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Granite

texture, rocks, cent, granites, rock, feldspar and qv

GRANITE (It. granito, granite, grained, from granire, to reduce to grains, from grano, Lat. Branum, grain; connected with Gott). kafirii, Icel., 011.G. morn, Ger. Korn, grain, AS., Eng. corn). An igneous rock or family of rocks characterized by acid composition and granitic texture (see IGNEOUS ROCKS ) , containing the minerals quartz and potash feldspar, and usually, also, either light or dark mica, or both, together with horn blende or augite. Other minerals are usually pres ent in less abundance, as lime-soda feldspar, mag netite, ilmenite, sphene, zircon, etc. The average composition of granite is as follows: Silica, 70 per cent.; alumina, 15 per cent.; sesquioxide and protoxide of iron, 4 per cent.; magnesia, 1 per cent.; lime, 2 per cent.; oxide of so dium, 4 per cent.; oxide of potassium, 4 per cent. The texture of granite, which gives its name to the texture of a considerable group of rock types, consists in a mosaic of interlock ing crystalline grains representing an essen tially uninterrupted period of crystallization.

Granites are classified into muscovite granite or alkali granite, muscovite-biotite granite or binary granite, hornblende granite, and augite granite, the first portion of the name generally indicating the mineral or minerals which with quartz and potash feldspar make up a large part of the rock's substance. By loss of the constit uent mineral quartz granite grades into syenite (q.v.) ; by reduction of the amount of potash feldspar and gain in the proportion of lime-soda feldspar it passes into diorite (q.v.). Augite diorites by a similar change pass into gabbro. These transitions are mentioned not alone to show the relationships which exist between the different families of igneous rocks possessing granitic texture, but because they indicate trans itions frequently observable within the same rock mass. Important variations in the relative proportions of the several mineral constituents of a rock within the same mass are, in fact, the rule rather than the exception, so that it is difficult to delimit with any sharpness the sev eral types.

In an earlier period of development of the science of geology, the question respecting the origin of granites was debated with great warmth by the rival schools of England and Germany.

Hutton (q.v.) and his followers maintained that granites were formed from fusion and consolida tion, whereas Werner and the Freiberg school of geologists, with even greater assurance, advocated the chemical precipitation theory of its forma tion. Time has shown the correctness of the view held by Hutton. though his theory has been modi fied in important ways. It was long a tradition that granite and rocks of granitic texture gener ally were all produced in the earlier geological ages, conditions necessary to their formation being assumed to have been attained which have not since been reproduced. This view was a natural one, since none of the granites then known was as young as the Tertiary age. When later granites were discovered which had been formed in the Tertiary or post-Tertiary periods, a different explanation became necessary. The view then became general that the special con ditions which are necessary for the formation of granite obtain only at considerable depths below the surface of the earth, and that granites of recent formation are only rarely found because, except under unusually favorable conditions, a long time interval is necessary to allow the forces of erosion and transportation to remove the rock cover under which they are buried. The granites which are now forming at great depths will in a future age be so far dissected as to appear at the surface.

In common usage the term granite is loosely applied to any rock of granitic texture, and among petrographers the term is sometimes used in a broad family sense to cover rocks of por phyritic texture but similar chemical composi tion, namely, rhyolites (q.v.) and rhyolite por phyries.

Granite is an important building-stone. It is much harder to quarry and tool than are sand stone, limestone, and marble, but has greater strength; the best grades are extremely resistant to weathering. Consult: Kemp, Handbook of Rocks (New York, 1900) ; Rosenbusch, Mikro skopische Physiographie der Mineralien and Ge steine (Stuttgart, 1896). See BUILDING-STONE.