Illustrations of the Huttonian Theory of the Earth

by John Playfair

Note xiv. § 75.

On Whinstone.

235. To the facts and reasonings given above, I shall, in this note, add a few remarks, tending to show, that whinstone is not of volcanic, nor of aqueous, but certainly of igneous origin.

It is asserted, ([§ 62,]) that carbonate of lime and zeolite are often contained in whinstone, but never in lava, and that this circumstance may sometimes serve to distinguish these stones from one another. With respect to carbonate of lime, in particular, it seems evident, that this substance cannot enter into the original composition of any lava, because the same heat which melted the lava, would, where there was no greater pressure than the weight of the atmosphere, expel the carbonic acid and produce quicklime. Notwithstanding this, rocks containing carbonate of lime, have often been considered as lavas, into the pores and cavities of which, calcareous matter having been carried by the infiltration of water, had crystallized into spar. Thus Spallanzani, in his account of the Euganean Hills, in Lombardy, describes some of the rocks as abounding at their surface, and even in their interior, with air-bubbles of various sizes, from such as are hardly perceptible, to some that are half an inch in diameter; and which, he says, are all of an oval figure, with their longest diameters in the same direction. This he considers as a proof that the rock is a genuine lava; for the air-bubbles prove the stone to have had its fluidity from fire; and by their elongation in the same direction they prove, that the mass when fluid was also in motion. Spallanzani adds, that many of these cavities are filled with crystals of the carbonate of lime, an effect of the infiltration of water.[120]

[120] Voyages dans les deux Siciles, tom. iii. p. 157. Edit. de Faujas de St Fond.

236. Though the argument here advanced for the igneous origin of the rock may be admitted as conclusive, the introduction of calcareous spar into it by infiltration must still be questioned. Lava, except in a state of decay or decomposition, is not readily penetrated by water; and, if it were, the filling of cavities with spar, by means of the water percolating through them, would still be subject to many difficulties, (§12.). Besides, whinstone rocks are frequently found so full of calcareous spar, or of zeolite, that they would become porous to such a degree, if the cavities filled with these latter substances were all empty, that they could hardly sustain their own weight, and much less that of the great masses of rock incumbent on them. In such cases, it is certain, that the crystallized substances were part of the original composition of the rock. The truth is, that the infiltration of the water is a mere gratuitous assumption, introduced for the purpose of explaining the existence of carbonated lime in a stone which had endured the action of intense heat: and this assumption ought of course to be rejected, if the phenomenon can be explained by a theory, that is in other respects conformable to nature. The spar, then, may be considered as a proof, that the rocks in question are to be numbered with those unerupted lavas which have flowed deep in the bowels of the earth, and under a great compressing force. This is the more probable, that the Euganean Hills, like some whinstone hills in our own country, have, in certain places, a covering of slaty and calcareous strata incumbent on them, even at their summits,[121] so that the torrent of melted stone, of which they are admitted to consist, cannot have flowed from the mouth of a volcano. I do not mean to say, that there are among these hills no vestiges of volcanic explosion. I am very far from having data sufficient for drawing this conclusion; but I believe it may be safely affirmed, that the bulk of them is no more composed of volcanic lava, than the basaltes of Staffa, or of the Giant's Causeway.

[121] Phil. Trans. 1775, p. 34.

237. But, besides the evidence deduced from calcareous spar and zeolite, against the rocks containing them being real lava, there are other marks, even less equivocal perhaps, that distinguish the lavas which we suppose to have flowed in the mineral regions, from those which have actually flowed on the surface. These are what we collect from the disposition, the organization, or, as we may say, the physical geography of whinstone countries, unlike, in so many respects, to that of volcanic countries. The shape of whinstone hills; their large flat terraces, rising one above another; their perpendicular faces, and the correspondence of their heights even at considerable distances; have nothing similar to them in the irregular torrents of volcanic lavas. The phenomena of the former are also on a scale of magnitude very far exceeding the latter, and clearly indicate, that though both have been produced by fire, it has been by fire in very different circumstances, and regulated by very different laws. The structure of the two kinds of rock agrees, in many respects, and so does their chemical analysis; but their disposition and arrangement are so dissimilar, that they cannot be supposed to be of the same formation.

238. This argument, I believe, was first stated by Mr Strange, in a letter to Sir John Pringle, published in the 65th volume of the Philosophical Transactions.[122] That intelligent observer, after visiting the countries in Europe most remarkable either for burning, or for what are accounted, extinguished volcanoes, and examining them with a very discriminating eye, remained convinced, that there are two distinct species of rock, which both owe their origin to fire; but to fire acting in circumstances and situations extremely different. The first is the common volcanic lava; the other, to which he gives the name of a basaltine rock, comprehends such rocks as the Giant's Causeway, the basaltes of the Vivarais, of the Euganean Hills, &c. and differs in nothing from that which is called here by the name of whinstone. Mr Strange conceived, that the one of these kinds of stone could, no more than the other, be accounted the work of aqueous deposition, but was led to the distinction just mentioned, by observing the organization and arrangement in the rocks of the latter kind, and comparing them with the disorder and ruin that every where mark the footsteps of volcanic fire He does not pretend to determine the nature of the fire to which the basaltine rocks owe their formation, nor the circumstances in which it has acted: he is satisfied with the negative conclusion, that it is not volcanic; and his paper affords a specimen of what is perhaps rare in any of the sciences, and certainly most rare of all in geology, viz. a philosophic induction carried just as far as the facts will bear it out, and not a single step beyond that point.

