27. It has been urged against this theory of the septaria, that these stones are sometimes found with the calcareous veins extending all the way to the circumference, and of course communicating with the outside. But it must be observed, that this fact does not affect the argument drawn from specimens in which no such communication takes place. It is at best only an ambiguous instance, that may be explained by two opposite theories, and may be reconciled either to the notion of igneous or of aqueous consolidation: but if there is a single close septarium in nature, it can, of course, be explained only by one of these theories, and the other must, of necessity, be rejected. Besides, it is plain, that a close septarium can never have been open, though an open septarium may very well have been close; and indeed, as this stone is, in certain circumstances, subject to perpetual exfoliation, it would be wonderful if no one was ever found with the calcareous veins reaching to the surface. With regard to the light, therefore, that they give into their own history, these two kinds of septaria are by no means on an equal footing; and this may serve to show, how necessary it is, in all inductive reasoning, and particularly in a subject so complex as geology, to separate with care such phenomena as admit of two solutions, from such as admit only of one. 28. The bituminous strata come next to be considered; and they are of great consequence in the present argument, because their dissimilarity in so many particulars to all other mineral substances, renders them what Lord Bacon calls an _instantia singularis_, having the first rank among facts subservient to inductive investigation. But though unlike in substance to other fossils, and composed, as has been shown, of materials that belonged not originally to the mineral kingdom, they agree in many material circumstances with the strata already enumerated. Their beds are disposed in the same manner, and are alternated indiscriminately with those of all the secondary rocks, and, being formed in the same region, must have been subject to the same accidents, and have endured the operation of the same causes. They are traversed too like the other strata, by veins of the metals, of spars, of basaltes, and of other substances; and, whatever argument may hereafter be derived from this to prove the action of fire on the strata so traversed, is as much applicable to coal as to any other mineral. The coal strata also contain pyrites in great abundance, a substance that is perhaps, more than any other, the decided progeny of fire. This compound of metal and sulphur, which is found in mineral bodies of every kind, I believe, without any exception, is destroyed by the contact of moisture, and resolved into a vitriolic salt. At the same time it is found in the strata, not traversing them in veins, which may be supposed of more recent formation than the strata