Gneiss

Metamorphic

Gneisses are foliated metamorphic rocks. The foliation forms from compressive stresses in the earth perpendicular to the layering. Gneisses contain abundant quartz, feldspar, and mafic minerals like augite. The distinctive banding is formed by recrystallization and the separation of quartz- and feldspar-rich layers from dark-colored layers that contain aligned prismatic crystals of mafic minerals like hornblende or augite. Many times in gneisses, porphyroblasts of garnet or feldspar grow within the gneiss — we term this augen gneiss.