Abstract
The Eastern Gneisses of South Uist which overlie the Outer Isles Thrust consist of three units; the Corodale Gneiss mass which is interpreted as an igneous body; the Mashed Gneiss; a group of quartzo-feldspathic gneisses and an intervening zone of phyllonites, the Usinish Mylonites. The phyllonites appear to have been formed by intense deformation at the margin of the igneous body.