Illustrations of the Huttonian Theory of the Earth

by John Playfair

Thus it appears, in general, that the same difficulties do not press against the two theories of aqueous and of igneous consolidation; and, that the latter employs an agent incomparably more powerful than the former, of more general activity, and, what is of infinite importance in a philosophical theory, vastly more definite in the laws of its operation.

18. A more particular examination of the different kinds of fossils will confirm this conclusion, and will show, that, wherever they bear marks of having been fluid, these marks are such as characterize the fluidity of fusion, and distinguish it from that which is produced by solution in a menstruum. Dr Hutton has enumerated many of these discovered in the course of that careful and accurate examination of fossils, in which he probably never was excelled by any mineralogist. It will be sufficient here to point out a few of the most remarkable examples.

19. Fossil wood, penetrated by siliceous matter, is a substance well known to mineralogists; it is found in great abundance in various situations, and frequently in the heart of great bodies of rock. On examination, the siliceous matter is often observed to have penetrated the wood very unequally, so that the vegetable structure remains in some places entire; and in other places is lost in a homogeneous mass of agate or jasper. Where this happens, it may be remarked, that the line which separates these two parts is quite sharp and distinct, altogether different from what must have taken place, had the flinty matter been introduced into the body of the wood, by any fluid in which it was dissolved, as it would then have pervaded the whole, if not uniformly, yet with a regular gradation. In those specimens of fossil wood that are partly penetrated by agate, and partly not penetrated at all, the same sharpness of termination may be remarked, and is an appearance highly characteristic of the fluidity produced by fusion.

20. The round nodules of flint that are found in chalk, quite insulated and separate from one another, afford an argument of the same kind; since the flinty matter, if it had been carried into the chalk by any solvent, must have been deposited with a certain degree of uniformity, and would not now appear collected into separate masses, without any trace of its existence in the intermediate parts. On the other hand, if we conceive the melted flint to have been forcibly injected among the chalk, and to have penetrated it, somewhat as mercury may, by pressure, be made to penetrate through the pores of wood, it might, on cooling, exhibit the same appearances that the chalk-beds of England do actually present us with.

The siliceous pudding-stone is an instance closely connected with the two last; in it we find both the pebbles, and the cement which unites them, consisting of flint equally hard and consolidated; and this circumstance, for which it is impossible to account by infiltration, or the insinuation of an aqueous solvent, is perfectly consistent with the supposition, that a stream of melted flint has been forcibly injected among a mass of loose gravel.

21. The common grit, or sandstone, though it certainly gives no indication of having possessed fluidity, is strongly expressive of the effects of heat. It is so, especially in those instances where the particles of quartzy sand, of which it is composed, are firmly and closely united, without the help of any cementing substance whatsoever. This appearance, which is very common, seems to be quite inconsistent with every idea of consolidation, except an incipient fusion, which, with the assistance of a suitable compression, has enabled the particles of quartz to unite into stone.

It has indeed been asserted, that the mere apposition of stony particles, so as to permit their corpuscular attraction to take place, was sufficient to form them into stone. To this Dr Hutton has very well replied, that, admitting the possibility of a hard and firm body being produced in this way of which, however, we have no proof, the close and compact texture, the perfect consolidation of the stones we are now speaking of, would still remain to be explained, and of this it is evident that the mere apposition of particles, and the force of their mutual attraction, can afford no solution.

22. These proofs that the strata must have endured the action of intense heat, though immediately deduced from those of the siliceous genus only, extend in reality to all the strata, of every kind, with which they are found alternated. It is impossible that heat, of the intensity here supposed, can have acted on a particular stratum, and not on those that are contiguous to it; and, as there are no strata of any kind with which the quartzy and siliceous are not intermixed, so there are none of which the igneous consolidation is not thus rendered probable. We need rest nothing, however, on this argument, as the fossils of every genus may be shown to speak distinctly for themselves.

23. Those of the calcareous genus do so perhaps more sparingly than the rest; yet even among them there are many facts, that, though taken unconnected with all others, are sufficient to establish the action of subterraneous fire. Such, for example, are the calcareous breccias, composed of fragments of marble or limestone, and not only adapted to each other's shape, but indented into one another, in a manner not a little resembling the sutures of the human cranium. From such instances, it is impossible not to infer the softness of the calcareous fragments when they were consolidated into one mass. Now, this softness could be induced only by heat; for it must be acknowledged, that the action of any other solvent is quite inadequate to the softening of large fragments of stone, without dissolving them altogether.

24. In many other instances it appears certain, that the stones of the calcareous genus have been reduced by heat into a state of fluidity much more perfect. Thus, the saline or finer kinds of marble, and many others that have a structure highly crystallized, must have been softened to a degree little short of fusion, before this crystallization could take place. Even the petrifactions which abound so much in limestones tend to establish the same fact; for they possess a sparry structure, and must have acquired that structure in their transition from a fluid to a solid state.[10]