[122] Account of Two Giants' Causeways in the Venetian State, &c. by John Strange, Esq. Phil. Trans. vol. lxv. (1775,) p. 5, &c.

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.

242. It can hardly be doubted, that the lava described by the same author as heaving up a mass of granite,[127] and including pieces of it, is a rock of real whinstone. The same may be said of many others; and, though I pretend not to affirm that there is nothing volcanic in the Vivarais, I must say, that nothing decidedly volcanic appears in the description of that country, but many things that are certainly of a very different origin.

[127] Volcains Eteints du Vivarais, fol. p. 365, &c.

In the present state of geological science, a skilful mineralogist could hardly employ himself better, than in traversing those ambiguous countries, where so much has been ascribed to the ancient operation of volcanic fire, and marking out what belongs either clearly to the erupted or unerupted lavas, and what parts are of doubtful formation, containing no mark by which they may be referred to the one of these any more than the other. Such a work would contribute very materially to illustrate the natural history of the earth.

243. One of the most ingenious attempts to support the volcanic theory, is the system of submarine volcanoes, imagined by the celebrated mineralogist Dolomieu. The phenomenon that led to this hypothesis, was what he had observed in the hills near Lisbon, and still more remarkably in those of the Val di Noto in Sicily, where the basaltine rocks had regular strata incumbent on them, and in some cases interposed or alternated with them.[128] It seemed from this evident, that the strata were of later formation than the stone on which they rested; and as they must, on every supposition, be held to be deposited by water, it was concluded, that the lava which they covered had been thrown out by volcanoes at the bottom of the sea; that the strata had afterwards been deposited on this lava; and that, in some cases, there had been frequent alternations of these eruptions and depositions.[129]

[128] Mémoire de Deodate de Dolomieu, sur les Volcains Eteints du Val di Noto, en Sicile. Journal de Phys. tom. xxv. (1784. Septembre.) p. 191.

[129] Near Vizini, in the Val di Noto, Dolomieu tells us, that he counted eleven beds, alternately calcareous and volcanic, in the perpendicular face of a hill, which at a distance appeared like a piece of cloth, striped black and white; ubi supra. He has since made similar observations in the Vicentine and in Tyrol. Journal de Phys. tom. xxxvii. (1790), partie 2, p. 200.

244. Though this hypothesis does certainly deliver the system of the Volcanists from one great difficulty, it is itself liable to insurmountable objections. I shall just mention some of the principal.

1. The regular and equidistant strata that we often see covering the tops of whinstone or basaltic rocks, could not have been deposited in the oblique and very much inclined position which they now occupy.

This is remarkable in the strata which cover the basaltic rock of Salisbury Craig, near Edinburgh, at its northern extremity. The strata are very regular, and must have been deposited in a plane nearly horizontal; yet the surface of the basaltes on which they now rest is very much inclined, dipping rapidly to the north-east. The necessity of a horizontal deposition in strata, which, though not now horizontal, have their planes nearly parallel to one another, has been proved at § 38.

2. If there is any truth in the principles established above, even the strata themselves have not been consolidated without the action of fire. By Dolomieu's system, therefore, the consolidation of the strata which cover the basaltes is not accounted for.

3. There are no means furnished by the hypothesis of submarine volcanoes for bringing the basalt, and the strata which cover it, above the level of the sea. If it is said that the waters of the sea have been drained off, the objections are all incurred that have been stated at [§ 37].[130] If it is said, that the rocks themselves have been elevated by a force, impelling them upwards, we say, that the existence of such a force, when admitted, furnishes another means of explaining the whole phenomenon, namely, that of the injection of melted matter among the strata, the same that is used in the Huttonian Theory.

[130] Dolomieu adopts this supposition; he thinks, that the surface of the sea must have been formerly 500 or 600 toises above its present level. Ibid. p. 196.

4. The phenomena of basaltic veins are not in the least explained by the hypothesis of submarine volcanoes. That hypothesis, then, even if the foregoing objections were removed, does not serve to explain all the facts respecting the rocks of this genus, and wants, of consequence, one of the most important characters of a true theory. It must be allowed, however, that it makes a considerable approach to such a theory, and that the submarine volcanoes of Dolomieu, have an affinity to the unerupted lavas of Dr Hutton.

245. Though in these remarks I have endeavoured to expose the errors of the volcanic system, I cannot but consider that system as coming infinitely nearer to the truth than the Neptunian. It has the merit of distinguishing an order of rocks, which bears no marks of aqueous formation, and in which the crystallized, sparry, or lava-like structure, bespeaks their primeval fluidity, and refers their origin to fire. The Neptunian system, on the other hand, strives to confound the most marked distinction in the mineral kingdom, and to explain the formation, both of the stratified and unstratified rocks, by the operation of the same element. Though chargeable with this inconsistency, it has become the prevailing system of geology; and the arguments which support it are therefore entitled to attention.

246. It will no doubt be thought singular, that the same mineralogist, whom we have just seen exerting his ingenuity in defence of the volcanic system, should now appear equally strenuous in defence of the Neptunian. Though Dolomieu contends for the volcanic origin of some basaltic rocks, he does not admit that all basaltes is volcanic, nor even all of igneous formation. Thus he states, that he had examined at Rome some of the most ancient monuments of art, executed in basaltes, brought from Upper Egypt, and that he could discover no mark of the action of fire in any of them.[131]

[131] Journal de Physique, tome xxxvii. (1790,) partie 2, p. 193.

