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. 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.