Skerryvore consists of acid gneiss with dark bands containing biotite and hypersthene. The mica was probably formed by the recrystallization of chloritic matter by heat from an underlying plutonic mass. The islet is cut by a 2 foot north-south dyke of olivine-dolerite.Dubh Artach forms part of a sill of ophitic olivine-dolerite and is probably of Tertiary age.Sule Skerry consists of andesine-anorthosite into which dark streaks consisting of hornblende and quartz have been injected at a later date to form a banded gneiss.