Hornfels is a massive, fine grained, contact metamorphic rock commonly showing conchoidal fracture and/or splintery debris. The heat of an igneous intrusion, like a granite pluton, can create a thermal aureole surrounding it. The country rocks within this aureole of heat may begin to metamorphose and recrystallize. This produces a texture called granoblastic, where the new grains are nearly all the same size and equant.

Hornfelses predominantly occur on Arran within metamorphic aureoles which surround the Northern Granite, although large sills and dykes may have their own minor accessory contact metamorphic rocks (the so-called chilled margin of the dyke shown on the left).