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Lewisian gneiss below Sron Godamull

Most of Lewis and the mountains of North Harris are made up of banded gneisses, but moving west and south there are increasingly more veins of hard pink granite and metamorphosed gabbro and related rocks, until at the extreme south of Harris there is a narrow band of metamorphosed sedimentary rock similar to that found at the extreme north of Lewis. Granite is less easily eroded than the surrounding gneiss and good examples can be seen in the sea stacks to the south of Uig Bay in Lewis and on the Ceapabhal promontory in the south-west corner of Harris. The coarsely crystallised pink and white granite found here is known as pegmatite and is largely made up of feldspar and quartz. It forms a distinct horizontal band across the hill that is obvious from quite a distance, especially when it catches the light. In addition to the large pink crystals of feldspar and white crystals of quartz, the rock is shot through with flakes of dark red garnet crystals and clear muscovite and glossy black biotite micas.