I. Introduction The Shetland Islands occupy a critical position in the Caledonian mountain area. They lie about 200 miles north-east of the post-Cambrian dislocations of Durness in Sutherland, the same distance north of the Dalradian outcrops of the Banffshire coast, and the same distance west of the metamorphic region of Bergen in Norway. The investigation of their tectonics and metamorphism is required in any synthesis of the Caledonian front. The island of Unst is the most northerly of the group. Its size is about 50 square miles. Its coastline reaches the great length of nearly 70 miles, and, though formed of cliffs usually hundreds of feet high, is for the most part readily accessible. Inland exposures are good, while even where peat partly obscures the solid rocks, as happens with some of the politic gneisses, outcrops are sufficient for the mapping of small divisions. During the field-seasons of 1929 and 1930, I mapped the island on the scale of 6 inches to a mile while a member of H.M. Geological Survey. Since then the material has been worked out at the University of Liverpool. In 1933 I spent three weeks in the island in order to test certain points that had arisen during the laboratory study. I desire to express my thanks to Sir John Flett, Director of the Survey, for permission to complete the Unst work and to present this account. Unst consists entirely of rocks that have undergone one or more metamorphisms under markedly different physical controls. The