried from their place, while the parts of the transferred mass remain undisturbed relatively to one another, they furnish a dear proof, that this change of plaice has not arisen from the falling in of the roofs of caverns, as some geologists suppose. The horizontal direction, and the regularity of the movement, are incompatible with the action of such a cause as this; and indeed it is highly interesting to remark, in the midst of the signs of disturbance which prevail in the bowels of the earth, that there reigns a certain symmetry and order, which indicate the action of a force of incredible magnitude, but slow and gradual in its effects. The parts of the mass moved are undisturbed relatively to one another; what has been broken has been cemented; the breaches of continuity have been filled up and healed; and every where we see the operation of a cause that could unite as well as separate. The twofold action of heat to expand and to melt, could scarce be pointed out more clearly by any system of appearances.
58. As a long period was no doubt required for the elevation of the strata, the rents made in them are not all of the same date, nor the veins all of the same formation. This is clear in the case of one vein producing a shift or slip in another; for the vein which forces the other out of its place, and preserves its own direction, is evidently the more recent of the two, and must have had its materials in a state of activity, when those of the other were inert. Sometimes, also, at the intersection of two veins, we may trace the current of the materials of the one, across those of the other; and here, of consequence, the relative antiquity is determined just as in the former instance.
59. The want of any appearance of stratification in mineral veins has already been taken notice of. There is, however, to be observed, in many instances, a tendency to a regular arrangement of the substances contained in them; those of the same kind forming coats parallel to the sides of the vein, and nearly of an equal thickness. This phenomenon is considered as one of the strongest arguments in favour of the Neptunian system, but has nothing in it, in the least incompatible with that theory which ascribes the formation of veins to the action of subterraneous heat. When melted matter from the mineral regions was thrown up into the veins, that which was nearest to the sides would soonest lose its heat. The similar substances, also, would unite while this process was going forward, and would crystallize, as in other cases of congelation, from the sides toward the interior. There is the more reason for supposing this to have been the case, that the same sort of coating is often observed on the inside of close cavities, which are, nevertheless, so constructed, as to afford a demonstration that no chemical solvent was ever included in them, ([§ 74.]) Some veins, it must also be considered, may have been filled by successive injections of melted matter, and this would naturally give rise to a variety of separate incrustations.[20]