Illustrations of the Huttonian Theory of the Earth

by John Playfair

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.