[12] Quart. Geol. Journ., vol. ix, p. 170, 1852.

[13] Logan, Geology of Canada, 1863.

CHAPTER XXXI.
PLUTONIC ROCKS.

General Aspect of Plutonic Rocks. — Granite and its Varieties. — Decomposing into Spherical Masses. — Rude columnar Structure. — Graphic Granite. — Mutual Penetration of Crystals of Quartz and Feldspar. — Glass Cavities in Quartz of Granite. — Porphyritic, talcose, and syenitic Granite. — Schorlrock and Eurite. — Syenite. — Connection of the Granites and Syenites with the Volcanic Rocks. — Analogy in Composition of Trachyte and Granite. — Granite Veins in Glen Tilt, Cape of Good Hope, and Cornwall. — Metalliferous Veins in Strata near their Junction with Granite. — Quartz Veins. — Exposure of Plutonic Rocks at the surface due to Denudation.

The Plutonic rocks may be treated of next in order, as they are most nearly allied to the volcanic class already considered. I have described, in the first chapter, these Plutonic rocks as the unstratified division of the crystalline or hypogene formations, and have stated that they differ from the volcanic rocks, not only by their more crystalline texture, but also by the absence of tuffs and breccias, which are the products of eruptions at the earth’s surface, whether thrown up into the air or the sea. They differ also by the absence of pores or cellular cavities, to which the expansion of the entangled gases gives rise in ordinary lava, never being scoriaceous or amygdaloidal, and never forming a porphyry with an uncrystalline base, nor alternating with tuffs.

From these and other peculiarities it has been inferred that the granites have been formed at considerable depths in the earth, and have cooled and crystallised slowly under great pressure, where the contained gases could not expand. The volcanic rocks, on the contrary, although they also have risen up from below, have cooled from a melted state more rapidly upon or near the surface. From this hypothesis of the great depth at which the granites originated, has been derived the name of “Plutonic rocks.” The beginner will easily conceive that the influence of subterranean heat may extend downward from the crater of every active volcano to a great depth below, perhaps several miles or leagues, and the effects which are produced deep in the bowels of the earth may, or rather must, be distinct; so that volcanic and Plutonic rocks, each different in texture, and sometimes even in composition, may originate simultaneously, the one at the surface, the other far beneath it. The Plutonic formations also agree with the volcanic in having veins or ramifications proceeding from central masses into the adjoining rocks, and causing alterations in these last, which will be presently described. They also resemble trap in containing no organic remains; but they differ in being more uniform in texture, whole mountain masses of indefinite extent appearing to have originated under conditions precisely similar.

The two principal members of the Plutonic family of rocks are Granite and Syenite, each of which, with their varieties, bear very much the same relation to each other as the trachytes bear to the basalts. Granite is a compound of feldspar, quartz, and mica, the feldspars being rich in silica, which forms from 60 to 70 per cent of the whole aggregate. In Syenite quartz is rare or wanting, hornblende taking the place of mica, and the proportion of silica not exceeding 50 to 60 per cent.

Granite and its Varieties.—Granite often preserves a very uniform character throughout a wide range of territory, forming hills of a peculiar rounded form, usually clad with a scanty vegetation. The surface of the rock is for the most part in a crumbling state, and the hills are often surmounted by piles of stones like the remains of a stratified mass, as in Figure 605, and sometimes like heaps of boulders, for which they have been mistaken. The exterior of these stones, originally quadrangular, acquires a rounded form by the action of air and water, for the edges and angles waste away more rapidly than the sides. A similar spherical structure has already been described as characteristic of basalt and other volcanic formations, and it must be referred to analogous causes, as yet but imperfectly understood. Although it is the general peculiarity of granite to assume no definite shapes, it is nevertheless occasionally subdivided by fissures, so as to assume a cuboidal, and even a columnar, structure. Examples of these appearances may be seen near the Land’s End, in Cornwall. (See Fig. 606.)

Feldspar, quartz, and mica are usually considered as the minerals essential to granite, the feldspar being most abundant in quantity, and the proportion of quartz exceeding that of mica. These minerals are united in what is termed a confused crystallisation; that is to say, there is no regular arrangement of the crystals in granite, as in gneiss (see [Fig. 622]), except in the variety termed graphic granite, which occurs mostly in granitic veins. This variety is a compound of feldspar and quartz, so arranged as to produce an imperfect laminar structure. The crystals of feldspar appear to have been first formed, leaving between them the space now occupied by the darker-coloured quartz. This mineral, when a section is made at right angles to the alternate plates of feldspar and quartz, presents broken lines, which have been compared to Hebrew characters. (See Fig. 608.) The variety of granite called by the French Pegmatite, which is a mixture of quartz and common feldspar, usually with some small admixture of white silvery mica, often passes into graphic granite.