Illustrations of the Huttonian Theory of the Earth

by John Playfair

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]