Illustrations of the Huttonian Theory of the Earth

by John Playfair

65. The Neptunists will no doubt allege, with Bergman, that, in the drying of starch, clay, and a few other substances, something analogous to basaltic columns is produced. Here, however, a most important difference is to be remarked, corresponding very exactly to one of the characters which we have all along observed to distinguish the products of aqueous, from those of igneous consolidation. The columns formed by the substances just mentioned, are distant from one another: they are separated by fissures which widen from the bottom to the top, and which arise from the shrinking and drying of the mass. In the basaltic columns, no such openings, nor vacuity of any kind is found; the pillars are in contact, and, though perfectly distinct, are so close, that the sharp edge of a wedge can hardly be introduced between them. This is a great peculiarity in the basaltic structure, and is strongly expressive of this fact, that the mass was all fluid together, and that its parts took their new arrangement, not in consequence of the separation of a fluid from a solid part, by which great shrinking and much empty space might be produced; but in consequence of a cause which, like refrigeration, acted equally on all the parts of the mass, and preserved their absolute contact after their fluidity had ceased.

66. A mark of fusion, or at least of the operation of heat, which whinstone possesses in common with many other minerals, is its being penetrated by pyrites, a substance, as has been already remarked, that is of all others most exclusively the production of fire. Another mark of fusion, more distinctive of whin, is, that both in veins and in masses it sometimes includes pieces of sandstone, or of the other contiguous strata, completely insulated, and having the appearance of fragments of rock, floating in a fluid sufficiently dense and ponderous to sustain their weight. Though these fragments have been too refractory to be reduced into fusion themselves, they have not remained entirely unchanged, but are, in general, extremely indurated, in comparison of the rock from which they appear to have been detached.

67. Similar instances of extraordinary induration are observed in the parts of the strata in contact with whinstone, whether they form the sides of the veins, or the floors, and roofs of the masses into which the whinstone is distributed. The strata whether sandy or argillaceous, in such situations, are usually extremely hard and consolidated; the former in particular lose their granulated texture, and are sometimes converted into perfect jasper. This interesting remark was first made by Dr Hutton, and the truth of it has been verified by a great number of subsequent observations.

68. To the same excellent geologist we are indebted for the knowledge of an analogous fact, attendant on the passage of whinstone veins through coal strata. As the beds of stone where they are in contact with veins of whin, seem to acquire additional induration, so those of coal, in like circumstances, are frequently found to have lost their fusibility, and to be reduced nearly to the condition of coke, or of charcoal. The existence of coal of this kind has been already mentioned, and considered as a proof of the operation of subterraneous heat. In the instances here referred to, that is, where the charring of the coal is limited to those parts of the strata which are in contact with the whin, or in its immediate vicinity, the heat is pointed out as residing in the vein; and this is to be accounted for only on the supposition of the melted whin, at a period subsequent to the consolidation of the coal, having flowed through the openings of the strata. The heat has been powerful enough, in many cases, to drive off the bituminous matter of the coal, and to force it into colder and more distant parts. Few facts, in the history of fossils, are more remarkable than this, and none more directly assimilates the operations of the mineral regions, with those that take place at the surface of the earth.

69. Again, the disturbance of the strata, wherever veins of whinstone abound, if not a direct proof of the original fluidity of the whinstone, is a clear indication of the violence with which it was introduced into its place. This disturbance of the position of the strata, by shifting, unusual elevation, and other irregularities, where they are intersected by whinstone veins, is a fact so well known to miners, that when they meet with any sudden change in the lying of the metals, they are wont to foretell their approach to masses, or veins of unstratified matter; and, in their figurative language, point them out as the causes of the confusion with which they are so generally accompanied.[21] The mineral veins likewise, as well as the strata, are often heaved and shifted by the veins of whinstone.

[21] A Trouble is the name which the colliers in this country give to a vein of whinstone.

70. Whinstone of every species is found frequently interposed in tabular masses, between beds of stratified rocks; and it then adds to the indications of its igneous origin, already enumerated, some others that are peculiar to it when in this situation. In such instances, it is not uncommon to find the strata in some places, contiguous to the whin, elevated, and bent with their concavity upward, so that they appear clearly to have been acted on by a force that proceeded from below, at the same time that they were softened, and rendered in some degree flexible: it is needless to remark, that these effects can be explained by nothing but the fusion of the whin; and that the great force with which it was impelled against the strata, could be produced by no cause but heat, acting in the manner that is here supposed.

71. Again, if it be true that the masses of whin, thus interposed among the strata, were introduced there, after the formation of the latter, we might expect to find, at least in many instances, that the beds on which the whinstone rests, and those by which it is covered, are exactly alike. If these beds were once contiguous, and have been only heaved up and separated by the irruption of a fluid mass of subterraneous lava, their identity should still be recognised. Now, this is precisely what is observed; it is known to hold in a vast number of instances, and is strikingly exemplified in the rock of Salisbury Crag, near Edinburgh.

This similarity of the strata that cover the masses of whinstone, to those that serve as the base on which they rest, and again the dissimilitude of both to the interposed mass, are facts which I think can hardly receive any explanation, on the principles of the Neptunian theory. If these rocks, both stratified and unstratified, are to be regarded as productions of the sea, the circumstances would require to be pointed out, which have determined the whinstone, and the beds that are all round it, to be so extremely unlike in their structure, though formed at the same time, and in the immediate vicinity of one another; as also those circumstances, on the other hand, which determined the stratified deposits above and below the whinstone, to be precisely the same, though the times of their formation must have been very different. The homogeneous substances, thus, placed at a distance, and the heterogeneous brought so closely together, are phenomena equally unaccountable, in a theory that ascribes their origin to the operation of the same element, and that necessarily dates their formation according to the order in which they lie, one above another.

72. If, indeed, in these instances, the gradation were insensible, as some have asserted it to be, between the strata and the interposed mass, so that it was impossible to point out the line where the one ended and the other began, whatever difficulties we might perceive in the Neptunian theory, we should find it hard to substitute a better in its room. But the truth seems to be, that, in the cases we are now treating of, no such gradation exists; and that, though where the two kinds of rock come into contact a change is often observed, by the strata having acquired an additional degree of induration, yet the line of separation is well defined, and can be precisely ascertained. This at least is certain, that innumerable specimens, exhibiting such lines of separation, are to be met with; and wherever care has been taken to obtain a fresh fracture of the stone, and to remove the effects of accidental causes, even where the two rocks are most firmly united, and most closely assimilated, I am persuaded that no uncertainty has ever remained as to the line of their separation. For these reasons, it seems probable that the gradual transition of basaltes into the adjoining strata, is in all cases imaginary, and is, in truth, a mere illusion, proceeding from hasty and inaccurate observation.