Illustrations of the Huttonian Theory of the Earth

by John Playfair

Note xv. § 83.

On Granite.

1. Granite Veins.

272. It is said above, [§ 77], that granite is found in unstratified masses, and in veins. In the former of these conditions, it constitutes entire mountains, and forms the central ridge of many of the greatest chains that traverse the surface of the earth. It is the granite of this kind that has been most generally described by travellers and mineralogists. The veins have not been so much attended to, though they are of peculiar importance for ascertaining the relation between granite and other fossils.

273. Though Dr Hutton was the first geologist who explained the nature of granite veins, and who observed with attention the phenomena which accompany them, he is not the first who has mentioned them. M. Besson found veins of this kind in the Limoges, in an argillaceous schistus, and unconnected, as far as appeared, with any large mass of granite.[139]

[139] Journal de Phys. tom. xxix. p. 89.

Saussure met with granite veins in the Valorsine, but did not see them distinctly. He ascribed them to infiltration.[140] The date of this observation is in 1776: He afterwards discovered similar appearances at Lyons.[141]

[140] Voyage au Alpes, tom. i. § 598, 599.

[141] Ibid. § 601.

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.

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.

284. In Invernessshire, between Bernera and Fort Augustus, the same phenomena occur on the north side of Loch Chloney, where some granite mountains rise from under the schistus. In travelling near this place, Lord Webb Seymour and myself were advertised of our approach to a junction of granite and schistus, by finding among the loose stones on the road many pieces of schistus, intersected with veins of feldspar and granite. We walked along this junction for more than a mile; and toward the east end, where the road leaves it, we saw, in the bed of a stream that runs into Loch Chloney, many beautiful specimens of granitic veins pervading the schistus, and branching out into very minute ramifications.

285. The last instance I have to mention from my own observation, is at St Michael's Mount in Cornwall. That mount is entirely of granite, thrust up from under a very hard micaceous schistus, which surrounds it on all sides. At the base of it, on the west side, a great number of veins run off from the granite, and spread themselves like so many roots fixed in the schistus: they are seen at low water. In the smaller veins, the granite is of very minute, though distinct parts; in the larger, it is more highly crystallized, and is undistinguishable from the mass of the hill.

Besides the above, Cornwall probably affords many other instances of the same kind, which I have not had an opportunity to examine. Such instances may in particular be looked for at the Land's End, where a promontory, consisting of a central part of granite, and covered by a micaceous schistus on both sides of it, is cut transversely by the sea coast, and the contact of the granite and schistus of course twice exposed to view.

286. Scotland also affords other examples of granite veins, and some of them have been actually described. Mr Jameson has taken notice of some which he saw in the bottom of the river Spey, at Glen Drummond, in Badenach, and has represented them in an engraving.[144] They traverse the strata in various directions, and inclose pieces of the micaceous schistus; and, from the great number of loose blocks which he found, exhibiting portions of such veins, it is probable, that they are very numerous in this quarter. The same mineralogist mentions some instances of similar veins in the Shetland Isles.[145]

[144] Mineralogy of the Scottish Isles, vol. ii. p. 3.

[145] Ibid. p. 216.

In Ross-shire, Sir George Mackenzie has observed a great variety of granite veins, some of them of large size. One of them, in particular, not far from Coul, when first discovered, was supposed to be a single mass, rising from under the schistus; but, on a more careful examination, has been found to be a part of a great system of veins which intersects the micaceous schistus of this tract in various directions.

287. The granite veins are not the only proof that this stone is more recent than some other productions of the mineral kingdom. Specimens of granite are often found, containing round nodules of other stones, as, for example, of gneiss or micaceous schistus. Such is the specimen of granite containing gneiss, which Werner himself is said to be in possession of, and to consider as a proof, that the schistus is of greater antiquity than the granite. Such also seemed to me some pieces of granite, which I met with in Cornwall, near the Land's End; and others which I saw in Ayrshire, in loose blocks, on the sea coast between Ayr and Girvan. It is impossible to deny that the containing stone is more modern than the contained. The Neptunists indeed admit this to be true, but allege, that all granite is not of the same formation; and that, though some granite is recent, the greater part boasts of the highest antiquity which belongs to any thing in the fossil kingdom. This distinction, however, is purely hypothetical; it is a fiction contrived on purpose to reconcile the fact here mentioned with the general system of aqueous deposition, and has no support from any other phenomenon.