On the contrary, he found that some of them consisted of green basaltes, which changes its colour to a bronze, when exposed even to a moderate heat, and which therefore, he argues, can never have endured any strong action of fire.

The answer to this argument is very plain, if we admit the effects ascribed by Dr Hutton to the compression which necessarily takes place in the mineral regions. If indeed the heat in those regions resembled exactly that of our fires at the surface, it would not be easy to deny the above conclusion, which therefore certainly holds good against the volcanic origin of the Egyptian basaltes. But there is no reason why, under strong compression, the colouring matter of these stones might not be fixed, and indestructible by heat, though it can be easily volatilized or consumed when such compression is removed. This argument then is against the volcanic; but not against what has been called the Plutonic formation of basaltes.

247. As to the other marks of fire which Dolomieu sought for and did not find in the above mentioned stones, we are not exactly informed in what they consisted. If the crystallized or spathose texture that belongs to this description of stones was wanting, the specimens were not to be considered as of the real basaltic or whinstone genus, whatever their name or history may seem to indicate. If they did possess that texture, they had the only mark of an igneous origin that could be expected, supposing that origin to have been in the bowels of the earth. No part, therefore, of the observations of this ingenious mineralogist, can be considered as inconsistent with the theory of basaltic rocks which has been laid down above.

248. Bergman had before reasoned on this subject precisely in the same manner, but from better data, as the stones from which he derived his argument were in their native place: "Trap," says that ingenious author, (that is whinstone,) "is found in the stratified mountains of West Gothland, in a way that deserves to be described. The lower stratum, which is several Swedish miles in circuit, (10½ of these miles make a degree,) is an arenaceous stone, horizontal, resting on granite, and having its particles agglutinated by clay. The stratum above this is calcareous, full of the petrifactions of marine animals, and above this is the trap. These three kinds of rock compose the greater part of the mountains just mentioned, though there are some other beds, particularly very thin beds of marl and of clay, which separate the middle stratum, both from that which is under it and over it, and are frequently so penetrated with bitumen that they burn in the fire. This schistus is black; when burnt it becomes red, and afterwards, when washed with water, affords alum. How can it be supposed," he adds, "that the trap has ever been violently heated, while the shistus on which it is incumbent retains its blackness, which however it loses by the action even of a very weak fire?"[132]

[132] Bergman de Productis Volcaniis, Opuscula, tom. iii. p. 214, &c.

The answer to this argument is already given. The reasoning, as in the former instance, is conclusive only against the action of volcanic fire, or fire at the surface; but not against the action of heat deep in the bowels of the earth, and under the pressure of the superincumbent ocean. In such a situation, the bituminous schistus might be in contact with the melted basalt, and yet there might be no evaporation of the volatile, nor combustion of the inflammable parts. It does not, however, always happen, that the bituminous substances, or substances alterable by fire, which are found in contact with basaltes, are without any mark of having endured the operation of fire. Instances in which such operation is apparent are given above, [§ 30]; and more will be added in the conclusion of this note.

249. The same mineralogist founds another argument for the aqueous formation of whin or trap on the existence of that stone in the form of veins, included in primeval rocks: "Invenitur hoc saxum (trap) in Suecia pluribus locis, sæpeque in montibus primævis, angustas implens venas, adeo subtilis structuræ, ut particulæ sint impalpabiles, et, dum niger est, genuinum efficit lapidem Lydium. In hisce montibus, nulla adsunt ignis subterranei vestigia."[133]

[133] Opuscula, ubi supra.

The phenomena here described, namely, a vein of compact whinstone traversing a primary rock, is, without doubt, as incapable of being explained by the operation of a volcano, as it is by that of aqueous deposition. It is, however, a most complete proof of the original softness of the substance of which the veins consist, and affords one of the strongest possible arguments for such an operation of fire as is supposed in the present theory. The main arguments, therefore, which have been proposed as subversive of the igneous origin of basaltes, are only subversive of their formation by one modification of fire, viz. of fire acting near the surface; and thus the weapons which directly pierce the armour of the Volcanist, and inflict a mortal wound, are easily turned aside by the superior temper of the Plutonic mail.

250. An argument founded on facts very similar to some of the preceding, and leading to the same conclusion, is employed by the mineralogist to whom the Neptunian system owes its chief support. Werner, in his observations on volcanic rocks and on basaltes, has rested his proof of the aqueous formation of the latter, on their interposition between beds of stone in mountains regularly stratified, and obviously formed by water. He describes an instance of this in the basaltic hill of Scheibenberg; and the facts, though most of them are not uncommon, are highly deserving of attention. Near the top of this hill, and above the basaltic rock which composes the body of it, he tells us, that there was a sand-pit; a circumstance which he appears to consider as not a little singular. It was, however, at the bottom of the hill, that he met with the appearances which chiefly attracted his notice: "First," says he; "or lowest, was a thick bank of quartzy sand, above that a bed of clay, then a bed of the argillaceous stone called wacke; and upon this last rested the basaltes." "When I saw," adds he, "the three first beds running almost horizontally under the basaltes, and forming its base; the sand becoming finer above, then argillaceous, and at last changing into real clay, as the argil was converted into wacke in the superior part; and, lastly, the wacke into basaltes; in a word, when I found a perfect transition from pure sand to argillaceous sand, from the latter to a sandy clay, and from this sandy clay, through many gradations, to a fat clay, to wacke, and at last basaltes, I was irresistibly led to conclude, that the basaltes, the wacke, the clay, and the sand, are all of one and the same formation; and that they are all the effect of a chemical precipitation during one and the same submersion of this country."[134]

