Gneiss

Gneiss, a term introduced from the German for a foliated crystalline-granular compound of quartz, felspar, and mica. The quartz is white or gray, and occurs in lenticular layers that vary from a mere line up to bands one foot or more in thickness. The felspar likewise forms folia, and is usually orthoclase, but plagioclase is often associated with it. Frequently the quartz and felspar are intimately commingled. The mica (usually Muscovite) occurs in laminae between the other minerals. In some varieties of gneiss the felspar occurs in lentil-shaped swellings, forming augen-gneiss ('eye-gneiss') or porphyritic gneiss. Varieties in composition are hornblende gneiss, in which hornblende replaces mica; protogine gneiss, with talc instead of mica; graphite gneiss, with graphite in place of mica. Gneiss belongs to the great class of schistose rocks, and in many cases can be shown to be the product of the metamorphism of clastic rocks, such as greywacke. In other cases it has been proved that gneiss has resulted from the metamorphism of granite—the one rock passing gradually into the other. The coarser-grained gneisses belong chiefly to the Archean System (q.v.), and concerning the origin of these geologists are still divided in opinion. The finer-grained varieties are met with in many regions which have been affected by local and regional metamorphism. See METAMORPHOSIS.