A general term for dense, igneous rock composed of quartz, feldspar, and ferromagnesium minerals which are so fine grained that the composition of the rock is difficult to determine. The group includes rhyolite, dacite, trachyte, and andesite.
Felsite is a very fine grained to cryptocrystalline igneous rock that may or may not contain larger crystals, called phenocrysts, that are typical of many porphyritic igneous rocks. The mass of the rock consists of a fine-grained matrix of felsic materials, particularly quartz, sodium and potassium feldspar, and may be termed a quartz felsite or quartz porphyry if the quartz phenocrysts are present. This rock is typically of volcanic origin, and may be found in association with obsidian and rhyolite.