The union of plagioclase feldspar, quartz, and either hornblende or mica is termed dacite, if the texture is dense. It is an extrusive lava, occurring mostly in sheets and dikes. It corresponds in composition to quartz-diorite. As the texture is dense it is difficult to distinguish dacite from rhyolite, for both have quartz and differ only in the character of the feldspar, so it is quite common to use the term felsite which does not distinguish between the two, and only states that the rock is dense, light-colored and extrusive. When, as often occurs, the texture is porphyritic, and the feldspars are the large crystals, then exact determination is fairly easy. Dacites are rather common, occurring on McClelland Peak, Nev., in the Eureka district, Nev., on Lassen’s Peak, Calif., Sepulchre Mt. in the Yellowstone Park, etc. Andesite The union of plagioclase feldspar with mica, hornblende or augite, makes andesite if the texture is dense. The lack of quartz, and so no mineral which has a hardness of over 5.5, makes it possible to distinguish andesite from dacite or rhyolite, but it is hard to distinguish this rock from trachite, which differs only on having orthoclase feldspar in place of plagioclase. When the texture is porphyritic and the feldspars are the large crystals, then it is easy to make the distinction. Andesite gets its name from being the characteristic lava of the Andes Mountains, and is the commonest of all the extruded, light-colored lavas, being the lava of hundreds of flows throughout the western United States. The union of plagioclase feldspar and biotite is the commonest type. Plagioclase with hornblende or augite is less common, and, when they do occur, they are usually distinguished as _hornblende-andesite_ or _augite-andesite_. Magnetite, apatite and zircon may be present as accessory minerals. The lavas of Mt. Hood, Shasta, Rainier and others of the volcanic peaks of the Cascade Range, those at Eureka and Comstock in Nevada, in the Yellowstone National Park, and the porphyries of many peaks in Colorado, like the Henry Mts., etc., which are exposed laccolithic intrusions, are all andesites, as are many more. Basalt The combination of plagioclase feldspar with olivine and augite (or any other pyroxene) makes a heavy, dark-colored, black to dark-brown rock which, if its texture is dense or porphyritic, is termed basalt. This usually has more or less magnetite in it as an accessory mineral, indeed the magnetite may be so abundant as to be a component part of the rock. This magnetite makes trouble for anyone trying to use a compass on or about basalt rocks. These are extrusive or intrusive rocks and correspond in composition to gabbro.