I. Introduction. One of the most conspicuous features upon a map of Wales is the great south-western extension of Carnarvonshire, known as Lleyn. Owing partly to difficulty of access, and partly to its thick mantle of Glacial Drift, this large area has never received the attention that it deserves, and is still somewhat of a terra incognita to geologists. The igneous rocks alone have found an investigator worthy of their interest, and it is to the researches of Dr. Harker, as embodied in his classic work ‘The Bala Volcanic Series of Carnarvonshire,’ that we turn for the best account of the petrology and of the general structure of the region. The sedimentary rocks have not been so fortunate, and despite the excellent foundation provided by the work of H.M. Geological Survey, the information collected during the journeys of E. B. Tawney, and the promising beginning made in the Aberdaron district by Dr. C. A. Matley, the age and relations of the rocks over the greater part of the country are still very obscure. It is hoped that the following account of the results obtained during the investigation of a small portion of this great promontory will show how promising is the outlook for research. Towards its extremity Lleyn expands southwards into two promontories, separated by the great bay of Hell's Mouth. The eastern and smaller of these is the St. Tudwal's Peninsula of the present paper. The area here described comprises all the peninsula situated south of the River Soch,