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.

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]

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.

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.