Illustrations of the Huttonian Theory of the Earth

by John Playfair

Another objection has been thrown out, namely, that the veins here referred to are not of true granite, according to the definition which mineralogists have given of that substance. The force of a fact, however, is not to be lessened by a change of names, or the use of arbitrary definitions. The general fact is, that the granitic mass, and the vein proceeding from it, constitute one continuous, and uninterrupted body, without any line of separation between them. The geological argument turns on this circumstance alone; and it is no matter whether the rock be a syenite, a granitelle, or a real granite. The phenomenon speaks the same language, and leads to the same conclusion, whatever be the technical terms the mineralogist employs in describing it.

280. It must, however, be admitted, that a difference of character is often to be observed between the granite mass and the veins proceeding from it; sometimes the substances in the latter are more highly crystallized than in the former; sometimes, but more rarely, they are less crystallized, and, in some instances, an ingredient that enters into the mass seems entirely wanting in the vein. These varieties, for what we yet know, are not subject to any general rule; but they have been held out as a proof, that the masses and the veins are not of the same formation. It may be answered, that a perfect similarity between substances that, on every hypothesis, must have crystallized in very different circumstances, is not always to be looked for; but the most direct answer is, that this perfect similarity does sometimes occur, insomuch that, in certain instances, no difference whatsoever can be discovered between the mass and the vein, but they consist of the same ingredients, and have the same degree of crystallization. Some instances of this are just about to be remarked.

281. A strong objection to the supposed origin of granitic veins from infiltration, and indeed to their formation in any way but by igneous fusion, arises from the number of fragments of schistus, often contained, and completely insulated in those veins. How these fragments were introduced into the fissures of the schistus, and sustained till they were surrounded by the matter deposited by water, is very hard to be conceived; but if they were carried in by the melted granite, nothing is more easily understood.

The following are some of the places where the phenomena of granite veins may be distinctly seen.

282. The island of Arran, remarkable for collecting into a very small compass a great number of the most interesting facts of geology, exhibits many instances of the penetration of schistus by veins of granite. A group of granite mountains occupies the northern extremity of the island, the highest of which, Goatfield, rises nearly to the height of 3000 feet, and on the south side is covered with schistus to the height of 1100. From thence, the line of junction, or that at which the granite emerges from under the schistus, winds, so far as I was able to observe, round the whole group of mountains, with many wavings and irregularities, rising sometimes to a greater, and descending sometimes to a much lower level, than that just mentioned. Along this line, particularly on the south, wherever the rock is laid bare, and cut into by the torrents, innumerable veins of granite are to be seen entering into the schistus, growing narrower as they advance into it; and being directed, in very many cases, from below upwards, they are precisely of the kind which the infiltration of water could not produce, even were that fluid capable of dissolving the substances which the vein consists of. From this south face of the mountain, and from the bed of a torrent that intersects it very deeply, Dr Hutton brought a block of schistus, of several hundred weight, curiously penetrated by granite veins, including in them many insulated fragments of the schistus.

From this point, the common section of the granite and schistus descends towards the west side of the mountain, and is visible at the bottom of a deep glen, (Glen-Rosa,) which detaches Goatfield from the hills farther to the west. The junction is laid bare at several places in the bed of the river which runs in the bottom of this glen; and in all of them exhibits, in a greater or less degree, the appearances of disturbance and violence which have accompanied the injection of the granite veins. Many circumstances render this spot interesting to a geologist, and, among others, an intersection of the granite, a little above its junction with the schistus, by a dike or vein of very compact whinstone.

The same line of junction is found on the opposite, or north-east, side of the mountain, where it is intersected by another little river, the Sannax, which on this side determines the base of the mountain. This junction is no less remarkable than the other two.

The island of Arran contains, I have no doubt, many other spots where these phenomena are to be seen; but I have had no opportunity of observing them, nor do I find that Dr Hutton met with any others in his visit to this island.

283. Another series of granite veins is found in Galloway, which was first discovered by Dr Hutton and his friend Mr Clerk, and afterwards more fully explored by Sir James Hall and Mr Douglas, the present Earl of Selkirk. The two last traced the line of separation between a mass of granite and the schistus incumbent upon it, all round a tract of country, about eleven miles by seven, extending from the banks of Loch Ken westward; and in all this tract they found, "that wherever the junction of the granite with the schistus was visible, veins of the former, from fifty yards, to the tenth of an inch in width, were to be seen running into the latter, and pervading it in all directions, so as to put it beyond all doubt, that the granite of these veins, and consequently of the great body itself, which was observed to form with the veins one uninterrupted mass, must have flowed in a soft or liquid state into its present position."[143] I have only farther to add, that some of these veins are remarkable for containing granite, not sensibly different, in any respect, from the mass from which they proceed.

[143] Trans. Royal Society Edin. vol. iii. p. 8.