A quartz porphyry is an acidic igneous rock containing phenocrysts of quartz crystals set in a medium or fine grained matrix such as microgranite or microgranodiorite. Feldspar microphenocrysts may also be present. At the Drmadoon sill on Arran, the phenocrysts (large white crystals) are mostly composed of both quartz and feldspar.

There are several quartz porphyry sills on Arran of Tertiary age. The most famous being Drumadoon Sill. Intruded into Triassic red marls and siltstones, and displaying excellent columnar-jointing, this 30m tall sill contains phenocrysts of quartz as well as plagioclase and alkali feldspar.

A striking feature of the Drumadoon Sill is the mixing of two quite different igneous rock types. The thick quartz-feldspar porphyry sill, an acid intrusion with a compsition similar to granite, has picked up some blocks of dolerite as the former was intruded. Dolerite is a basic rock, similar to basalt in composition. This mixing of two rock types can be easily seen in blocks lying on the foreshore at Drumadoon (as shown on right).

Other quartz porphyries include sills at Bennan Head and Brown Head, as well as the prominent hill known as Dun Dubh in the Corrygills district, which is vertical plug-like intrusion.