[134] "Combien je fus surpris de voir en arrivant au fond, un epais banc de sable quartzeux, puis au-dessus une couche d'argile, enfin une couche de la pierre argileuse nommée Wacke, et sur celle-ci reposer le basalte. Quand je vis les trois premiéres couches s'enfoncer presqu' horizontalement sous le basalte, et former ainsi sa base; le sable devenir plus fin au-dessus, puis argileux, et se changer enfin en vraie argile, comme l'argile se convertissoit en wacke dans sa partie supérieure; et finalement la wacke en basalte; en un mot, de trouver ici une transition parfaite du sable pur au sable argileux, de celui-ci à l'argile sablonneuse, et de l'argile sablonneuse, par plusieurs gradations, à l'argile grasse, à la wacke et enfin au basalte.

"A cette vue, je fus sur le champ et irrésistiblement entrainé à penser, (comme l'auroit été sans doute tout connoisseur impartial frappé des conséquences de ce phénomène;) je fus, dis je, irrésistiblement entrainé aux id es suivantes: Ce basalte, cette wacke, cette argile, et ce sable, sont d'une seule et même formation; ils sont tous l'effet d'une precipitation par voie humide dans une seule et même submersion de cette contrée; les eaux qui la couvroient alors transportoient d'abord le sable, puis deposoient l'argile, et changoient peu-à-peu leur précipitation en wacke, et enfin en vraie basalte.—Journal de Physique, tom. xxxviii. (1791,) Partie i. p. 415.

First, as to the sand on the top of this basaltic hill, it is most probably the remains of certain sandstone strata that originally covered the basaltic part, but are now worn away. We are therefore to consider this as an instance of a basaltic rock, interposed between strata that are undoubtedly of marine origin. In this, however, there is nothing inconsistent with Dr Hutton's theory of basaltes; on the contrary, it is one of the principal facts on which that theory is founded. It has indeed been argued by some mineralogists, that bodies thus contiguous must owe their origin to the same element, and that a mineral substance cannot be of more recent formation than that which lies above it. But the maxim, that a fossil must have the same origin with those that surround it, does not hold, unless they have a certain similarity of structure. It is, for instance, the want of this similarity, that authorizes us to assign different periods of formation to mineral veins, and to the rocks in which they are included.

In a succession of strata, no one can doubt, that the lowest were the first formed, and the others in the order in which they lie; but, when between two strata of sandstone or of limestone we find an intermediate rock, so different as to resemble lava, and to have nothing schistose or stratified in its composition, the same instrument cannot be supposed to have been employed in the formation of both; nor is there any reason why we may not suppose, that the intermediate body was interposed between the other two, by some action subsequent to their formation. It was thus that Dolomieu concluded, when he saw a lava-like stone interposed between calcareous strata in the Val di Noto, that, though contiguous, these two rocks could not possibly be of the same formation; and thus far it is certain, that every unprejudiced observer must agree with him.

251. But the circumstance on which Werner seems to lay the greatest stress, is the gradual transition from the sand to the basalt, through the intermediate steps of clay and wacke; this gradual transition he considers as a direct proof, that they are all of the same formation.

A gradual transition of one body into another, can only be said to take place, when it is impossible to define their common boundary, or to determine the line where the one begins and the other ends. Now, if this be the proper notion of gradual transition, I must say, that after much careful examination, I have never seen an instance, in which such a transition takes place between whinstone and the contiguous strata. The line of separation, though in some places less evident than in others, has, on the whole, been marked out with great precision; and, though the stones have been firmly united, or, as one may say, welded one upon another, yet, when a fresh fracture was obtained, the stratified and unstratified parts have rarely failed to be distinguished. The fresh fracture is indeed often necessary, for many species of whinstone get by decomposition a granulated texture at the surface, so as hardly to be distinguished from real sandstone.

Some of the kinds of primary schistus also, particularly the argillaceous, when much indurated, have in their structure a considerable resemblance to whinstone; they are slightly granular, or laminated, and have a tendency to a sparry texture. Where it happens that this sort of schistus and whinstone are contiguous, it is natural to expect, that their common boundary will be traced with difficulty, and in many parts will be quite uncertain. Still, however, if a careful examination is made; if the effects of accidental causes are removed; and, above all, if the more ambiguous instances are compared with the more decisive, and interpreted by them, though single specimens may be doubtful, we will hardly ever find that any uncertainty remains with respect to entire rocks.

252. This general fact, which I state on much better authority than that of my own observations, viz. on those of Dr Hutton, is not given as absolutely without exception. The theory of whinstone which has been laid down here, leads us indeed to look for some such exceptions. It is certain, that the basis of whinstone, or the material out of which it is prepared by the action of subterraneous heat, is clay in some state or other, and probably in that of argillaceous schistus. It follows, of consequence, that argillaceous schistus may by heat be converted into whinstone, or the material out of which is prepared by the action of subterraneous heat, is clay in some state or other, and probably in that of argillaceous schistus. It follows, of consequence, that argillaceous schistus may by heat be converted into whinstone. When, therefore, melted whinstone has been poured over a rock of such shistus, it may, by its heat, have converted a part of that rock into a stone similar to itself; and thus may now seem to be united, by an insensible gradation, with the stratum on which it is incumbent; and phenomena of this kind may be expected to have really happened, though but rarely, as a particular combination of circumstances seems necessary to produce them. Hence it is evident, that stones may graduate into one another, without being of the same formation; and that it is fallacious to conclude, from the insensible transition of one kind of rock into another, without any other circumstance of affinity, that they have both the same origin.

