What is a gneiss in geology ?

Gneiss : definition

A gneiss is a rock of general mesozonal to catazonal metamorphism, composed of an alternation of dark beds of ferromagnesian minerals (micas, amphiboles...) and clear beds of quartz and feldpars.

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.