GNEISS, in geology a term originally used by the miners of the Erzgebirge to designate the country rock in which the mineral veins occur. The word is of Slavonic origin meaning "rotted," or "decomposed," in allusion to the altered character of the country rock in the immediate vicinity of the ore veins. It has gradually passed into acceptance as a generic term signifying a large and varied series of rocks with a banded and usually foliated struc ture in which layers of minerals with a granular texture alternate with thin layers composed of lamellar or fibrous minerals, usually in parallel arrangement. The foliation may be frequently inter rupted and the ease of splitting of the rock is usually much less in evidence than in the case of schists (q.v.). Gneisses, however, may also be built up wholly of granular minerals, the gneissose structure being given by the alternation of bands of different mineral composition, e.g., pyroxene-gneiss.
As used in its widest sense, gneiss is a structural term rather than a name applied to rocks of a particular mineral composition or genesis. Thus gneisses may be of igneous or metamorphic origin, and have a great range of chemical composition. The minerals of the granular bands usually consist of quartz, felspar (orthoclase, microcline, plagioclase) or both, and the lamellar or fibrous bands are usually composed of chlorite, mica (muscovite, biotite), graphite, amphibole, sillimanite, etc.
According to their origin, gneisses are sub-divided broadly into three groups : (a) Primary gneisses, (b) Ortho-gneisses and (c) Para-gneisses.
Primary gneisses are plutonic igneous rocks possessing a banded structure, in which a parallel arrangement of the lamellar or fibrous minerals (if present) is evident. These rocks owe their structures to a flow movement in a magma in which crystallization has already progressed. Primary gneisses are often of granitic composition and build up great areas of Archaean terranes. Much of the Lewisian gneiss of Scotland, the Laurentian gneiss of Canada and the igneous gneisses of other Continental shields are rocks of this character. The setting up of gneissic banding in a fluid magma by flow movement pre-supposes a magmatic hetero geneity which in nature arises either by imperfect differentiation or by the incorporation of foreign material within the magma. In many Archaean shields the granite-gneisses are characterized by containing numerous bands of rock, usually of the nature of amphibolites or hornblende-schists, representing basic igneous rocks of earlier date incorporated in the magma during intrusion. These basic bands become injected lit par lit by the granitic material and ultimately in places become so intimately inter mingled with the magma as to produce a gneiss of hybrid origin. Less advanced stages of this process where injection takes place along the foliation planes of inclusions or of the country rock adjacent give rise to injection gneisses.
While gneisses are most prevalent in Archaean tracts, they are by no means absent from later formations. Some of the best known primary gneisses have been described from the Inner Hebrides (Skye and Rum). These are of Tertiary age. The banded gabbros of Druim an Eidhne, Skye, illustrate gneisses arising from imperfect differentiation, and the gneisses of central Rum, also of Tertiary age, have been produced by an intimate commingling of eucrite and granite.
The term ortho-gneiss refers strictly to igneous rocks in which a gneissic structure has been superimposed by metamorphism, but the name is loosely used by some writers to include also primary gneisses. Criteria for the distinction of ortho-gneisses from pri mary gneisses are sometimes difficult to establish, and are chiefly provided in the textural and structural relations of the rocks. They may be evidenced by signs of crushing (cataclastic structure), relict textures, or where the whole rock has been totally recrystal lized by the textural relations of the minerals. In primary gneisses the form-development of the crystals is largely dependent on the order in which the minerals have crystallized from the magma, while in totally recrystallized ortho-gneisses the growth of the minerals has taken place in an essentially solid environment, and the form-development is dependent on the crystallizing power of the several minerals (crystalloblastic texture, see METAMOR PHISM). Some of the best known ortho-gneisses are those of the granulite districts of Saxony and the Austrian Waldviertel near Krems.
Many gneisses are undoubtedly sedimentary rocks brought to their present state by such agents of metamorphism as heat, movement, crushing and recrystallization. This may be demon strated partly by their mode of occurrence ; they accompany limestones, graphite-schists, quartzites and other rocks whose sedimentary origin is never in doubt. In many cases bulk chemi cal composition is a certain clue to their origin, since they cor respond in this particular to normal sediments and not to any known igneous rocks. Structural or textural criteria, such as bed ding, evidence of original pebbly or clastic character are not infrequently to be found. The chemical composition of para gneisses is reflected in their mineralogical constitution. Gneisses derived from argillites may be rich in biotite, muscovite, cordierite, almandine-garnet, staurolite, chloritoid, kyanite and sillimanite, some of which minerals are practically unknown in metamorphosed igneous rocks, while gneisses derived from limestones or dolomites carry such characteristic minerals as grossularite, idocrase, wol lastonite, scapolite or forsterite. Some para-gneisses are rich in felspar (orthoclase, microcline and plagioclase) and quartz and may show so close a resemblance to gneisses of igneous origin that by no single character, chemical or mineralogical, can their original nature be definitely established. (C. E. T.)