Illustrations of the Huttonian Theory of the Earth

by John Playfair

297. Saussure was so strongly impressed with the appearances of what he accounted regular stratification, such as water only can produce, and such as must have been in the beginning horizontal, that, placed as he now was, on one of the highest points of the earth's surface, he formed the bold conception, that the summit on which he was standing had been once buried under the surface, to the depth at least of half the diameter of the mountain, and horizontally distant from its present place by a line not less than the whole height of the mountain; the granite beds which compose that mountain, having been raised by some enormous power from their horizontal position, and turned as on an axis, till they were brought into the vertical plane. In this notion, which suits so well with the nature of mountains really composed of vertical strata, and which does credit to the extent of Saussure's views, it is wonderful that he did not see the overthrow of the geological system be had adopted, which is provided with no means whatsoever of explaining these great effects.

Such, then, were the ideas suggested to Saussure, by viewing the mountains of the Alps from the highest of their summits. His great experience, his accurate knowledge of the objects before him, and the power he had acquired of dissipating those illusions, to which, in viewing mountainous tracts, the eye is peculiarly subject, all conspire to give great weight to his opinion. Yet, as this opinion is opposed by that which he himself had so long entertained, before it can be received with perfect confidence, it will require to be verified by new observations. It seems certain, that the beds of rock here described, differ from all ordinary strata, both horizontal and vertical, in the circumstance of their vast thickness, three of them being so large as to form the main body of a mountain. Their parallelism cannot easily be ascertained; and they have at best but a very slight resemblance to such beds as water is known to produce.

298. Their parallelism is difficult to be ascertained; for, on account of the magnitude and inaccessibility of the objects, it is impossible to place the eye in any situation, where it shall not be much nearer to one part of the planes whereof the parallelism is to be estimated, than to another. Indeed, one can perceive a cause which may have rendered the parallelism of the plates of granite which compose the aiguilles, more accurate in appearance than in reality, when viewed from a point so elevated as the summit of Mont Blanc. For, even on the supposition that the comparison of those plates to leaves of artichokes was just, and that the planes of their separation converged toward one another, in ascending to the top, when they were viewed from a point more elevated than that top, this convergency would be diminished, and, by the force of the perspective, might even be converted into parallelism. We cannot at present ascertain what effect this cause of deception may have actually produced.

299. The observations of Saussure concerning the stratification of granite, are not, however, in all instances, liable to these objections; and it seems to be on much less exceptionable grounds that he pronounces the granite of St Gothard to be stratified. The gneiss and micaceous schistus which constitute the lower part of that mountain, are succeeded by a granite without any schistose appearance, but divided into large plates, exactly parallel to the beds of the former gneiss. These he regards as real strata. On studying them in detail, he says, considerable irregularities were to be observed, but not greater than in the case of limestone or micaceous schistus.[153] It may be inferred from this, that these plates of granite are not so thick but that they admit of comparison with beds that are known with certainty to be of aqueous formation, and I am therefore disposed to believe, that the granite of St Gothard, in this part at least, is stratified. The transition from gneiss to granite en masse, is not uncommon, as Saussure has observed in other instances, and as we are just about to consider more particularly.

[153] Voyages aux Alpes, tom. iv. § 1830.

300. In the mountains of our own country, some difficulties concerning the stratification of granite have also occurred. In Arran, for instance, the mountain of Goatfield, which I have mentioned above as affording an instance of granite sending out many veins into the schistus, and rivetted, as it were, by means of them to the superincumbent rock, when I visited it, with, a view of verifying on the spot the interesting observations which Dr Hutton had there made, appeared to me to be without any vestige of stratification in its granitic part, as did also the whole group of mountains to which it belongs. It was, therefore, not without a good deal of surprise, that I lately read, in an account of that island, by a very accurate and ingenious mineralogist, that Goatfield consists of stratified granite.[154] The impression which the appearance of that mountain made on my mind, is just the reverse; and though I saw large tabular masses, sometimes nearly vertical, separated by fissures, they appeared to be much too irregular, too little extended in length and height, and vastly too much in thickness, to be reckoned the effects of stratification. For all this, I would by no means be understood to set my observations in opposition to those of Mr Jameson. In my visit to Arran, I did not direct my inquiries much toward this point; the general appearance of the rocks did not suggest the necessity of doing so, and I was not perfectly aware how much the stratification of granite had been insisted on by some mineralogists; so that I applied myself entirely to study some other of the interesting phenomena which this little island offers in so great abundance. I therefore carry my confidence in the appearances which seemed to indicate a want of stratification in the granite of Arran no further than to remain sceptical both as to Mr Jameson's conclusions and my own, till an opportunity shall occur of verifying the one or the other by actual observation.