I am disposed, therefore, to make some limitation to what is said in [§ 72], where I have expressed an absolute incredulity as to such transitions as are here referred to. The great skill and experience of the mineralogist who has described the strata at Scheibenberg, do not allow us to doubt of his exactness, though some of the appearances are such as decomposition and wearing might well enough be supposed to produce. The fairest way is to take Mr Werner's observations just as they are given us, and to try whether they cannot be explained without the assistance of his theory. In effect, the wacken which he describes, rests, it would seem, on an unconsolidated bed of clay; and it may be supposed, that a part of this bed has been converted into wacken by the heat of the incumbent mass, and has thus produced the apparent gradation from the one substance to the other. As the appearances of the rocks of Scheibenberg seem to be considered by Werner as furnishing a very strong, and even an unexpected confirmation of his system, I cannot help thinking, that an explanation of them, on the principles of Dr Hutton, without any straining or forcing of those principles, contributes not a little toward extending the empire of the latter over all the phenomena of geology.

253. Another fact, which has been much infilled on of late, in proof of the aqueous formation of basaltic rocks, is that shells are found in them. Of the reality of this fact, however, or at least of the instances hitherto produced, great doubts I think may be reasonably entertained. The specimens of the supposed basaltes, with shells included in them, that are chiefly relied on, are found at Portrush in Ireland, a rocky promontory to the westward of the Giant's Causeway, and separated from it by a considerable body of calcareous strata. Some of these specimens were brought to Edinburgh about a year ago, and were supposed, I believe, to contain an irrefragable proof of the Neptunian origin of the basaltic promontory where they were found. I went to see these specimens in company with Lord Webb Seymour and Sir James Hall; and, on examining them carefully, we were all of opinion, that the stones which contained the shells, or the impressions of the shells, were no part of the real basaltes. They were all very compact, and had all more or less of a siliceous appearance, such as that of chert; they had nothing of a sparry or crystallized structure; their fracture was conchoidal, and but slightly uneven. In two of them, one of which bore the impression of a cornu ammonis, the schistose texture might be distinctly perceived. A specimen which accompanied them, but in which there was no shell, served very exactly to explain the relation between these stones and the true basaltes. Part of this specimen was a true basalt, and the rest a sort of hornstone, exactly the same with that in which the shells were, and not unlike the jasper that is under the whinstone of Salisbury Crag, and in contact with it; so that on the whole it was evident, that the rock containing the shells is the schistus or stratified stone, which serves as the base of the basaltes, and which has acquired a high degree of induration, by the vicinity of the great ignited mass of whinstone.

This solution of the difficulty has since been confirmed by observations made on the spot by Dr Hope, who discovered two or three alternations of the basaltic rock, with the beds of the schistus in which the shells are contained.

254. This also explains some observations of Spallanzani, made in the island of Cerigo, on the coast of Greece, the Cythæra of the ancients.[135] The base of that island is limestone; but it abounds also in unstratified rocks, which the Italian naturalist supposes to be of volcanic origin; but which, if I mistake not, we would regard as whinstone, or perhaps porphyry; and they are said to contain oyster-shells and pectinites of a large size, perfectly mineralized. These petrifactions, however, Spallanzani says, are not contained in the lava that has actually flowed, but in stones which have only endured a slighter action of fire. Without the commentary afforded by the Portrush specimens, it would be difficult to make out any thing very precise from this description. By help of the information derived from those specimens, we may conclude, that the condition of the shells in them, and in the rocks of Cerigo, is perfectly alike; and that, in both cases, the shells are involved in parts of the rock which are truly stratified, but which have been, in some degree, assimilated to the basaltes by the heat which they have endured. Spallanzani would probably have used exactly the same terms which he employs in speaking of Cerigo, if he had been required to describe the petrified shells at Portrush.

[135] Journal de physique, tom. xlviii. (1798,) p. 278.

255. In the instances just mentioned, the petrified marine objects are not found in the real whinstone; but if they were found in it, when it borders on stratified rocks containing such objects, the thing would not be at all surprising, nor furnish any argument against the igneous consolidation of the stone. If a torrent of melted matter was poured in among the strata, by a force which at the same time broke up and disordered those strata, nothing could be more natural, than that this matter should contain fragments of them, and of the objects peculiar to them.

In one instance, mentioned by Mr Strange, this seems actually to have taken place. In the Veronese, a country remarkable for a mixture of limestone strata, containing marine objects, with volcanic or basaltine hills, he assures us, that he had seen a mass of stone, which had evidently concreted from fusion, in which the marine fossil bodies, originally, as he supposes, contained in the strata, were perfectly distinguishable, though variously disfigured.[136] It may be, that in this, as in the foregoing examples, it was not real basaltes, or real lava, which contained the shells, but the conterminal rock; but, supposing it to be as Mr Strange represents it, there appears to be no inconsistency between the phenomenon, and the igneous origin of the rock in which the shells were included. Here, however, it should be remarked, that the presence of great pressure, to prevent the conversion of the shells into quicklime, seems absolutely necessary; and that the phenomenon of these basaltic petrifactions, requires the application of heat to have been deep under the surface of the earth.

