A schist is a regional metamorphic rock of pelitic composition which displays a pervasive schistocity. The minerals defining the foliation are often the micas: muscavite and biotite; although elongate quartz crystals may also produce schistocity. The photo shows the well developed folding in these rocks

Starting out life as mudstones, shales or sandy mudstones, the rocks are then subjected to intense pressure and heat during continental collision. The mineral assemblage becomes unstable at these altered conditions, and begins to change via the diffusion of atoms. New, more stable minerals grow as unstable minerals "dissolve".

The growth and alignment of platey, micaceous minerals like chlorite and biotite give the distinctive sheen on the cleavage surfaces of schists. On Arran, the oldest rocks to be found are schists of Dalradian age. Produced by the Grampian Orogeny, fine examples of schist can be found outcropping along the west and northwest coast from Dougarie to Lochranza.