The Probert Encyclopaedia of Rocks & Minerals

GRANITE

Granite is a plutonic, igneous, crystalline, granular rock, consisting generally of quartz, feldspar, and mica, mixed up without regular arrangement of the crystals, and usually of a whitish, greyish, or flesh-red colour. The grains vary in size from that of a pin's head to a mass of almost one meter, but they seldom exceed about two centimetres. When they are of this size, or larger, the granite is said to be 'coarse-grained.'.

Granite is one of the most abundant of the igneous rocks seen at or near the surface of the earth, and was formerly considered as the foundation rock of the globe, or that upon which all sedimentary rocks repose; but it is now known to belong to various ages from the Pre-Cambrian to the Tertiary, the Alps of Europe containing granite of the later age. In Alpine situations it presents the appearance of having broken through the more superficial strata; the beds of other rocks in the vicinity rising towards it at increasing angles of elevation as they approach it. It forms some of the most lofty of the mountain chains of the eastern continent, and the central parts of the principal mountain ranges of Scandinavia, the Alps, the Pyrenees, and the Carpathian Mountains are of this rock.

Granite supplies one of the most durable materials for building, as many of the ancient Egyptian monuments testify. It varies much in hardness as well as in colour, in accordance with the nature and proportion of its constituent parts, so that there is much room for care and taste in its selection. Granite in which feldspar predominates is not well adapted for buildings, as it cracks and crumbles down in a few years. The Aberdeen bluish-gray granite is celebrated for its great durability, and also for its beauty. The Peterhead red granite, the hue of which is due to its feldspar being the flesh-coloured potash variety called orthoclase, is highly esteemed for polished work, as columns, pillars, graveyard monuments, etc. Granite in which mica is replaced by hornblende is called syenite; when both mica, and hornblende are present it is called syenitic granite; when talc supplants mica it is called protogene, talcose, or chloritic granite; a mixture of quartz and hypersthene, with scattered flakes of mica, is called hypersthenic granite; and the name of graphic granite, or pegmatite, is given to a variety composed of feldspar and quartz, with a little white mica, so arranged as to produce an irregular laminar structure. When a section of this latter mineral is made at right angles to the alternations of the constituent materials, broken lines resembling Hebrew characters present themselves; hence the name.

Granite abounds in crystallized earthy minerals; and these occur for the most part in veins traversing the mass of the rock. Of these minerals beryl, garnet, and tourmaline are the most abundant. It is not rich in metallic ores. The oriental basalt, found in rolled masses in the deserts of Egypt, and of which the Egyptians made their statues, is a true granite, its black colour being caused by the presence of hornblende and the black shade of the mica. The oriental red granite chiefly found in Egypt, and of which Pompey's Pillar and Cleopatra's Needles were constructed, is composed of large grains or imperfectly formed crystals of flesh-coloured feldspar, of transparent quartz, and of black hornblende.
Research Granite