[8] Homo naturæ minister, et interpres tantùm facit et intelligit, quantùm de naturæ ordine re, vel mente, observaverit: nec amplius scit, aut potest.—Nov. Org. lib. i. aph. 1.
14. Such are the more obvious, but I think unanswerable objections, that may be urged against the aqueous consolidation of the strata. It is true, that stony concretions, some of them much indurated, are formed in the humid way under our eyes. Very particular conditions, however, are required for that purpose, and conditions such as can hardly have existed at the bottom of the sea. First. The water must dissolve the substance of which the concretion is to be formed, as it actually does in the case of calcareous, and in certain circumstances, in that of siliceous, earth. Secondly. It must be separated from that substance, as by evaporation, or by a combination of the matter dissolved with some third substance, to which it has a greater affinity than to water, so as to form with it an insoluble compound. Lastly, the water that is deprived of its solution must be carried off, and more of that which contains the solution must be supplied, as sometimes happens where water runs in a stream, or drops from the roof of a cavern. The two last conditions are peculiarly inapplicable to the bottom of the sea, where the state of the surrounding fluid would neither permit the water that was deprived of its solution from being drawn off, nor that which contained the solution from succeeding it.
It is further to be observed, that the consolidation of stalactitical concretions, that is, the filling up of their pores, is always imperfect, and is brought about by the repeated action of the fluid running through the porous mass, and continuing to deposit there some of the matter it holds in solution. This, which is properly infiltration, is incompatible with the nature of a fluid, either nearly, or altogether quiescent.
15. In order to judge whether objections of equal weight can be opposed to the hypothesis of igneous consolidation, we must attend to a very important remark, first made by Dr Hutton, and applied with wonderful success to explain the most mysterious phenomena of the mineral kingdom.
It is certain, that the effects of fire on bodies vary with the circumstances under which it is applied to them, and, therefore, a considerable allowance must be made, if we would compare the operation of that element when it consolidated the strata, with the results of our daily experience. The materials of the strata were disposed, as we have already seen, loose and unconnected, at the bottom of the sea; that is, even on the most moderate estimation, at the depth of several miles under its surface. At this depth, and under the pressure of a column of water of so great a height, the action of heat would differ much from that which we observe here upon the surface; and, though our experience does not enable us to compute with accuracy the amount of this difference, it nevertheless points out the direction in which it must lie, and even marks certain limits to which it would probably extend.
The tendency of an increased pressure on the bodies to which heat is applied, is to restrain the volatility of those parts which otherwise would make their escape, and to force them to endure a more intense action of heat. At a certain depth under the surface of the sea, the power even of a very intense heat might therefore be unable to drive off the oily or bituminous parts from the inflammable matter there deposited, so that, when the heat was withdrawn, these principles might be found still united to the earthy and carbonic parts, forming a substance very unlike the residuum obtained after combustion under a pressure no greater than the weight of the atmosphere. It is in like manner reasonable to believe, that, on the application of heat to calcareous bodies under great compression, the carbonic gas would be forced to remain; the generation of quicklime would be prevented, and the whole might be softened, or even completely melted; which last effect, though not directly deducible from any experiment yet made, is rendered very probable, from the analogy of certain chemical phenomena.
16. An analogy of this kind, derived from a property of the barytic earth, was suggested by that excellent chemist and philosopher, the late Dr Black. The barytic earth, as is well known, has a stronger attraction for fixed air than common calcareous earth has, so that the carbonate of barytes is able to endure a great degree of heat before its fixed air is expelled. Accordingly, when exposed to an increasing heat, at a certain temperature, it is brought into fusion, the fixed air still remaining united to it: if the heat be further increased, the air is driven off, the earth loses its fluidity, and appears in a caustic state. Here, it is plain, that the barytic earth, which is infusible, or very refractory, per se, as well as the calcareous, owes its fusibility to the presence of the fixed air; and it is therefore probable, that the same thing would happen to the calcareous earth, if by any means the fixed air were prevented from escaping when great heat is applied to it. This escape of the fixed air is exactly what the compression in the subterraneous regions is calculated to prevent, and therefore we are not to wonder if, among the calcareous strata, we find marks of actual fusion having taken place.[9]
17. These effects of pressure to resist the decomposition, and augment the fusibility of bodies, being once supposed, we shall find little difficulty in conceiving the consolidation of the strata by heat, since the intervals between the loose materials of which they originally consisted may have been closed, either by the softening of those materials, or by the introduction of foreign matter among them, in the state of a fluid, or of an elastic vapour. No objection to this hypothesis can arise from the considerations stated in the preceding case; the solvent here employed would want no pores to lodge in after its work was completed, nor would it find any difficulty in making its retreat through the densest and most solid substances in the mineral kingdom. Neither can its incapacity to dissolve the bodies submitted to its action be alleged. Heat is the most powerful and most general of all solvents; and, though some bodies, such as the calcareous, are able to resist its force on the surface of the earth, yet, as has just been shown, it is perfectly agreeable to analogy to suppose, that, under great pressure, the carbonic state of the lime being preserved, the purest limestone or marble might be softened, or even melted. With respect to other substances, less doubt of their fusibility is entertained; and though, in our experiments, the refractory nature of siliceous earth has not been completely subdued, a degree of softness and an incipient fusion have nevertheless been induced.