Granite-Gneiss

a holocrystalline shale rock consisting, like granite, of quartz, potash feldspar, acid plagioclase, and mica (predominantly biotite).

A majority of researchers believe granite-gneisses to be granites that have crystallized deep in the earth’s crust either during the cooling of the magmatic melt under directed pressure or during the process of the magma’s movement, as a result of which there is parallel arrangement of the mica (more rarely, of the other minerals). It is extremely difficult to distinguish the magmatic granite-gneisses from the meta-morphic gneisses. For this reason, the name granite-gneiss is sometimes used to denote macrocrystalline gneisses that are rich in granitic material.