[154] Mineralogy of the Scottish Isles, vol. i. p. 35, 36.

301. The stratification of granite, though it made no part of Dr Hutton's system, does by no means embarrass his theory with any new difficulty. Rocks, of which the parts are highly crystallized, are already admitted as belonging to the strata, and are exemplified in marble, gneiss, and veined granite. In the two last, we have not only stratification, but a schistose, united with a crystallized structure, and the effects of deposition by water, and of fluidity by fire, are certainly no where more singularly combined. The stratification of these substances is therefore more extraordinary than even that of the most highly crystallized granite. Neither the one nor the other can be explained but by supposing, that while such a degree of fluidity was produced by heat, as enabled the body when it cooled to crystallize, the whole mass was kept in its place by great pressure acting on all sides, so that the shape was preserved as originally given to it by the sea. As we cannot, however, suppose, that the intensity of the heat, or the fusibility of the substance through all the parts of a stratum, were precisely the same, we may expect to find in the same stratum, or in the same body of strata, that in some parts the marks of stratification are completely obliterated while in others they remain entire. It is thus that veined granite, of what I think should be called granitic schistus, often graduates into granite in mass, that is, granite without any schistose or fissile texture. Saussure says, that to be veined or not veined, is an affection of granite, that seems, in many cases, accidental;[155] as, in the midst of rocks of that substance, most clearly fissile, large portions appear without any vestige of stratification. Of this phenomenon, which is frequent in the Alps, instances are also to be met with in the granite rocks of Scotland, and the adjacent isles; and I know that Dr Hope, in a mineralogical excursion which he lately made among the Hebrides, observed many interesting and curious examples of it. Indeed, when rocks were so much fused as to crystallize, and so compressed, at the same time, as to remain stratified, they were evidently on the verge of change; two opposite forces were very nearly balanced, and each carried as far as it could go without entirely overcoming the other; so that a small alteration in the conditions may have made a great alteration in the effects. Hence a sudden transition from a stratified to an unstratified texture, which is only found in rocks highly crystallized, and such as have endured the most violent action of the mineralizing powers.

[155] Voyages aux Alpes, tom. iv. § 2143.

302. Now, though the stratification of granite, or the mixture of the stratified with the unstratified rocks of that genus, is not only reconcilable with the principles of the Huttonian geology, but might even have been deduced as a corollary from those principles, before it was actually observed, it may be considered as inconsistent with the theory of granitic veins that has just been given. A stratum, though soft or fluid, could not invade the surrounding strata with violence, nor send out veins to penetrate into them. It might, if, strongly compressed by another stratum less fluid than itself, fill up any fissures or cracks that were in that other, but this would hardly produce, such large veins, and of such considerable length, as often: penetrate from the granite into the schistus, nor could it give rise to any appearance of disturbance. If, therefore, veins were found proceeding from such stratified granite as that of Chorley Forest or Lammermuir, I should think, that the explanation of them was still a desideratum in geology. The Neptunian theory of infiltration would indeed be as applicable to them as to any other veins; for it is but little affected by the condition of the phenomena to be explained. Indeed, it is very difficult to set any limits to the explanations which this theory affords; and it would certainly puzzle a Neptunist, to assign any good reason why infiltration has not produced veins of one schistus running into another, or veins of schistus running into granite, as well as of granite running into, schistus. He will find it a hard task to restrain the activity of his theory, and to confine its explanations to those things that really exist.