Geologically, its northern half is composed of Torridonian sandstone, with basalt at points between the West coast and the centre, of gabbro in the south-east, with a belt of gneissose rocks on its east seaboard and of quartz-porphyry in the south-west.
The so-called "iron roses" (Eisenrosen) of Switzerland are rosette-like aggregates of hexagonal tabular crystals, from fissures in the gneissose rocks of the Alps.
They comprise Seil, Easdale, Torsay, Luing and Shuna, and owe their name to the fact that they are composed mainly of metamorphic rocks, Easdale, Torsay and Luing being entirely slate, Seil mostly slate with some porphyrite in the north, and Shuna gneissose.