What is a gneiss in geology ?
Gneiss : definition
Gneisses are common rocks resulting from the extensive metamorphism of pelitic or arkositic sedimentary rocks (paragneiss), or granitic to dioritic magmatic rocks (orthogneiss). Gneisses can also contain garnet, sillimanite, cordierite, etc... depending on the metamorphic gradient to which they have been subjected.
Augen (photo) are characterized by the presence of feldspars porphyroblasts ; they are very often orthogneiss, deriving from the metamorphism of porphyroid granites.