[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.