239. Several other hints contained in this paper are highly deserving of notice; for we not only find in it the notion of a formation of basaltic rocks, igneous though not volcanic, but also that of their simultaneous crystallization,[123] together with the suggestion, that granite and basalt are of the same origin,[124] These opinions had not, I believe, occurred at that time to any mineralogist except Dr Hutton, nor had they been communicated by him to any but a few of his most intimate friends; so that Mr Strange has without doubt all the merit of a first discoverer. Indeed, without the knowledge of the principle of compression, such as it is laid down by Dr Hutton, it was hardly possible for him to proceed further than he has done. He remarked the unburnt limestone that lies on the tops of some of the Euganean basaltes, and seems to have been aware of the great difficulty, which it was reserved for the Huttonian Theory to overcome. His letter contains also some excellent general remarks on the rocks of the Vivarais and Velay, which he had visited, before FAUJAS DE ST FOND had published his curious and elaborate description of these countries.
[123] Phil. Trans, ubi supra, p. 17.
[124] Ibid. p. 36 and 37.
240. The cause of the peculiar structure which has just been observed to distinguish whinstone from volcanic countries, is easily assigned in the Huttonian Theory. According to that theory, the whinstone rocks were formed, in the bowels of the earth, of melted matter poured into the rents and openings of the strata. They were cast, therefore, in those openings, as in a mould; and received the impression and character of the rocks by which they were surrounded. Hence the tabular masses of whinstone, which when soft have been interposed between strata, and compressed by their weight, so as almost to have themselves acquired the appearance of stratification. Hence the perpendicular faces of the same rocks, produced by their being abutted when yet soft, against the abrupt sides of the strata. The rocks which formed those moulds have, in many cases, entirely disappeared; in others, a part still remains, surrounding, or even covering, the basaltes, as in the Euganean Hills, in those of the Val di Noto in Sicily, the rocks near Lisbon,[125] and in different parts of Great Britain.
[125] Recherches sur les Volcans Eteints du Vivarais; Lettre du Dolomieu, p. 443.
Above all, the veins of whinstone which intersect the strata, are the completest proofs of the theory here given of these rocks, and the most inconsistent, in all respects, with the hypothesis of their volcanic origin.
241. If these criteria are applied to what are called extinguished volcanoes, I have no doubt that many which have been reckoned of that number, will be found to derive their origin more directly from the fire of the mineral regions. The basaltic rocks of the Vivarais, I am well persuaded, belong to this class; and I conclude that they do so, not only from the account of them given by Mr Strange, but from the description of Faujas himself, who, though under the influence of the opposite theory, seems very fair and accurate in his description of phenomena. The most unequivocal mark of real whinstone rock, and of a formation in the strictest sense mineral, is where veins of that kind of rock intersect the strata. Now, in a letter to Buffon, on the streams of lava found in the interior of certain calcareous rocks in the lower Vivarais, Faujas describes what can be accounted nothing else but a vein or dike of whinstone, accompanied with several of its most remarkable and characteristic appearances: "Figurez-vous un courant de lave, de la nature du basalte noir, dur et compacte, qui a percé à travers les masses calcaires, et s'est fait jour dans quelques parties, paroissant et disparoissant alternativement: Cette coulée de matière volcanique s'enfonce sous une partie de la ville, bâtie sur le rocher; elle reparoit dans la cave d'un maréchal, se cache et se montre encore de temps en temps en descendant dans le vallon, &c. Ce qu'il y a d'admirable, c'est que la lave forme deux branches bien extraordinaires, dont l'une s'éleve sur la crête du rocher, tandis que l'autre coupe horizontalement de grands bancs calcaires escarpés, qui sont à découvert, et bordent le chemin.
"Quels efforts n'-a-t-il pas fallu pour forcer cette lave se prendre une telle direction, et se percer cette suite de rochers calcaires? Si cette longue coulée de lave avoit eu 200 ou 300 toises de largeur, je ne serois pas surpris qu'un torrent de matière en fusion de ce volume eut pu produire, des effets extraordinaires et violens; mais figurez-vous, Monsieur, que dans les endroits les plus larges, elle n'a tout-au-plus qu'environ 12 ou 15 pieds; elle n'en a que 3 ou 4 dans certaines parties".[126]
[126] Volcains Eteints du Vivarais, p. 328, &c.
This narrow stream is to be traced across the strata for more than a league and a half; and the whole appeared to Faujas so marvellous, that he says he almost doubted the testimony of his senses. He would have done much better, however, to have doubted the conclusions of his theory; for it was by them that the phenomena before him were rendered so mysterious and incredible. While he continued to regard what is described above as a stream of melted lava, which had descended from the top of one mountain, and climbed up the sides of the opposite, like water in a conduit pipe, piercing occasionally through vast bodies of solid rock, it is no wonder that he considered as marvellous what is indeed physically impossible. Had his belief in the volcanic theory permitted him to see in all this, not a superficial current, but one of indefinite depth, he would have beheld the object divested, not of what was curious and interesting, but of what was incredible or absurd, and reduced to the same class of things with mineral veins. That it belongs really to this class, and is no more than a vein or dike of whinstone, intersecting the strata to an unknown depth, and most probably, like other veins, communicating with the mineral regions, cannot be doubted by any one who has studied the subject of basaltine rocks, through any other medium than the volcanic theory. The ramifications which run from it into the calcareous rock, contrived, Faujas says, just as if on purpose to perplex mineralogists, is one of the well known and characteristic appearances of basaltic veins.