2. Granite of Portsoy.

288. The granite of Portsoy is one of the most singular varieties of this stone, and is remarkable for this circumstance, that the feldspar is the substance which has assumed the figure of its proper crystal, and has given its form to the quartz, so that the latter is impressed both with the acute and obtuse angles belonging to the rhombic figure of the former. The angular pieces of quartz thus moulded on the feltspar, and ranged by means of it in rows, give to this stone the appearance of rude alphabetical writing.

Now, Dr Hutton argued, that substances precipitated from a solution, and crystallizing at liberty, cannot be supposed to impress one another in the manner here exemplified; and that they could do so only when the whole mass acquired solidity at the same time, or at the same time nearly.[146] Such simultaneous consolidation can be produced in no way that we know of, but by the cooling of a mass that has been in fusion.

[146] Theory of the Earth, vol. i. P. 104.

289. A granite, brought from Daouria by M. Patrin, and described by him in the Journal de Physique for 1791, p. 295, under the name of pierre graphique, seemed to Dr Hutton to have so great a resemblance to the granite of Portsoy, that he ventured to consider them both as the same stone, and as both containing quartz moulded on feltspar.[147] It should seem, however, from further explanations, which M. Patrin has since given, but Dr Hutton was mistaken in his conjecture, and that, in the pierre graphique of the former mineralogist, the quartz gives its form to the feltspar, preserving in its crystals their natural angle of 120 degrees[148] It is impossible, I think, to doubt of the accuracy of this statement; and the graphical stone of Portsoy must therefore be admitted to differ materially from that of Daouria. They are not, however, without some considerable affinity, besides that of their outward appearance; for, though the quartz in the former is generally moulded on the feltspar, the feldspar is also occasionally impressed by the quartz, and sometimes even included in it. They may be considered as varieties of the same species of granite; and the pierre graphique of Corsica is probably a third variety, different from them both.

[147] Trans. Royal Society Edin. vol. iii. p. 83.

[148] Journal Britainnique, (of Geneva,) 1798, vol. viii. Sciences et Arts, p. 78.

290. It would seem, however, that all these stones lead exactly to the same conclusion. M. Patrin describes his specimen as containing quartz crystals, that are for the most part only cases, having their interior filled with feltspar "Le feltspath en masse contient des crysteaux quartzeux, qui n'ont le plus souvent que la carcasse, et dont l'interieur est rempli de feltspath; souvent il manque à ces carcasses quelques unes de leurs faces, et souvent la section de cette pierre dans un sens transversal aux crysteaux, presente une suite de figures qui sont des portions d'hexagones, et qui ne resemblent pas mal à des caractères Hebraiques."[149]

[149] Journal Britannique, Ibid.

These imperfect hexagonal cases of quartz, filled with feldspar, certainly indicate the crystallization of substances, which all assumed their solidity at the same time, and, in doing so, constrained the figures of one another. To use the words of Dr Hutton, "whether crystallizing quartz inclose a body of feltspar, or concreting feltspar determine the shape of fluid quartz, particularly if we have, as is here the case, two solid bodies including and included, it amounts to a demonstration, that those bodies have concreted from a fluid state of fusion, have not crystallized, in the manner of salts, from a solution."[150]

[150] Trans. Royal Society Edin. ubi supra, p. 84.

