Porphyry

Porphyry (Gr., 'purple'), a term originally confined to an Egyptian rock used in sculpture and known as porfido rosso antico. It occurs as a dyke or vein some 65 to 85 feet thick in the granite of Jebel Dokhan (formerly called Mons Porphyrites) in Egypt, between Siout and the Red Sea. It is composed of a felspathic base, in which are disseminated crystals of oligoclase feldspar, with some plates of dark hornblende, and grains of an iron oxide. The beautiful pink or red colour of the porphyritic feldspar and the fine-grained base is due to the diffusion of the red variety of epidote, called Withamite or Piedmontite. The term porphyry is not now used to denote any particular rock, but is applied by architects and others to any igneous rock which, like the porfido rosso antico, has a homogeneous, compact base or fine-grained ground-mass, through which are scattered distinct crystals of one or more minerals. By geologists the term porphyry is seldom used without some descriptive word bracketed with it, as quartz-porphyry, orthoclase-porphyry, augite-porphyry, &c.