The New International Encyclopædia/Gneiss
GNEISS (Ger., probably connected with OHG. gneista, Icel. gneisti, AS. gnāst, Eng. gnast, spark). A family of rocks belonging to the metamorphic series, and resembling granite in composition. Gneisses are granular aggregates of feldspar and quartz, with mica, hornblende, or pyroxene, and some of the rarer metals. Their structure is characterized by a parallel arrangement of the constituents: the light and dark minerals alternate in-bands or layers, which are sometimes so regular and distinct as to give the appearance of stratification. Owing to this peculiarity, many geologists hold that they are metamorphosed sediments. There is conclusive evidence, however, that the parallel arrangement may be brought about in rocks of truly igneous origin, either as a result of movements of the constituents while the magma is in process of solidification, or by compression and shearing strains after the rock mass has solidified. Some gneisses, doubtless, have resulted from the metamorphism of sediments; but in such cases the proof is not based primarily upon the gneissoid character. Neither the igneous nor the sedimentary theory of origin is to be accepted for gneisses as a class, and each occurrence must be studied by itself. For this reason, geologists have come to use the term gneiss in its structural sense, without implying anything further as to origin or constituent minerals. When it is desired to define the composition of a particular type, other rock names are united with the term; for example, granite-gneiss, syenite-gneiss, gabbro-gneiss, or granitic gneiss, syenitic gneiss, gabbroic gneiss. Gneisses are the most widely distributed of metamorphic rocks (q.v.), and are found underlying the earliest sediments in almost all parts of the world. Consult: Kemp, Handbook of Rocks (New York, 1900); Rosenbusch, Mikroskopische Physiographie der Mineralien und Gesteine (Stuttgart, 1896). See Geology.