[136] Phil. Trans. 1775, p. 25.

256. The phenomena we have been considering, have been selected as the most unfavourable to the igneous origin of basaltic rocks; and we have seen, that when duly examined, they are not at all inconsistent with it. We are now to take a view of some appearances, that seem quite irreconcilable with the aqueous formation of these rocks.

Where whinstone rocks are found in masses, bounded by the strata, and insulated among them, they subject the Neptunian system to great difficulties. For, supposing it true that this stone may be produced by the precipitation and crystallization of mineral substances dissolved in water, yet it seems unaccountable, that this effect has been so local and limited in extent, as often to be confined to an irregular figure of a few acres, while, all round, the substances deposited have had no tendency to crystallization, and have been formed into the common secondary strata. The rock of Salisbury Craig, for instance, is a mass of whinstone, having a perpendicular face eighty or ninety feet high toward the west, and extending from north to south with a circular sweep about 900 yards. The whole of this rock rests on regular beds of secondary sandstone, not horizontal, but considerably depressed toward the north-east: the rock is loftiest in the middle, and decreases in thickness toward each end, terminating at its northern extremity in a kind of wedge. It is covered at top, toward that extremity, with regular beds of sandstone, perfectly similar to those on which it is incumbent; and it is not improbable, that this covering formerly extended over the whole.

Now, what cause can have determined the column of water, which rested on the base at present occupied by this rock, to deposit nothing but the materials of whinstone, while the water on the south, west, and north, was depositing the materials of arenaceous and marly strata? Wherefore, within this small space, was the precipitate every where chemical, to use the language of Werner, while close to it, on either side, it was entirely mechanical? Why is there, in this case, no gradation? and why is a mere mathematical line the boundary between regions where such different laws have prevailed? Whence also, we may ask, has the basaltic deposit been abruptly terminated toward the west, so as to produce the steep face which has just been mentioned? The operation of currents, or of any motion that can take place in a fluid, will furnish no explanation whatever of these phenomena; yet they are phenomena far from being peculiar to a single hill; they are among the most general and characteristic appearances in the natural history of whinstone mountains; and a geological theory which does not account for them, is hardly entitled to any consideration.

257. The basaltic rock, just described, is also covered, at least partly, with strata perfectly similar to those that lie under it. Now, it appears altogether unaccountable, that after the water had done depositing the materials of the whin on the spot in question, the former order was so quickly resumed, and a deposition of sand, and of the other materials of the strata, took place just as before. All this is quite unintelligible; and the principles of the Neptunian system seem here to stand as much in need of explanation, as any of the appearances which they are intended to account for.

258. The unequal thickness, and great irregularity in the surface of the whinstone mass, here treated of, and of many rocks of the same kind, is also a great objection to the notion of their aqueous formation. This seems to have been perceived by Werner, in the instance of the rocks formerly mentioned; and he endeavours to explain it, by supposing, that much of these rocks has been destroyed by waste and decomposition, so that an irregularity of their surface, and want of correspondence has been given to them, which they did not originally possess. In the instance of Salisbury Craig, however, we have a proof, that the great irregularity of surface, and the inequality of thickness, do not always arise from these causes. The thinnest part of that rock, toward its northern extremity, is still covered by the strata in their natural place, and has been perfectly defended by them from every sort of wearing and decay. The cuneiform shape, therefore, which this rock takes at its extremities, and the great difference of its thickness at them and in the middle, is a part of its original constitution, and can be attributed to nothing casual, or subsequent to its consolidation.

The same may be said of many other basaltic rocks, where an inequality of thickness, most unlike to what belongs to aqueous deposits, is known to exist in beds of whinstone that are still deep under the surface. Thus the toadstone of Derbyshire, even where it has a thick covering of strata over it, has been found, by the sinking of perpendicular shafts, to vary from the thickness of eighteen yards to more than sixty, within the horizontal distance of less than a furlong. Nothing of this kind is ever found to take place in those beds of rock which are certainly known to originate from aqueous deposition, and no character can more strongly mark an essential difference of formation.

259. We have had frequent occasion to consider the characters of those masses of whinstone which are so often found interposed between stratified rocks. These have been found in general very adverse to the Neptunian system; and two of them which yet remain to be mentioned, are even more so than any of the rest.

Where a bed or tabular mass of whinstone is interposed between strata, and wherever an opportunity offers of seeing its termination, if the strata under it are not broken, it may be remarked, that they do not abut themselves bluff and abrupt against the whin. On the contrary, if we mark the course of the stratum which covers the whinstone, and of that which is the base of it, we shall find they converge toward one another, the interposed mass growing thinner and thinner, like a wedge. When the latter terminates, the two former come in contact, and have no stratum interposed between them. Thus the roof and base of the whinstone rock are contiguous beds, that appear as if they had been lifted up and bent, and separated by an interposed mass. Had the whole been an effect of simultaneous deposition, the regular strata must have been abruptly terminated by the whin, like two courses of different forts of masonry where they meet with one another.

260. From this wedge-form of the whinstone masses, and in general from the irregularity of their surfaces, another conclusion follows, similar to the preceding, and one which has been already mentioned. Where the surface of the interposed mass is greatly inclined to the horizon, the strata which rest on this inclined plane, are nevertheless as exactly parallel to that plane, and to one another, as if they were really horizontal. It is certain, therefore, that they were not deposited on the same inclined plane on which they now rest; for, if so, they would have been still nearly horizontal, and by no means parallel to the inclined side of the whinstone. This follows from the nature of aqueous deposition, as already explained.

