25. In accounting, by the operation of heat, for these appearances of fluidity, Dr Hutton has proceeded on the principle already laid down, as conformable to analogy, that calcareous earth, under great compression, may have its fixed air retained in it, notwithstanding the action of intense heat, and may, by that means, be reduced into fusion, or into a state approaching to it. In all this I do not think that he has departed from the strictest rules of philosophical investigation. The facts just stated prove, that limestone was once soft, its fragments retaining at the same time their peculiar form, an effect to which we know of none similar but those of fire; and, therefore, though we could not conjecture how heat might be applied to limestone so as to melt it, instead of reducing it to a calx, we should, nevertheless, have been forced to suppose, that this had actually taken place in the bowels of the earth; and was a fact which, though we were not able to explain it, we were not entitled to deny. The principle just mentioned relieves us therefore from a difficulty, that would have embarrassed, but could not have overturned, this theory of the earth. 26. From the arguments which the argillaceous strata afford for the igneous consolidation of fossils, I shall select one on which Dr Hutton used to lay considerable stress, and which some of the adversaries of his system have endeavoured to refute. This argument is founded on the structure of certain ironstones called _septaria_, often met with among the argillaceous schistus, particularly in the vicinity of coal. These stones are usually of a lenticular or spheroidal form, and are divided in their interior into distinct _septa_, by veins of calcareous spar, of which one set are circular and concentric, the other rectilineal; diverging from the centre of the former, and diminishing in size as they recede from it. Now, what is chiefly to be remarked is, that these veins terminate before they reach the surface of the stone; so that the matter with which they are filled cannot have been introduced from without by infiltration, or in any other way whatsoever. The only other supposition, therefore, that is left for explaining the singular structure of this fossil, is, that the whole mass was originally fluid, and that, in cooling, the calcareous part separated from the rest, and afterwards crystallized.