Quartz-porphyry

Quartz-porphyry, an igneous rock, consisting of crystals of quartz and felspar scattered porphyritically through a compact or very finely crystalline ground-mass of the same minerals. It occurs both as an intrusive rock and in the form of lavas which have flowed out at the surface. Some of the quartz-porphyries which have a very compact or microfelsitic ground-mass appear to have been originally volcanic vitreous rocks—the glass having subsequently become devitrified. The non-porphyritic varieties which have a flinty or hornstone-like aspect are called Felsite (q.v.). 'Quartzlers-porphyry' is a name for Orthoclase-porphyry (q.v.).