Illustrations of the Huttonian Theory of the Earth

by John Playfair

Werner also, in enumerating the substances of which veins are formed, reckons granite as one of them.

274. Veins of granite may be considered as of two kinds, according as they are connected, or not connected apparently with any large mass of granite, it is probable, that these two kinds of veins only differ in appearance, and that both are connected with masses of the same rock, though that connection is visible in some instances, and invisible in others. The distinction, however, whatever it be with respect to the thing observed, is real with respect to the observer; and, as it is right, in a description of facts, to avoid every thing hypothetical, I shall speak of these veins separately.

275. Veins of granite, having no communication, so far as can be discovered, with any mass of the same rock, are found in the Western Islands of Scotland, peculiarly in that of Coll, where they traverse the beds of gneiss and hornblende schistus, which compose the main body of the island. They are sometimes several fathoms in thickness, obliquely intersecting the planes of the strata just mentioned, which are nearly vertical. In these veins the feldspar is predominant; it is very highly crystallized, and of a beautiful flesh colour. Many smaller veins are also to be met with in the same place; but no large mass of granite is found, either in this or the adjacent island of Tiree.

276. The Portsoy granite, of which mention has been already made, § 80, also constitutes a vein or dike, traversing a highly indurated micaceous schistus, about a mile to the eastward of the little town of Portsoy, and not visibly connected with any large mass of the same kind. More dikes than one of this granite have been observed near the same spot.

A similar granite is likewise found inland, in the neighbourhood of Huntly, about eighteen miles south of Portsoy; but whether in the shape of a vein or a mass, I have not been able to learn.

277. Veins of granite are also frequent in Cornwall, where they are known by the name of lodes, the same name which is applied in that country to metallic veins. The granite veins frequently intersect the metallic, and are remarkable for producing shifts in them, or for throwing them out of their natural direction. The mineral veins, particularly those that yield copper and tin, run nearly from east to west, having the same direction with the beds of the rock itself, which is a very hard schistus. The granite lodes, as also those of porphyry, called elvan in Cornwall, are at right angles nearly to the former; and it is remarked, that they generally heave the mineral veins, but that the mineral veins seldom or never heave the cross-veins. In this country, therefore, the veins of granite and porphyry are posterior in formation to the metallic veins. These veins of granite may perhaps be connected with the great granitic mass that runs longitudinally through Cornwall, from Dartmoor to the Land's End. This much is certain, that their directions in general are such, that, if produced, they would intersect that mass, nearly at right angles.

278. The granite veins in Glentilt, where Dr Hutton made his first observations on this subject, are not, I believe, visibly connected with any large mass of the same rock.[142] The bed of the river Tilt, in the distance of little more than a mile, is intersected by no less than six very powerful veins of granite, all of them accompanied with such marks of disorder and confusion in the strata, as indicate very strongly the violence with which the granite was here introduced into its place. These veins very probably belong to the great mass of granite which is known to form the central ridge of the Grampians further to the north; but they are several miles distant from it, and the connection is perhaps invisible in the present state of the earth's surface.

[142] Trans. Royal Society Edin. Vol iii. p. 77, &c.

279. The second kind of granite vein, is one which proceeds visibly from a mass of that rock, and penetrates into the contiguous strata. The importance of this class of veins, for ascertaining the relation between granite and other mineral bodies, has been pointed out, § 82; and by means of them it has been shown, that the granite, though inferior in position, is of more recent formation than the schistus incumbent on it; and that the latter, instead of having been quietly deposited on the former, has been, long after its deposition and consolidation, heaved up from its horizontal position, by the liquid body of granite forcibly impelled against it from below.

It has been alleged, in order to take off the force of the argument derived from granite veins, that these veins are formed by infiltration, though, to give any probability to this supposition, it would be necessary to show, that water is able to dissolve the ingredients of granite; and even if this could be done, the direction which the veins have, in many instances, rising up from the granite, is a proof, as remarked [§ 82], that they cannot be the effect of infiltration.