Where the granite was little decomposed, the quartz was not easily disengaged from the mass it was imbedded in, and often broke in pieces before it could be extricated. The crystallization of the quartz, therefore, would not have been discovered, but for the decomposition of the feltspar; and it is probable, that similar crystallizations exist in many granites where they are not perceived.
292. Some mineralogists are inclined to think, that the regular crystallization of quartz is to be found only in what they call secondary granites, or in those that are of a formation subsequent to the great masses which constitute the granite mountains. It is indeed true, that in the instances given here, both from Cornwall and Daouria, the granites containing quartz crystals are from veins that intersect the primary schistus, and are therefore, on every hypothesis, of a formation subsequent to that schistus. But it does not follow from thence, that they are less ancient than the great masses of unstratified granite; with these last they are most probably coëval, nor can there be any reason for thinking the crystallization of quartz a mark of more recent formation than that of feltspar.
3. Stratification of Granite.
293. What are the various modes in which granite exists, is a question not absolutely decided among mineralogists. 1. That it exists as a schistose stone of a fissile texture, in gneiss and veined granite, is on all hands admitted, though in this state the name of granite is generally withheld from it. 2. That it exists often without any indication of a fissile texture, and altogether unstratified, is likewise acknowledged. 3. That it is found in veins, intersecting the strata, has been shown above. The only mode of its existence subject to dispute, is that in which it is said to be stratified in its outward configuration, but not schistose in its texture. On this point mineralogists do not perfectly agree: Dr Hutton did not think that this was a state in which granite ever appears. When not schistose in its structure, he supposed it to be unstratified altogether; and he considered it as a body which, like whinstone, was originally in a state of igneous fusion, and, in that condition, injected among the strata. The school of Werner, on the other hand, maintain, that granite, if not always, is generally stratified, and disposed in beds, sometimes horizontal, though more frequently vertical, or highly inclined.
In forming an opinion where there are great authorities on opposite sides, a man must trust chiefly to his own observations, and ought to esteem himself fortunate if these lead to any certain conclusion. Mine incline me to differ from Dr Hutton, on the one hand, and from the Neptunists, on the other, as they convince me, that granite does form strata where it has no character of gneiss; and, at the same time, induce me to suspect, that the stratification ascribed by the Neptunists to the granite mountains, is, in many instances, either an illusion, or at least something very different from what, in other stones, is accounted stratification.
294. The first example I ever saw of granite that was stratified, and yet had no character of gneiss, was at Chorley Forest, in Leicestershire. The greater part of that forest has for its base a hornstone schistus, primary and vertical; and, on its eastern border, particularly near Mount Sorrel, are beds of granite, holding the same direction with those of the schistus. The stone is a real granite; it has nothing in its internal structure of a schistose or fissile appearance; and its beds, which it is material to remark, are no thicker than those of the hornstone strata in the neighbourhood. This granite is remarkable, too, for being close to the secondary sandstone strata; I did not see their contact, but traced them within a small distance of one another; so that I think it is not likely that any body of rock intervenes. At the same time that I state my belief of this rock of granite being in regular strata, I must acknowledge, that a very intelligent mineralogist, who viewed these rocks at the same time, and whose eye was well practised in geological observation, remained in doubt concerning them.
295. Another instance of a real granite, disposed in regular beds, but without any character of gneiss, is one which I saw in Berwickshire, in Lammermuir, near the village of Priestlaw. The little river of Fassnet cuts the beds across, and renders it easy to observe their structure. The beds are not very thick; they run from about S.S.W. to N.N.E. like the schistus on either side of them. I was in company with Sir James Hall when I saw these rocks; we examined them with a good deal of attention, and traced them for more than a mile in the bed of the river; and, if I mistake not, our opinions concerning them were precisely the same.
296. What exists in two instances may exist in many, and, after these observations, I should be guilty of great inconsistency, in refusing to assent to the accounts of Pallas, De Luc, Saussure, and many other mineralogists, who so often represent granite as formed into strata. In some cases, however, it is certain, that the stratification they describe is extremely unlike that in the two instances just mentioned, and indeed very unlike any thing that is elsewhere known by the name of stratification. For example, the stratification must be very ambiguous, and very obscurely marked, that was not discovered till after a series of observations, continued for more than twenty years, by a very skilful and distinguishing mineralogist. Yet such undoubtedly is the stratification of Mont Blanc, and of the granite mountains in its neighbourhood, as it escaped the eyes of Saussure, in the repeated visits which he made to them, during a period of no less extent than has just been mentioned. It was not till near the conclusion of those labours, to which the geologists of every age will consider themselves as highly indebted, that, having reached the summit of Mont Blanc, he perceived, or thought that he perceived, the stratification of the granite mountains. The Aiguilles or Needles which border the valley of Chamouni, and even Mont Blanc itself, appeared to be formed of vast tabular masses of granite, in position nearly vertical, and so exactly parallel, that he did not hesitate to call them by the name of strata. Till this moment, these same mountains, viewed from a lower point, had been regarded by him as composed of great plates of rock, nearly vertical indeed, but applied, as it were, round an axis, and resembling the leaves of an artichoke;[151] and the fissures by which they are separated from one another, had been considered as effects of waste and degradation. "But now," (says he, speaking of the view from the top of Mont Blanc,) "I was fully convinced, that these mountains are entirely composed of vast plates of granite, perpendicular to the horizon, and directed from N.E. to S.W. Three of these plates, separated from each other, formed the top of the Aiguille du Midi, and other similar plates, decreasing gradually in height, compose its declivity to the south."[152]
[151] Voyages aux Alpes, tom. ii. § 910, &c.
[152] Voyages aux Alpes, tom. iv. § 1996.