The Geology of the Country around Mynydd Rhiw and Sarn, South-Western Lleyn, Carnarvonshire

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Abstract
I. I ntroduction I n a recent communication to this Society ( Matley, 1928 ) I showed that the Pre-Cambrian Complex of south-western Lleyn is separated from the Palæozoic region to the east of it by the Boundary Thrust, which runs from the south coast of the Lleyn Peninsula at Wîg, on the east side of Aberdaron Bay, in a northerly to north-easterly direction as far as Porth Nevin, where it passes below the sea. The present paper is concerned with about 9 square miles of the Palæozoic country that lies immediately east of the Boundary Thrust. The strip of coast between Aberdaron Bay and Porth Neigwl forms the southern base of the area, which extends northward as far as the most northern exposures of the Sarn granite. Its eastern boundary runs northward from the western side of Porth Neigwl till it meets the Thrust. The lower ground of this tract is, for the most part, deeplycovered by glacial drift, but a broad stretch of high ground that extends from Mynydd Penarfynydd on the south coast northward through Mynydd y Graig to Mynydd Cefnamwlch, affords many exposures, chiefly of igneous rocks. Mynydd Rhiw, in the centre of this group of hills, rises to 994 feet O.D.; south of it lie Mynydd y Graig, between 700 and 750 feet, and Mynydd Penarfynydd, over 500 feet. The height of the granite hill of Mynydd Cefnamwlch is 598 feet. The streams are small and unimportant rivulets, except the Afon Soch, of which, however,