291. The quartz in granite so generally receives the impressions of all the other substances, particularly of the feldspar and schorl, and appears to be so passive a body, that it has been doubted by some mineralogists, whether in this stone it ever assumes its own figure, except where cavities afford room for its crystallization. But it is certain that, beside the Daourian granite just mentioned, there are others, in which the quartz is completely crystallized. Of this sort are some specimens, found in a granite vein on the west side of the hill of St Agnes, in Cornwall. The vein traverses the primitive schistus, of which that hill consists, from south to north nearly: the stone is much decomposed, and the feldspar in general is almost reduced to the state of clay. In this decomposed mass, quartz crystals are found, having the shape of double hexagonal pyramids, perfectly regular and complete. The side of the hexagon, which is the base of the two opposite pyramids, varies from half a tenth to a tenth of an inch in length, and is the same with the altitude of each of the pyramids. In some few specimens, the two pyramids do not rest on the same base, but are separated by a very short, though regular, hexagonal prism. The surfaces of these crystals are rough, and somewhat opaque, with slender spiculæ of schorl frequently traversing them. This roughness is occasioned by slight furrows on the surface of the crystal, very regularly disposed, and parallel to one another, being without doubt impressions from the thin plates of the feldspar, which surrounded the crystal, and slightly indented it. They very much resemble some impressions, remarked by Dr Hutton in the granite of Portsoy, and ascribed by him also to a similar cause. He has represented these in his Theory of the Earth, vol. i. plate ii. fig. 4. The action and reaction of two crystallizing bodies, hardly admits of a stronger and more unequivocal expression, than in these two instances.

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.

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.

303. As the Huttonian system cannot boast of theories of equal versatility, it would be not a little embarrassed to account for veins of great magnitude proceeding from a rock distinctly stratified, and accompanied with marks of having disturbed the rocks through which they pass. I am, however, inclined to believe, that this embarrassment will never occur; and that the granite veins do not proceed from the rocks that are really stratified, but from such as have never been deposited by water, and where the appearances of stratification, if there are any, are altogether illusory. This anticipation, however, requires to be verified by future observation; and it remains to be seen, whether granitic veins ever accompany real granitic strata, or are peculiar to those in which the appearances of regular beds are either ambiguous, or are entirely wanting. The decision of this question is an object highly worthy of the attention of geologists.

304. An argument, directed at once against the igneous origin and unstratified nature of all granite, is given in a work already mentioned. "If granite had flowed from below, how does it happen, that, after it had burst through the strata of micaceous schistus, &c. it did not overflow the neighbouring country? If this hypothesis were true, Mont Blanc could never have existed."[156]

[156] Mineralogy of the Scottish Isles, vol. ii. p. 166.

A theory is never more unfairly dealt with, than when those parts are separated which were meant to support one another, and each left to stand or fall by itself. This, however, is precisely what is done in the present instance; for Dr Hutton's theory of granite would not deserve a moment's consideration, if it were so inartificially constructed, as to suppose that granite was originally fluid, and yet to point out no means of hindering this fluid from diffusing itself over the strata, and settling in a horizontal plane. The truth is, that his theory, at the same time that it conceives this stone to have been in fusion, supposes it to have been, in that state, injected among the strata already consolidated; to have heaved them up, and to have been formed in the concavity so produced, as in a mould. Thus Mont Blanc, supposing that it is unstratified, is understood to consist of a mass that was melted by subterraneous heat under the strata, and being impelled upwards by a force, that may stand in some comparison with that which projected the planets in their orbits, heaved up the strata by which it was covered, and in which it remained included on all sides.

305. The covering of strata, thus raised up, may have been burst asunder at the summit, where the curvature and elevation were the greatest; but the melted mass underneath may have already acquired solidity, or may have been sustained by the beds of schistus incumbent on its sides. This schistus, forming the exterior crust, was immediately acted on by the causes of waste and decomposition, which have long since stripped the granite of a great part of its covering, and are now exercising their power on the central mass. That even Mont Blanc itself, as well as other unstratified mountains, was once covered with schistus, will appear to have in it nothing incongruous, when we consider the height to which the schistus still rises on its sides, or in the adjacent mountains; and when we reflect, that, from the appearances of waste and degradation which these mountains exhibit, it is certain, that the schistus must have reached much higher than it does at present.

It is obvious, therefore, that when the corresponding parts are brought together, and placed in their natural order, no room is left for the reproach, that this system is inconsistent with the existence of granite mountains. I have no pleasure in controversial writing; and, notwithstanding the advantages which a weak attack always gives to a defender, I cannot but regret, that Dr Hutton's adversaries have been so much more eager to refute than to understand his theory.

