■514: DR. C. A. MATLEY OX THE [Oct. I913, 



23. The Geology of Bardsey Island. By Charles Alfred 

 Matley, D.Sc., F.G.S. ; with an Appendix on the Petrography, 

 by John Smith JFlett, M.A., D.Sc, F.R.S., F.G.S. (Read 

 February 26th, 1913.) 



[Plates XLIX & L.] 



Contents. 



Page 



I. Introductory Remarks 514 



II. History of Previous Researches 515 



III. Stratigraphy of the Pre-Cambrian Rocks 516 



IV. Post-Movement Intrusions , 525 



V. Stratigraphical Summary 525 



VI. Glaciology 526 



VII. The Post-Glacial Raised Beach (?) 528 



VIII. Appendix on the Petrography , 528 



I. Introductory Remarks. 



Bardsey, an island a mile and three-quarters long, and covering 

 •approximately an area of 500 acres, is the westernmost fragment 

 of North Wales, and lies about 2 miles from the promontory of 

 the Lleyn (Western Carnarvonshire), from which it is separated by 

 Bardsey Sound. Its Welsh name, Ynys Enlli, or the Island 

 of the Current, has reference to the strong tides that sweep 

 ■between its shore and the mainland. 



Mynydd Enlli, the dominant topographical feature of Bardsey, 

 lies in the north-east of the island, and forms a bold ridge running 

 nearly due north and south. It rises precipitously above the eastern 

 line of sea-cliffs to a height of 548 feet O.D., and descends more 

 gradually on its western side to about 100 feet O.D., where it 

 passes into cultivated drift-covered ground. At the harbour of 

 Henllwyn, the larger and northern part of the island in which this 

 'mountain' is situated is connected by an isthmus, at its narrowest 

 .■some 35 yards wide, with the smaller southern portion, which 

 consists of a rather flat tract having an elevation of about 50 to 

 60 feet O.D. ; on this portion the lighthouse is built. The southern 

 •extremity has been so much dissected by sea-erosion, that the 

 strip of slates between the heads of the opposing inlets of Ogof 

 Lladron and Ogof Diban is only 40 feet wide. 



Geologically, Bardsey has long been recognized as forming the 

 isolated extremity of the 18-mile-long strip of ancient rocks that 

 borders the western coast of the Lleyn from Kevin south-westwards. 

 The geology of the whole of this strip has been under investigation 

 by me for some time, and I hope to present my results at a not 

 far distant date. Meanwhile, an account of the structure and 

 rocks of Bardsey may be of interest to geologists, as affording an 

 ■epitome of the geological phenomena and yielding many of the 

 rock- types of the adjacent mainland. 



