Many of the rocks of this district have been already described by the late Mr. E. B. Tawney, in a series of papers entitled “Woodwardian Laboratory Notes,” contributed to the Geological Magazine a few years ago. The following additional remarks are to a certain extent supplemental to the above-mentioned papers, and are founded on a microscopical examination of a large series of rocks, collected some few years ago during a brief visit to the Lleyn district.