Granite is one of the most easily recognisable and ubiquitous of igneous rocks on Arran. It is a light coloured, coarse grained, intrusive igneous rock, acidic in composition. It consists of quartz, alkali feldspar, mica and variable amounts of amphibole. These can all be identified with the naked eye.

Granites can form from the partial melting of continental crust, by differentiation of basaltic magma, by localised metasomatic effects on continental crust, or by a combination of the above. On Arran, the granites are associated with the British Tertiary Province (link to Tertiary?) , an episode of intense magmatism linked to the break-up of the North American and Eurasian continents.

The North Granite forms the prominent hills on Arran, including the highest peak Goat Fell. The granite is in fact the roots of an ancient volcano, exposed after 60Ma years of erosion. This large, circular pluton covers an area of 146 square kilometres, and is made up of two separate periods of intrusion (an earlier coarse grained outer member, and a younger finer grained inner member). Aplite veining is pervasive. The veins of aplite, a type of fine grained microgranite, represent the intrusion of the residual magmatic liquids which were poor in volatiles.

Streams emanating from glens in the area of the Northern Granite are chock full of granite boulders. Many granite boulders, some estimated to be around 400 tons in wieght, are found around the coast of Arran. They are though to be erratics, and to have been moved by ice (see image on right).