306. A remark which Dr Hutton has made on the quantity of granite that appears at the surface, compared with that of other mineral bodies, has been warmly contested. Having affirmed, that the greater part of rocks bear marks of being formed from the waste and decomposition of other rocks, he alleges that granite, (a stone which does not contain such marks) does not, for as much as appears from actual observation, make up a tenth, nor perhaps even a hundredth part of the mineral kingdom.[157] Mr Kirwan contends, that this is a very erroneous estimate, and that the quantity of granite visible on the surface, far exceeds what is here supposed.[158] The question is certainly of no material importance to the establishment of Dr Hutton's theory: it is evident, too, that an estimation, which varies so much as from a tenth to a hundredth part, cannot have been meant as any thing precise; yet it may not be quite superfluous to show, that the truth probably lies nearer to the least than the greatest of the limits just mentioned.

[157] Theory of the Earth, vol. i. p. 211.

[158] Geol. Essays, p. 480.

307. Though granite forms a part, generally the central part, of all the great chains of mountains, it usually occupies a much less extent of surface than the primary schistus. Thus in the Alps, if a line be drawn from Geneva to Ivrea, it will be about eighty-five geographical miles in length, and will measure the breadth of this formidable chain of mountains, at the place of its greatest elevation. Now, from the observations of Saussure, who crossed the Alps exactly in this direction, it may be collected, that less than nine miles of this line, or not above a tenth part of it, in the immediate vicinity of Mont Blanc, is occupied by granite.

308. In some sections of the Alps, no granite at all appears. Thus, in the route from Chambery to Turin, across Mont Cenis, which measures by the road not less than ninety miles, no granite is found, at least of that kind which is distinctly in mass, and different from gneiss or veined granite.[159]

[159] Voyages aux Alpes, tom. iii. § 1190, &c.

309. In some other places of the same mountains, the granite is more abundant. A line from the lake of Thun, along the course of the Aar, and over the mountains to the upper end of Lago Maggiore, crosses a very elevated tract, and passes by the sources of the Rhone, the Rhine, and the Tessino, which last runs into the Po. A good deal of granite is discovered here, in the mountains of Grimsel and St Gothard; but by far the greater part of it is the veined granite, the granite in mass being confined chiefly to the north side of the Grimsel. Both together do not occupy more than one-third of the line, and therefore the latter less than one-sixth.

310. The essay on the mineralogy of the Pyrenees, by the Abbé Palasso, contains a mineralogical chart of those mountains. From this chart I have found, by computation, that the granite does not occupy one-fifth of the horizontal surface on the north side of the ridge, reckoning from one end of it to the other. Indeed, many great tracts, even of the central parts of the Pyrenees, contain no granite whatsoever; and not a few of the highest mountains consist entirely of calcareous schistus. A large deduction should be made from the fraction 1/5 on account of the substances unknown, which, from the construction of the chart, are often confounded with the granitic tract.

311. I might add other estimations of the same kind, all confessedly rude and imperfect, but still conveying, by means of numbers, a better idea of the limit to which our knowledge approximates, than could be done simply by words; and, on the whole, it would appear, that if we state the proportion of granite to schistus to be that of one to four, we shall certainly do no injustice to the extent of the former.

It remains to form a rough estimate from maps, and from the accounts of travellers, of what proportion of the earth's surface consists of primary, and what of secondary rocks. After supplying the want of accurate measurement by what appeared to me the most probable suppositions, I have found, that about 1/18 of the surface of the old continent may be conceived to be occupied by primitive mountains; of which, if we take one-fifth, we have 1/90 for the part of the surface occupied by granite rocks, which differs not greatly from the least of the two limits assigned by Dr Hutton.

312. In estimating the granite of Scotland, Dr Hutton has certainly erred considerably in defect,[160] and Mr Kirwan, who always differs from him, is here nearest the truth; though he is right purely by accident, as the information on which he proceeds is vague and erroneous.

[160] Dr Hutton in this case no doubt made a very loose estimate. He says, the granite does not perhaps occupy more than a 500dth part of the whole surface. The whole surface of Scotland is not much more than 23,000 geographical miles, the 500dth part of which is exactly 46; and this is exceeded by the granite in Kirkcudbrightshire done, as may be gathered from what is said [§ 283].

