GNEISS, a term introduced from the German, as the name for a variety of metamor hie rock, which has the same component materials as granite, and differs from it only to these materials being arranged in layers, rather than in an apparently confused aggre gated mass. The Minerals .o.f which it is composed are quartz; feldspar,: and mica. The mica is sometimes replaced by hornblende, producing a gneiss corresponding to the variety of granite called syenite. , The different ingredients occur in various propor tions, altering the character and appearance of the gneiss accordingly. It is often diffi cult to determine hand specimens of gneiss; for, on the one hand, they are sometimes so crystalline that they resemble granite, while. on the other, the schistose varieties approach so near to mica-schist, that even in the field, under the most favorable circum stances, it is not easy positively to separate them.
Gneiss was originally deposited as sand or mud, and has been converted into a hard, tough crystalline rock by long and continuous subjection to metamorphic action, induced, perhaps, chiefly by heat. It has generally been considered as an azoic rock,
that is deposited before the existence of life on the globe. The older strata, classified by Logan under the title Laurentian, the equivalents of which have been recently observed by.Murchison in Scotland, have as yet proved destitute of fossils, but this may be to the extreme metamorphism they have undergone. The Cambrian and Silurian strata of the n. of Scotland have also been to a large extent converted into gneissose rocks, which contain intercalated with them fossiliferous limestones. It would seem, indeed, that gneiss and its allied stratified rocks are not necessarily "primary rocks," but may occur wherever an agency sufficiently powerful has acted upon ordi nary sandstone and shale.