Fine-grained igneous rock, typically porphyritic, consisting of quartz and alkali feldspar (often as phenocrysts), with minor plagioclase and biotite, in a microcrystalline, cryptocrystalline or glassy groundmass. Flow texture is common. Includes rocks defined modally in QAPF fields 2 and 3 or chemically in TAS Field R as rhyolite.