GRANITE, a genus of stones of the order of petra, belonging to the class of saxa. The principal constituent parts of this stone are felt-spar or rhombic quartz, mica, and quartz. These ingredients constitute the hardest sort of granite, and that most anciently known. That into which felspar enters is more subject to decomposition. They never have any particular texture or regular form, but consist of enormous shapeless masses extremely hard. In the finer granites the quartz is transparent; in others generally white or grey, violet or brown. The felt-spar is generally the most copious ingredient, and of a white, yellow, red, black, or brown colour. The mica is also grey, brown, yellow, green, red, violet, or black; and commonly the least
copious. The felspar is generally black, and abounds in the granites that contain it. Hence the colour of the granites depends principally on that of the spar or felspar. The red granites consist commonly of white quartz, red felt-spar, and grey mica; the grey ones of white quartz, grey or violet felt-spar, and black mica. The black granites commonly contain felspar instead of felt-spar; and the green usually contain green quartz.
On expoling granite to the flame of a blow-pipe, the component ingredients separate from one another. Mr Gerhard having melted some in a crucible, found the felt-spar run into a transparent glass; below it the mica lay in form of a black slag, the quartz remaining unaltered. It melted somewhat better when all the three were powdered and mixed together; though even then the quartz was still discernible by a magnifying glass. Hence we may explain the reason why grains of a white colour are sometimes found in volcanic lavas. The mixture of mica prevents the felspar or quartz from splitting or cracking; and hence its infusibility and use in furnace-building.
Granites are seldom flat or laminated. In those which are of a close texture, the quartz and felspar predominate. They take a good polish; for which reason the Egyptians formerly, and the Italians still work them into large pieces of ornamental architecture, for which they are extremely fit, as not being liable to decay in the air. Farber, in his letters from Italy, mentions a kind of stone named granitone, composed of felt-spar and mica: a substance of this kind, which moulders in the air, is found in Finland; which is said to contain salt-petre, and sometimes common salt. In that country it is called rapakiri. Wallerius describes 18 species of granites, besides many others akin to this genus. Those described by Cronstedt are, 1. Loose and friable, which comes from France, and is used at the brass-works for casting that metal in. 2. Hard or compact, of which there are two varieties, red and grey. The former is met with of two kinds; viz. fine-grained from Swappari in Lapland, or coarse-grained from the province of Dalarna in Sweden. The grey, with other colours, is met with on the coast round Stockholm and Norland in Sweden.