 It is a quartz-rich metamorphic marine sandstone that may be unique to Scotland and which is nearly 5,000 metres thick.South of Rubh' a' Mhail there are outcrops of quartzite, and a strip of mica schist and limestone cuts across the centre of the island from The Oa to Port Askaig.Further south is a band of metamorphic quartzite and granites, a continuation of the beds that underlie Jura.