When subjected to high temperatures and great pressure, granite is transformed into gneiss.

Gneiss is a metamorphic rock. Its chemical composition is still that of the original granite. However, the distribution of minerals such as quartz, feldspar and mica, which in granite is more or less random, shows a typical orientation along parallel planes. This orientation is called schistosity and is the result of stress sustained during the formation of the Alps. Gneiss tends to break up into flat fragments along the planes of schistosity.

Gneiss in the Macun area shows a well-developed schistosity. The parallel planes of schistosity run perpendicular to the direction of stress sustained. Gneiss is the oldest rock in the region.