79. That granite has undergone a change from a fluid to a solid state, is evinced from the crystallized structure in which some of its component parts are usually found. This crystallization is particularly to be remarked of the feldspar, and also of the schorl, where there is any admixture of that substance, whether in slender spiculæ, or in larger masses. The quartz itself is in some cases crystallized, and is so, perhaps, more frequently than is generally supposed. The fluidity of granite, in some former period of its existence, is so evident from this, as to make it appear singular that it should ever have been considered as a fossil that had remained always the same, and one, into the origin of which it was needless to inquire. If the regular forms of crystallization are not to be received as proofs of the substance to which they belong having passed from a fluid to a solid state, neither are the figures of shells and of other supposed petrifactions, to be taken as indications of a passage from the animal to the mineral kingdom; so that there is an end of all geological theories, and of all reasonings concerning the ancient condition of the globe. To an argument which strikes equally at the root of all theories, it belongs not to this, in particular, to make any reply.
80. We shall, therefore, consider it as admitted, that the materials of the granite were originally fluid; and, in addition to this, we think it can easily be proved, that this fluidity was not that of the elements taken separately, but of the entire mass. This last conclusion follows, from the structure of those specimens, where one of the substances is impressed by the forms which are peculiar to another. Thus, in the Portsoy granite,[27] which Dr Hutton has so minutely described, the quartz is impressed by the rhomboidal crystals of the feldspar, and the stone thus formed is compact and highly consolidated. Hence, this granite is not a congeries of parts, which, after being separately formed, were somehow brought together and agglutinated; but it is certain that the quartz, at least, was fluid when it was moulded on the feldspar. In other granites, the impressions of the substances on one another are observed in a different order, and the quartz gives its form to the feldspar. This, however, is more unusual; the quartz is commonly the substance which has received the impressions of all the rest; and the spiculæ of schorl often shoot both across it and the feldspar.
[27] Theory of the Earth, vol. i. p. 104.
The ingredients of granite were therefore fluid when mixed, or at least when in contact with one another. Now, this fluidity was not the effect of solution in a menstruum; for, in that case, one kind of crystal ought not to impress another, but each of them should have its own peculiar shape.
81. The perfect consolidation of many granites, furnishes an argument to the same effect. For, agreeably to what was already observed, in treating of the strata, a substance, when crystallizing, or passing from a fluid to a solid state, cannot be free from porosity, much less fill up completely a space of a given form, if, at the same time, any solvent is separated from it; because the solvent so separated would still occupy a certain space, and, when removed by evaporation or otherwise, would leave that space empty. The perfect adjustment, therefore, of the shape of one set of crystallizing bodies, to the shape of another set, as in the Portsoy granite, and their consolidation into one mass, is as strong a proof as could be desired, that they crystallized from a state of simple fluidity, such as, of all known causes, heat alone is able to produce.
82. This conclusion, however, does not rest on a single class of facts. It has been observed in many instances, that where granite and stratified rocks, such as primary schistus, are in contact, the latter are penetrated by veins of the former, which traverse them in various directions. These veins are of different dimensions, some being of the breadth of several yards, others of a few inches, or even tenths of an inch; they diminish as they recede from the main body of the granite, to which they are always firmly united, constituting, indeed, a part of the same continued rock.
These phenomena, which were first distinctly observed by Dr Hutton, are of great importance in geology, and afford a clear solution of the two chief questions concerning the relation between granite and schistus. As every vein must be of a date posterior to the body in which it is contained, it follows, that the schistus was not super-imposed on the granite, after the formation of this last. If it be argued, that these veins, though posterior to the schisti, are also posterior to the granite, and were formed by the infiltration of water in which the granite was dissolved or suspended; it may be replied, 1mo, That the power of water to dissolve granite, is a postulatum of the same kind that we have so often, and for such good reason, refused to concede; and, 2do, That in many instances the veins proceed from the main body of the granite upwards into the schistus; so that they are in planes much elevated in respect of the horizon, and have a direction quite opposite to that which the hypothesis of infiltration requires. It remains certain, therefore, that the whole mass of granite, and the veins proceeding from it, are coeval, and both of later formation than the strata.
Now, this being established, and the fluidity of the veins, when they penetrated into the schistus, being obvious, it necessarily follows, that the whole granite mass was also fluid at the same time. But this can have been brought about only by subterraneous heat, which also impelled the melted matter against the superincumbent strata, with such force as to raise them from their place, and to give them that highly inclined position in which they are still supported by the granite, after its fluidity has ceased. Thus a conclusion, rendered probable by the crystallization of granite, is established beyond all contradiction by the phenomena of granitic veins.[28]
83. With the granite, we shall consider the proof of the igneous origin of all mineral substances as completed. These substances, therefore, whether stratified or unstratified, owe their consolidation to the same cause, though acting with different degrees of energy. The stratified have been in general only softened or penetrated by melted matter, whereas the unstratified have been reduced into perfect fusion.