Granite (Ital. granito, 'gritty'; Lat. granum, 'grain'). This well-known rock is a thoroughly crystalline-granular aggregate of quartz, felspar, and mica. The felspar is generally orthoclase (pink or gray), but some plagioclase is often present. The mica may be muscovite or biotite, and other varieties also occur, but the most common perhaps is muscovite. There is no base or matrix in this rock—the several crystals and crystalline granules, confusedly commingled, being bound together by their faces. In crystallising out, the felspar and mica have interfered with each other's development, so that these minerals rarely assume perfect crystalline forms. The quartz still more rarely appears in the form of perfect or even approximately perfect crystals, but occurs as irregular crystalline granules, or seems to be moulded upon and hemmed in between the other minerals. Fluid cavities are generally plentiful in the quartz. As a general rule the component crystals of granite have separated out in the following order: mica, felspar, quartz. Occasionally, however, it is found that the felspar and the quartz have crystallised together, and thus mutually interfered with each other's form. More rarely the formation of the quartz has even preceded that of the felspar. All varieties of texture are met with among granites, from very fine-grained up to coarsely-crystalline rocks, in which the component crystals may be several inches in diameter. The coarser-grained kinds are called pegmatite. In the variety known as graphic granite the quartz is crystallised in the orthoclase, forming alternate zigzag-shaped laminae, which, on a cross-fracture, present the appearance of Hebrew writing. The accessory minerals, such as beryl, topaz, tourmaline, garnet, sphene, &c., are met with chiefly in irregular cavities, and in such cavities very fine crystals of the essential minerals often occur. Scattered through the body of the rock, however, accessory minerals are not uncommon, especially apatite and sphene, and less frequently zircon—these three minerals occurring as inclusions in the essential minerals. The relative proportion of mica, felspar, and quartz varies; in many granites felspar forms more than half of the bulk of the rock—quartz coming next, and mica last. In other granites there is extremely little quartz, while mica is more plentiful. Sometimes the rock is rendered porphyritic by the appearance of large crystals of orthoclase, embedded in a granitoid or finely-crystalline ground-mass. It is generally the felspar which gives the prevalent colour to a granite—the rock being red or gray according as flesh-coloured or white felspar predominates. Very often dark patches and nodules occur in granite. Sometimes these are fragments of foreign rocks more or less altered; at other times they are composed of the same minerals as the granite itself, but in different proportions—mica often predominating. Veins of similar composition are also found ramifying through granite. These and the patches together are supposed to be 'the result of differentiation accompanying the crystallisation of the original magma'—the dark portions being more basic in composition than the rock in which they occur. Most granites are traversed by lighter-coloured veins—some of which are finer grained and others coarser than the rock in which they appear. The origin of these veins is uncertain. They would appear to be of contemporaneous origin with the granite, and to have sometimes formed in rents of the original pasty magma, possibly by segregation of the minerals from the surrounding mass. The fine-grained veins, on the other hand, were probably injected before the granite had become quite consolidated. It seems certain at least that the rock of the veins and the granite itself originally formed portions of one and the same molten mass.
Amongst varieties of granite may be mentioned hornblende granite, in which hornblende is added to the other constituents. When this is the case, mica is only sparingly present. When schorl (black tourmaline) replaces mica, we have schorlaceous granite. Greisen is a granular aggregate of quartz and mica. Aplite is a fine-grained aggregate of quartz and orthoclase, with sometimes a little mica. These three last-mentioned varieties are met with chiefly in veins proceeding from masses of ordinary granite.
Granite usually occurs in great bosses or amorphous masses—and frequently forms the nuclei of mountain-chains. Its petrographical characters and behaviour in the field prove it to be of igneous origin, at all events in the great majority of cases, and to have consolidated at considerable depths in the earth's crust. Hence it belongs to the Plutonic class of igneous rocks. Some writers have held that certain granites are of metamorphic origin, but the appearances which seem to support this view have of recent years received another interpretation. And although, in the present state of our knowledge, it cannot be asserted that no granite is of metamorphic origin, yet it would appear that granites of demonstrably metamorphic origin have not yet been discovered. Those which are supposed to be of such origin are intimately associated with crystalline schists, which themselves are believed to be the result of metamorphic changes. At one time granite was looked upon as the oldest of primitive rocks, but it is now known to be of various ages. Its presence at the surface is due of course to denudation, which has removed the great masses of rock that originally covered it.
The more durable kinds of granite are largely used as building materials in bridges and engineering-works, and also in public buildings and dwellings. The difficulty of working it makes it expensive, but this is counterbalanced by its great durability. It cannot be cut, like the majority of building-stones, with saws, but is worked first with large hammers, and then with pointed chisels. The success with which the Egyptians operated upon this refractory stone is very extraordinary. They worked and polished it in a way that we cannot excel, if, indeed, we can come up to it, with all the appliances of modern science; and not content with polishing, they covered some of the blocks with the most delicate and sharply-cut hieroglyphics!
The granites best known in the British Islands for ornamental purposes are the gray Aberdeen granite and the reddish-coloured Peterhead granite. Of this last-mentioned variety handsome polished columns for public halls have been constructed. On the Continent granite has been quarried for similar purposes in several countries: as near Baveno in Italy, and in the islands of Sardinia and Elba; in Normandy and Brittany; in southern Sweden, Finland, the Tyrol, Switzerland, &c. In North America granites are worked at a number of places, as in Maine, New Hampshire, Massachusetts, Connecticut, New York, Michigan, and California, and at various places in the Canadian province of Quebec. The rock would probably be more abundantly used than it is, were it not for the fact that in many cases it occurs at elevations and in districts more or less difficult of access.
The soil produced by the weathering of granitic rocks should be fertile, as their component ingredients yield the necessary elements. But in hilly districts, where granite is chiefly developed, the fine clay which results from the decomposition of the felspar is washed away, so that only the quartz sand is left on the slopes—forming a thin, ungrateful soil. In the hollows and flats whither the clay is transported we find generally a cold, stiff, and wet subsoil, which is only worked with difficulty. In low-lying granitic tracts, especially under genial climatic conditions, the soil which results from the weathering of granite is sometimes very fertile. See Geo. F. Harris, Granite and the Granite Industries (1888).