GNEISS (Ger., probably connected with OHG. gneista, Icel. gneisti, AS. gnast, Eng. guest, 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, pyroxenc, and some of the rarer metals. Their structure is characterized by a parallel arrange ment of the constituents; the light and dark minerals alternate in bands or layers, which tire sometimes so regular and distinct ns to give the appearance of stratification. Owing to this pe culiarity, many geologists hold that they are metamorphosed sediments. There is conclusive evidence, however, that the parallel arrange ment 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 meta morphism of sediments; but in such cases the proof is not based primarily upon the gneissoid character. Neither the igneous nor the sedimen
tary 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 ex ample, granite - gneiss, syenite - gneiss, gabbro gneiss, or granitic gneiss, syenitic gneiss, gab broic gneiss. Gneisses are the most widely dis tributed of metamorphic rocks (q.v.), and are found underlying the earliest sediments in almost all parts of the world. Consult: Kemp, Hand book of Rocks (New York, 1900) ; Rosenbusch, Mikroskopische Physiographie der Mimeralien und Gesteine (Stuttgart, 1896). See GEOLOGY.