The Student's Elements of Geology

by Sir Charles Lyell

Talcose Granite, or Protogine of the French, is a mixture of feldspar, quartz, and talc. It abounds in the Alps, and in some parts of Cornwall, producing by its decomposition the kaolin or china clay, more than 12,000 tons of which are annually exported from that country for the potteries.

Schorl-rock, and Schorly Granite.—The former of these is an aggregate of schorl, or tourmaline, and quartz. When feldspar and mica are also present, it may be called schorly granite. This kind of granite is comparatively rare.

Eurite, Feldstone.—Eurite is a rock in which the ingredients of granite are blended into a finely granular mass, mica being usually absent, and, when present, in such minute flakes as to be invisible to the naked eye. It is sometimes called Feldstone, and when the crystals of feldspar are conspicuous it becomes Feldspar porphyry. All these and other varieties of granite pass into certain kinds of trap—a circumstance which affords one of many arguments in favour of what is now the prevailing opinion, that the granites are also of igneous origin. The contrast of the most crystalline form of granite to that of the most common and earthy trap is undoubtedly great; but each member of the volcanic class is capable of becoming porphyritic, and the base of the porphyry may be more and more crystalline, until the mass passes to the kind of granite most nearly allied in mineral composition.

Syenitic Granite.—The quadruple compound of quartz, feldspar, mica, and hornblende, may be termed Syenitic Granite, and forms a passage between the granites and the syenites. This rock occurs in Scotland and in Guernsey.

Syenite.—We now come to the second division of the Plutonic rocks, or those having less than 60 per cent of silica, and which, as before stated (p. 552), are usually called syenitic. Syenite originally received its name from the celebrated ancient quarries of Syene, in Egypt. It differs from granite in having hornblende as a substitute for mica, and being without quartz. Werner at least considered syenite as a binary compound of feldspar and hornblende, and regarded quartz as merely one of its occasional minerals.

Miascite.—Miascite is one of the varieties of syenite most frequently spoken of; it is composed chiefly of orthoclase and nepheline, with hornblende and quartz as occasional accessory minerals. It derives its name from Miask, in the Ural Mountains, where it was first discovered by Gustavus Rose. Zircon-syenite is another variety closely allied to Miascite, but containing crystals of Zircon.

Connection of the Granites and Syenites with the Volcanic Rocks.—The minerals which constitute alike the Plutonic and volcanic rocks consist, almost exclusively, of seven elements, namely, silica, alumina, magnesia, lime, soda, potash, and iron (see Table [p. 449]); and these may sometimes exist in about the same proportions in a porous lava, a compact trap, and a crystalline granite. The same lava, for example, may be glassy, or scoriaceous, or stony, or porphyritic, according to the more or less rapid rate at which it cools.

It would be easy to multiply examples and authorities to prove the gradation of the Plutonic into the trap rocks. On the western side of the Fiord of Christiania, in Norway, there is a large district of trap, chiefly greenstone-porphyry and syenitic-greenstone, resting on fossiliferous strata. To this, on its southern limit, succeeds a region equally extensive of syenite, the passage from the trappean to the crystalline Plutonic rock being so gradual that it is impossible to draw a line of demarkation between them.

“The ordinary granite of Aberdeenshire,” says Dr. MacCulloch, “is the usual ternary compound of quartz, feldspar, and mica; though sometimes hornblende is substituted for the mica. But in many places a variety occurs which is composed simply of feldspar and hornblende; and in examining more minutely this duplicate compound, it is observed in some places to assume a fine grain, and at length to become undistinguishable from the greenstones of the trap family. It also passes in the same uninterrupted manner into a basalt, and at length into a soft claystone, with a schistose tendency on exposure, in no respect differing from those of the trap islands of the western coast.” The same author mentions, that in Shetland a granite composed of hornblende, mica, feldspar, and quartz graduates in an equally perfect manner into basalt.[^[4]] In Hungary there are varieties of trachyte, which, geologically speaking, are of modern origin, in which crystals, not only of mica, but of quartz, are common, together with feldspar and hornblende. It is easy to conceive how such volcanic masses may, at a certain depth from the surface, pass downward into granite.

Granitic Veins.—I have already hinted at the close analogy in the forms of certain granitic and trappean veins; and it will be found that strata penetrated by Plutonic rocks have suffered changes very similar to those exhibited near the contact of volcanic dikes. Thus, in Glen Tilt, in Scotland, alternating strata of limestone and argillaceous schist come in contact with a mass of granite. The contact does not take place as might have been looked for if the granite had been formed there before the strata were deposited, in which case the section would have appeared as in Fig. 610; but the union is as represented in Fig. 611, the undulating outline of the granite intersecting different strata, and occasionally intruding itself in torturous veins into the beds of clay-slate and limestone, from which it differs so remarkably in composition. The limestone is sometimes changed in character by the proximity of the granitic mass or its veins, and acquires a more compact texture, like that of hornstone or chert, with a splintery fracture, and effervescing freely with acids.