Volcanic, Fine-Grained, Igneous Rocks

    Because of their very fine-grained structure volcanic rocks cannot in general  be readily told apart. A number of different types are recognized, the distinction between them being based, however, chiefly upon microscopic study. In the field only an approximate classification, depending upon whether the rock is light or dark in color, can be made. A brief description of these types of volcanic rocks follows.

    Felsite. This is a dense fine-grained rock type with a stony texture and includes all colors except dark gray, dark green or black. These rocks may, by the aid of a lens, still show a very fine-grained structure or there mineral constituents may occurs in such small particles as to give them a dense and homogeneous, often a flinty, appearance. By microscopic study the felsites have been derived into the following groups; rhyolite, consisting chiefly of alkaline feldspars and quartz; dacite, lime-soda feldspars and quartz; trachyte, alkaline feldspars with little or no quartz; andesite, soda-lime feldspars with little or no quartz; phonolite, alkaline feldspars and nephelite. As a rule these varieties are not to be distinguished from each other in the field. The felsites are widespread in their occurrence, being found as dikes and sheets intruded into the upper part of the earth’s crust or as lava flows which have been poured out upon the earth’s surface.

    Basalt. The basals are dense fine-grained rocks that are of very dark color, gr een or black. They are composed of microscopic grains of a soda-lime feldspar with pyroxene, iron ore, often more or less olivine and at times biotite or hornblende. These rocks are formed under the same conditions as the felsites and are to be found occurring in the same ways.

    Glassy Rocks. Some of the volcanic rocks have cooled so rapidly that they are wholly or in part made up of a glassy material in which the different elements have not had the necessary opportunity to group themselves into definite minerals. In the entire rock is composed of glass it is called obsidian, when it has a bright and vitreous luster; pichstone when its luster is dull and pitchy; perlite if it is made up of small spheroids; and pumice if it has a distinctly cellular structure. These rocks may also have distinct crystals of various minerals embedded in the glass, in which case they are known as glass porphyries (see below for a definition of a porphyry) or vitrophyres.

    Porphyries. Igneous rocks as times show distinct crystals of certain minerals which lie embedded in a much finer-grained material. These larger crystals are known as phenocrysts, and the finer-grained material as the groundmass of the rock. Rocks exhibiting such a structure are known as porphyries. The phenocrysts may vary in size from crystals an inch or more across down to quite small individuals. The groundmass may also be composed of fairly coarse-grained material or its grains may be microscopic in size. It is the distinct difference in size existing between the phenocrysts and the particles of the groundmass that is the distinguishing feature of a pophyry. This peculiar structure is due to certain conditions prevailing during the formation of the rock which permitted some crystals to considerable size before the main mass of the rock consolidated into a finer-and uniform-grained material. The explanation of the reasons why a certain rock should assume a porphyritic structure would involve a more detailed discussion than it is expedient to give in this place. Any one of the above described types of igneous rocks may have a porphyritic variety, such as granite-porphyry, diorite-porphyry, felsite-porphyry, etc. Porphyritic varieties are more liable to occur in connection with volcanic rocks, and they are also found most frequently in the case of the acid types.