Under the name of granite are included many rocks differing largely in appearance, properties, and mode of origin, but agreeing in their general petrological character. The granites are all distinctly crystalline, the size of the crystals varying from a few inches in length, as in the porphyritic granites of shap, to an almost microscopic size in some of the very finely grained granites. Granite is composed essentially of three minerals - quartz (SiO2). usually white and glassy; felspar (a silicate of alumina, and potash, or some other base), often in large crystals; and mica (a complex silicate of alumina and other bases), iu flaky crystals, usually of small size; scattered through the mass there are very often crystals of garnet and other secondary or accessory minerals. The colour of tin-rock depends mainly on the colour of the felspar and the mica. When pink felspar is present, the colour is pink; whilst when the felspar is white and the mica black, the granite is grey. The colours vary considerably, according to the proportions in which the various constituents are present.

Granite is usually classed as an igneous rock; but whilst it is probable that some of the granites have been formed by fusion, there are others which have certainly been produced by the metamorphism of stratified rocks without fusion, probably under the combined influence of heat, water, and pressure. Granite is largely used for heavy work, where great durability is required, and for ornamental columns and other parts of structures, being then usually polished. It is only used as a building stone in neighbourhoods where it occurs in abundance. It is hard and difficult to work, and therefore is expensive. Granite is usually regarded as being a very durable stone but whilst on account of its hardness it is undoubtedly good for resisting heavy wear, it does not resist the corroding influences of the atmosphere so powerfully as is often supposed. Felspar, especially the pink potash variety, yields in time to atmospheric influences, breaking down ultimately to a soft, incoherent mass of kaolin or china clay, and it is by no means uncommon to find beds of granite which have been exposed to the air for ages weathered in this manner to a considerable depth.

The corrosion that has been observed in granite structures is, of course, much less, being mainly confined to a loss of the polish and a roughening of the surface, due to the corrosion of the felspar crystals. If iron be present in any form, it may accelerate decay, especially if it be irregularly distributed in the form of marcasite (FeS2.). This is indicated by the production of iron stains on the surface of the stone on exposure to the weather. As a general rule the smaller the grain of a granite the more durable it is likely to be, aud at the same time the more easily will it be worked. Syenite closely resembles granite, except that the mica is replaced by hornblende; or if both mica and hornblende are present, it is a syenitic granite. The syenites are often darker in colour than true granites, and are hard aud tough. Igneous rocks other than granite are not used to any large extent, except in localities where they are abundant. The porphyrites are compact rocks of igneous origin, consisting of a felspathic base, in which are crystals of quartz, felspar, and other minerals. They contain from 50 to 80 per cent, of silica, and vary in colour and in chemical and mineralogical composition.

Porphyrites are mainly used in England for road metal.