Texture Excess of light colored minerals Excess of dark colored minerals Glassy obsidian, perlite, pumice, pitchstone scorias, trachylyte, basalt-obsidian Feldspar orthoclase Feldspar Plagioclase No feldspar Mica and/or hornblende and/or augite Mica and/or hornblende with pyroxene augite and/or hornblende and/or mica +quartz -quartz +quartz -quartz +olivine -olivine +olivine -olivine Dense rhyolite trachite dacite (felsite) andesite (felsite) basalt augitite or hornblendite Porphyritic rhyolite-porphyry trachite-porphyry dacite-porphyry andesite-porphyry basalt-porphyry augitite-porphyry Granitoid granite syenite quartz-diorite diorite olivine-gabbro gabbro peridotite pyroxenite Fragmental rhyolite, tuff trachite, tuff Dacite, tuff or andesite tuff or Basalt tuffs and breccias or breccia or breccia breccia breccia When it is located as igneous, turn to the key on page 177 and decide as to which type of texture is present. If glassy, the color, luster and type of construction will place it. If the rock is crystalline, first decide whether feldspar is present, and if present, what type: then determine the dark mineral, and lastly whether quartz or olivine is present. In dense rocks the presence of quartz may be determined by trying the hardness, for none of the other constituents of igneous rocks have so great hardness. For example, if it is found that a rock is composed of orthoclase hornblende and quartz, and the texture is granitoid, it is granite: or if the rock is plagioclase feldspar and pyroxene of any sort, it is gabbro, etc. Granite Pl. 53