Gneiss

Gneiss, a foliated rock, composed essentially of orthoclase felspar, quartz, and mica. Bubble cavities containing water and liquid carbon dioxide occur in the quartz; plagioclase felspars and garnet are common accessory minerals, and hornblende, talc, and graphite are sometimes so abundantly present, replacing in part the mica, as to give names to recognised hornblendic, talcose, and graphitic varieties. Lithologically gneiss hardly differs from granite (q.v.), except in its foliation (q.v.), and some of the coarser varieties do not exhibit this structure in hand-specimens. Gneiss is said to graduate in the field either into mica schist, slates, or less altered sedimentary rocks; or, on the other, into true granite. It has been generally considered a highly metamorphosed aqueous rock; but may, at least in some cases, be a granite molecularly rearranged. Most gneiss is associated with Archaean rocks (q.v.).