Orthoclase-porphyry

Orthoclase-porphyry, a crystalline igneous rock, of variable colour, but generally reddish. It is fine-grained and compact in texture. The ground-mass is felspathic, and micro- or cryptocrystalline; now and again it shows a little glassy or devitrified matter. Scattered through this ground-mass are microscopic crystals of orthoclase, and usually some hornblende and biotite in small granules, crystals, and scales. The rock is met with amongst Palæozoic strata, both as contemporaneous lava-flows and as intrusive masses. See also FELSPAR, and IGNEOUS ROCKS.