We have a remarkable instance of the phenomenon here referred to, in the rock of Salisbury Craig, of which mention has been so often made, and in which almost every circumstance is united, that can serve to elucidate the natural history of basaltic rocks. The north end of that rock is in the figure of a wedge, with its inclined side considerably steep, and covered by strata of grit, perfectly regular, and parallel to the surface on which they lie. The inspection of them will convince any one, that they were not deposited by the water, on a bottom so highly inclined as that on which they now rest. They are of a structure very schistose; their layers very thin; so that any inaccuracy of their parallelism would be readily observed. The appearances of the horizontal deposition of these strata, are indeed so clear, and so impossible to be misunderstood, that the followers of the Huttonian system would not risk much, if they were to leave the whole theory of whinstone to the decision of this single fact, and should agree to abandon that theory altogether, if the Neptunists can shew any physical or statical principle, on which the deposition now described can possibly have been made; or will point out the rule, by which nature has given a structure so nicely stratified to arenaceous beds deposited on a surface so highly inclined. If no such principle can be pointed out, though we cannot conclude that the Huttonian Theory is true, we certainly may conclude that the Neptunian is false.

261. Proofs of the igneous formation of whinstone, still more direct, are derived from the induration of the contiguous strata; from their disturbance when interfered by veins of whinstone; and from the charring of the coal which happens to be in contact with these veins. These are considered above at § [66], [67], &c.; and it is particularly taken notice of at [§ 66], that pieces of sandstone are sometimes found as if floating in the whinstone, and, at the same time, greatly altered in their texture. One of the best and most unequivocal instances of this sort which I have seen, is to be found on the south side of Arthur's Seat, near Edinburgh. The rock which composes the upper part of the hill, on that side, is a whinstone breccia, such as we have many examples of, and, I believe, very much resembling what is called a lava brecciata by the volcanic geologists. The stony fragments included in this compound mass, are for the greater part rounded; and some of them are of whinstone, others of porphyry, strongly characterized by rectangular maculæ of feldspar, and many seem to be of sandstone, but so considerably altered, as to leave it at least disputable whether they really are so or not. In one part, however, where the face of the rock is nearly perpendicular, a narrow ridge is seen standing out from the rest, and of a different colour, being more entirely covered with moss than the rock round about it, and, as may be presumed from that circumstance, less liable to decomposition. On examination I found, that this ridge does not consist of whinstone, but of a very hard and highly consolidated sandstone. It appears to be the edge of a stratum, of the thickness of about nine or ten inches, and of the height of fifteen or sixteen feet. It is not perfectly straight, but slightly waved, its general direction being nearly vertical; and it is on both sides firmly embraced by the whinstone. When broken, it appears that this sandstone resembles in colour, and in every thing but its greater consolidation, and more vitreous structure, the common grit found at the bottom of the hill, and over all the adjacent plain.

262. If all these circumstances are put together, there appears but one conclusion that can be drawn from them. We have here the manifest marks of some power which could lift up this fragment of rock from its native place, distant at least several hundred yards from its present situation, place it upright on its edge, encompass it with a solid rock, of a nature quite heterogeneous to itself, and bestow on it, at the same time, a great addition of solidity and induration. If the mass in which this stone is now imbedded, be supposed to have been once in fusion, and forcibly thrown up from below, invading the strata, and carrying the fragments along with it, the whole phenomena now described admit of an explanation, and all the circumstances accord perfectly with one another; but, without this supposition, they are so many separate prodigies, which have no connection with one another, nor with any thing that is known. It is indeed impossible, that the effects of motion and heat can be more clearly expressed than they are here, or the subject in which these powers resided more distinctly pointed out.

263. The preceding facts being susceptible but of one interpretation, are on that account extremely valuable. The phenomena of Salisbury Craig, near the same place, are almost equally free from ambiguity. The basaltic rock which forms that precipice, rests on arenaceous or marly strata; and these, in their immediate contact with the former, afford an instance of what is mentioned [§ 67], namely, the conversion of the strata in such situations into a kind of petrosilex, or even jasper. The line which separates the one rock from the other, is, at the same time, so well defined, as, in the eyes even of the most determined Neptunist, to exclude all idea of insensible gradation.

264. The same rock affords some remarkable instances of the disturbance of the strata contiguous to the whinstone. The beds of the former are bent upwards in several places; and, at one in particular, form an arch, with its convexity downward, so as to make it evident, that the force which produced this bending was directed from below upwards.

265. It is, however, where whinstone takes the form of veins, intersecting the strata, that the induration of the latter is most conspicuous. The coast of Ayrshire, and the opposite coast of Arran, exhibit these veins in astonishing variety and abundance. The strata are, in many instances, so reticulated by the veins, and intersected at such small distances, that it seems necessary to suppose, that the fissures in them were hardly sooner made than filled up. This at least is true, if the veins are to be accounted all of the same formation; and, in the greatest number of instances by far, there is no mark of the one being posterior to the other.

266. The induration of the sides of these veins, in some cases, has been such, that the sides have become more durable than the vein itself; so that the whinstone has been worn away by the washing of the waves, and has left the sides standing up, with an empty space, like a ditch, between them. One of these I remarked on the south side of Brodick Bay, in Arran, which, where it met the face of an abrupt cliff was not less than forty or fifty feet in depth.

