I. I ntroduction . T he ancient parish of Ynyscynhaiarn lies along the northern shore of Cardigan Bay, at the mouth of the Afon Glaslyn, and forms the south-eastern corner of the county of Carnarvon. The area to be described in the present paper is the country represented upon the 6-inch Ordnance Survey maps of Carnarvonshire, Sheet XXXIV S.E. (Criccieth) and Sheet XXXIV S.W. (Portmadoc), with portions of Sheets XLII N.E. & N.W. and XXXIV N.E. & N.W. to the north and south of these. It includes portions of all four quarter-sheets of Sheet 75 of the 1-inch Geological Survey map. This area is a rectangular block of some 20 square miles, and includes the townships of Portmadoc and Tremadoc (now united as the Urban District of Ynyscynhaiarn) and the flourishing health-resorts of Borth-y-gest and Criccieth. It is traversed by the Cambrian Railway, between Barmouth and Pwllheli, and is approached either by that railway or by the London and North-Western Railway (through Bangor and Afonwen), or again by the narrow-gauge railway from Ffestiniog. For its scenery the district has long been famous, and there are few British districts which can show so great a variety of tide-flat, salt-marsh, sea-cliff, sand-dune, park, coppice, meadow, cornland, bog, moor, lake, and mountain, even over an area of many times its size. The bigger hills of the district, rising abruptly almost to 1000 feet above sea-level, occur in the north and east; and while, both geologically and topographically, Tremadoc may he said to lie among the Snowdonian