This is found on the isle of Iona which also has a band of rare, originally algal limestone which is now a serpentinised marble. Torridonian sandstones are also found on the island. Moine biotite schists and psammites are found in the Ross of Mull with more psammites at Gribun. They were metamorphosed during the Caledonian orogeny and occupy the Garnet zone of Barrow's zones of metamorphism. Dalradian schists followed unconformably by Devonian lavas are seen in the Loch Don inlier.
Devonian granite is seen in the Ross of Mull.
The Mesozoic basement sediments are best exposed at Gribun and Carsaig. Mull's famous Tertiary igneous history, which has been very well mapped and researched, contains amongst many complexities of great interest, two important geological phenomena found nowhere else in the world. Within the igneous complex three centres have been discovered with cone sheets and ring dykes of older centres being cut by those of younger centres. The magmatic history of Mull Central igneous complex had four phases, acid, basic, acid, basic. There are a larger than normal amount of acid rocks for magma chambers associated with a mantle plume and this attributed to two causes.