Field book of common rocks and minerals

by Frederic Brewster Loomis

Rhyolites are abundant in the western states, as in the Black Hills, the Yellowstone Park, Colorado, Nevada, California, etc.

[Trachite]

The combination of orthoclase feldspar with mica, hornblende or augite is termed trachite, if the texture is dense. It is usually an extrusive lava of light color (green, red or gray), and corresponds in composition to syenite. It can be distinguished from rhyolite by having no quartz, and so nothing to show a hardness above 5.5; but it is difficult to distinguish it from andesite, which differs only in having plagioclase feldspar in place of orthoclase. It sometimes occurs with a porphyritic structure, in which case the feldspar crystals are usually large enough to be distinguished.

Trachites are not abundant in America, but some are found in the Black Hills of South Dakota, in Custer Co., Colo., and in Montana.

[Dacite]

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.