63. But, in order to see whether this hypothesis will explain the diversity of the two substances, without which it will not be entitled to much attention, we must remark, that the presence of carbonate of lime in a body that has been fused, argues, agreeably to the principles formerly explained, that the fusion was brought about under a great compressing force, that is to say, deep in the bowels of the earth, or in the great laboratory of the mineral regions. We are, therefore, to suppose that the fusion of the whin was performed in those regions, where the compression was sufficient to preserve the carbonic gas in union with the calcareous earth, so that these two substances melted together, and, on cooling, crystallized into spar. In the lavas, again, thrown out by volcanic eruption, the fusion, as we know, wherever it may begin, continues in the open air, where the pressure is only that of the atmosphere: the calcareous earth, which, therefore, may have been, in the form of a carbonate, among the materials of this lava, must be converted into quicklime, and become infusible; hence the want of calcareous spar in lavas that have flowed at the surface. Thus, whinstone is to be accounted a subterraneous, or _unerupted_ lava; and our theory has the advantage of explaining both the affinity and the difference between these stony bodies, without the introduction of any new hypothesis. In the Neptunian system, the affinity of whinstone and lava is a paradox which admits of no solution. 64. The columnar structure sometimes found in that species of whinstone called basaltes, is a fact which has given rise to much discussion; and it must be confessed, that though one of the most striking and peculiar characters of this fossil, it is not that which gives the clearest and most direct information concerning its origin. One circumstance, however, very much in favour of the opinion that basaltic rocks owe their formation to fire, is, that the columnar form is sometimes assumed by the lava actually erupted from volcanoes. Now, it is certainly of no small importance, to have the synthetic argument on our side, and to know, that basaltic columns can be produced by fire; though, no doubt, to give absolute certainty to our conclusion, it would be necessary to show, that there are in nature no other means but this by which these columns can be formed. This sort of evidence is hardly to be looked for; but since the power of fusion, to produce the phenomena in question, is perfectly established, and since the production of the same phenomena in the humid way is a mere hypothesis, if there be the least reason to suspect the action of subterraneous heat as one of the causes of mineralization, every maxim of sound philosophy requires that the basaltic structure, in all cases, should be ascribed to it.