Illustrations of the Huttonian Theory of the Earth

by John Playfair

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.