The places in Scotland where granite is found, are very well known; but the extent of some of the most considerable of them is not accurately ascertained. In the southern parts, except the granite of Galloway, which is found in two pretty large insulated tracts, there is no other of any magnitude. The granite of the north extends over a large district. If we suppose a line to be drawn, from a few miles south of Aberdeen to a few miles south of Fort William, it will mark out the central chain of the Grampians in its full extent, passing over the most elevated ground, and by the heads of the largest rivers, in Scotland. Along this line there are many granite mountains, and large tracts in which granite is the prevailing rock. There are, however, large spaces also in which no granite appears, though, if we were permitted to speak theoretically, and if the question did not entirely relate to a matter of observation, we might suppose, that, in no part of this central ridge is the granite far from the surface, notwithstanding that in some places it may be covered by the schistus.

313. A great part of the Grampian mountains is on the south side of the line just mentioned, but hardly any granite is found in this division of them, except such veins as those of Glentilt. On the north side of the line, the granite extends in various directions; and, if from Fort William a line is drawn to Inverness, the quadrilateral figure, bounded on two sides by these lines, and on the other two by the sea, will be found to contain much granite, and many districts consisting entirely of that stone. This is in fact the great granite country of Scotland: it is a large tract, containing about 3170 square geographical miles, or about a seventh part of the whole: but the proportion of it occupied by granite cannot at present be ascertained with any exactness, nor will, till some mineralogist shall find leisure to examine the courses of the great rivers, the Dee, the Spey, &c. which traverse this country. If we call it one-fourth of the whole surface, its extent is certainly not underrated, and will amount to 790 square miles nearly; to which adding 150, as a very full allowance for all the other granite contained in Scotland, exclusive of the isles, we shall have 940 square miles, between a twenty-fourth and twenty-fifth part of the surface of the whole.

This computation, it must be observed, aims at nothing precise, but I think it is such, that a more accurate survey would rather diminish than increase the proportion assigned in it to the granite rock.

314. This result may perhaps fall as much short of Mr Kirwan's notion, as it exceeds the estimate made by Dr Hutton. If it shall not, and if the former has, in this instance, come nearest the truth, it cannot be ascribed to the accuracy of his information, or the soundness of the principles which directed his research. Mr Williams, whom he quotes, was a miner, of great skill and experience in some branches of his profession, to which, if he had confined himself, he might have written a book full of useful information. What he says on the subject of granite, is, in the main I believe just; but it is far too general to authorize the conclusion which Mr Kirwan derives from it. Dr Ash, for whose judgment I have great respect, cannot, I think, have meant, when he used the expression granitic rocks, to describe granite strictly so called. He says, in the passage quoted by Mr Kirwan, that "from Galloway, Dumfries, and Berwick, there is a chain of mountains, commonly schistose, but often also granitic." Now, the fact is, that the great belt of primary rock, here alluded to, which traverses the south of Scotland, consists of vertical schistus of various kinds; but except in Galloway, and again in Lammermuir, near Priestlaw, it appears, as already mentioned, to contain no granite whatsoever. If the German mineralogist quoted by Mr Kirwan, when he says that the Grampian mountains consist of micaceous limestone, gneiss, porphyry, argillite, and granite, alternating with one another, means only to affirm that all these stones are found in the Grampians, he is certainly in the right, and the catalogue might easily be enlarged; but, if he either means to say, that these are nearly in equal abundance, or that the granite is commonly found in strata alternating with other strata, I must say, that these are propositions quite contrary to any thing I have ever seen or heard of those mountains. But it is probable that this is not meant, and that the fault lies in understanding the expressions much too literally. Mr Kirwan accuses Dr Hutton of not knowing where to look for the granite; not aware of how much, notwithstanding any error committed in the present estimate, he was skilled in the art of mineralogical observation; an art, which those who have not practised do not always know how to appreciate. But, however imperfect Mr Kirwan's knowledge of this subject has been, he has here had the good fortune to correct a mineralogist of very superior information. The mere disposition to oppose is not always without its use: no man is in every thing free from error, and, to controvert indiscriminately all the opinions of any individual, is an infallible secret for being sometimes in the right.