These unstratified rocks are not, however, all the work of the same period; they differ evidently in the date of their formation, and it is not unusual, to find tabular masses of one species of whin, intersected by veins of another species. Indeed, of all the fossil bodies which compose the present land, the veins of whin appear to be the most recently consolidated.[25]
Porphyry may so properly be regarded as a variety of whin, distinguished only by involving crystallized feldspar, that, in a geological sketch like the present, it is hardly entitled to a separate article. Like the other kinds of whin, it exists both in veins and in tabular masses, having, no doubt, an origin similar to that which has just been described. Porphyry, however, has the peculiarity of being rarely found in any but the primary strata; it seems to be the whinstone of the old world, or at least that which is of the highest antiquity in the present. It no where, I believe, assumes a columnar, or basaltic appearance, of any regularity; but this is also true of many other varieties of whin, of all, indeed, except the most compact and homogeneous. These differences are not so considerable as to require our entering into any particular detail concerning the natural history of this fossil.
3. Granite.
77. The term Granite is used by Dr Hutton to signify an aggregate stone, in which quartz, feldspar, and mica are found distinct from one another, and not disposed in layers. The addition of hornblende, schorl, or garnet, to the three ingredients just mentioned, is not understood to alter the genus of the stone, but only to constitute a specific difference, which it is the business of lithology to mark by some appropriate character, annexed to the generic name of granite.
The fossil now defined exists, like whinstone and porphyry, both in masses and in veins, though most frequently in the former. It is like them unstratified in its texture, and is regarded here, as being also unstratified in its outward structure.[26] One ingredient which is essential to granite, namely, quartz, is not contained in whinstone; and this circumstance serves to distinguish these genera from one another, though, in other respects, they seem to be united by a chain of insensible gradations, from the most homogeneous basaltes, to granite the most highly crystallized.
[26] Those rocks that consist of the ingredients here enumerated, if they have at the same time a schistose texture, or a disposition into layers, are properly distinguished from granite, and called Gneiss, or Granitic Schistus. But it has been questioned whether a stone does not exist composed of these ingredients, and destitute of a schistose texture, but yet divided into large beds, visible in its external form. Dr Hutton supposes such a stone not to exist, or at least not to constitute any such proportion of the mineral kingdom, as to entitle it to particular consideration, in the general speculations of geology.
Whether this supposition is perfectly correct, may require to be farther considered: this, however, is certain, that a rock, in all respects conformable to it, composes a great proportion of what are usually called the granite mountains. See Note xv.
78. Granite, it has been just said, exists most commonly in masses; and these masses are rarely, if ever, incumbent on any other rock: they are the basis on which others rest, and seem, for the most part, to rise up from under the ancient, or primary strata. The granite, therefore, wherever it is found, is inferior to every other rock; and as it also composes many of the greatest mountains, it has the peculiarity of being elevated the highest into the atmosphere, and sunk the deepest under the surface, of all the mineral substances with which we are acquainted.
Notwithstanding the circumstance of not being alternated with stratified bodies, which constitutes a remarkable difference between granite and whinstone, the affinity of these fossils is such as to make the similarity of their origin by no means improbable. Accordingly, in Dr Hutton's theory, granite is regarded as a stone of more recent formation than the strata incumbent on it; as a substance which has been melted by heat, and which, when forced up from the mineral regions, has elevated the strata at the same time.