267. I shall pass over whatever argument might be drawn in favour of our system, from the slender ramifications of the veins, and the varieties of their sizes, from a few inches to many fathoms in diameter, and also from the connection which they often appear to have with the great tabular masses of basaltes; and shall only add a few remarks on the charring of coal in the vicinity of veins or masses of whinstone. The connection between the charring of coal and the presence of whinstone, was first observed by Dr Hutton; and, as far as opportunities of verifying the observation have yet occurred, appears to be a fact no less general than it is curious and interesting. In the coal mines of Scotland, it certainly holds remarkably, particularly in those about Saltcoats in Ayrshire, where a whinstone dike is known to stretch across the whole of the coal country, and to be every where accompanied with blind or uninflammable coal. At Newcastle, dikes of the same kind are met with, and one, in particular, in what is called the Walker Colliery, has proved the action of subterraneous fire, to the satisfaction of mineralogists nowise prejudiced in favour of the Huttonian system.

The coal found under basaltes, in the Island of Sky, has been already mentioned, [§ 140]. To what was said concerning the fibrous structure of the parts of that fossil in immediate contact with the whin, it may be added, that it is also charred in those parts, so as to have hardly any flame when it is burnt, though further down it is of the nature of ordinary coal. Indeed, if there be any truth in Mr Kirwan's general remark, that it is common to find wood coal under basaltes, it must be understood to arise from this, that the coal in contact with the basaltes is frequently charred, and its fibrous structure, by that means, rendered more visible.

268. It has been objected to the supposition of coal having its bituminous part driven off by the heat of the whinstone, that this ought not, on Dr Hutton's principles, to happen in the mineral regions. But it may be replied, as has been done above, that the local application of heat might certainly produce this effect, and might drive off the volatile ports from a hotter to a colder part of the same stratum. The bitumen has not been so volatilized and expanded as entirely to escape from the mineral regions; but it has been expelled from some parts of a mass, only to be condensed and concentrated in others. This supposition coincides exactly with the appearances.

269. The native or fossil coke which accompanies whinstone, has been distinguished into two varieties. The first is the most common, in which, though the coal is perfectly charred, it is solid, and breaks with a smooth and shining surface. The second is also perfect charcoal, but is very porous and spongy. This substance is much rarer than the other. Dr Hutton mentions an instance of it at the mouth of the river Ayr, where there is a whinstone dike.[137] I had the satisfaction of visiting it along with him. It was in the bed of the river, below the high water mark; the specimens had the exact appearance of a cinder.

[137] Theory of the Earth, vol. i. p. 611.

In the banks of the same river, some miles higher up, he found a piece of coal, belonging to a regular stratum, involved in whinstone, and extremely incombustible. It consumed very slowly in the fire, and deflagrated with nitre like plumbago. This be considered as the same fossil which has been described under the name of plombagine. Near it, and connected with the same vein of whinstone, was a real and undoubted plumbago.

From these circumstances he also concluded, that plumbago is the extreme of a gradation, of which fossil-coal is the beginning, and is nothing else than this last reduced to perfect charcoal This agrees with the chemical analysis, which shows plumbago to be composed of carbon, combined with iron.

In confirmation of this theory, he mentions a specimen, in his possession, of steatical whinstone, from Cumberland, containing nodules of a very perfect and beautiful plumbago; and he also takes notice of a mine of this last, in Ayrshire, which, on the authority of Dr Kennedy, who has examined it with great care, I can state as being contained, or enveloped in whinstone; and I hope the public will soon be favoured with a particular description of this very interesting spot, by the same ingenious and accurate observer.

270. Thus the mineralogical and chemical discoveries agree in representing coal, blind coal, plombagine, plumbago, as all modifications of the same substance, and as exhibiting the same principle, carbon, in a state of greater or less combination. As the last and highest term of this series should be placed the diamond; but we are yet unacquainted with the matrix of this curious fossil, and its geological relation to other minerals. When known, they will probably give to this substance the same place in the geological, as in the chemical arrangement: in the mean time, it is hardly necessary to remark, how well all the preceding facts agree with the hypothesis of the igneous formation of whinstone, and how anomalous and unconnected they appear, according to every other theory.

271. Notwithstanding all this accumulated and unanswerable evidence for the igneous formation of basaltes, a great objection would still remain to our theory, were it not for the very accurate and conclusive experiments concerning the fusion of this fossil, referred to above, § 75. A strong prejudice against the production of any thing like a real stone by means of fusion, had arisen, even among those mineralogists, who were every day witnesses of the stony appearance assumed by volcanic lava. They still maintained, on the authority of their own imperfect experiments, that nothing but glass can ever be obtained by the melting of earths or of stones, in whatever manner they are combined.

An ingenious naturalist, after describing a block of basaltes, in which he discovered such appearances, as inclined him to admit its igneous consolidation, rejects that hypothesis, merely from the imaginary inability of fire to give to any substance a stony character: "Quelque mélange,"says he, "de terres que l'on suppose, quelque soit le degré de feu que l'on imagine, quelque soit le tems que l'on emploie, il est très certain que l'on n'obtiendra pas, par le seul fluide igné, ni basalte, ni rien qui lui ressemble."[138]

[138] Journal de Phys. tom. xlix. (1799,) p. 36.

Sir James Hall's experiments have completely demonstrated the contrary of what is here asserted: they have added much to the evidence of the Huttonian system; and, independently of all theory, have narrowed the circle of prejudice and error.