The mass of iron pyrites worked at Cae Côch, Trefriw (Carnarvonshire) appears to be unique in some ways, so far as Great Britain is concerned, consequently its geological characters are of some interest. In August, 1917, Mr. T. C. Cantrill and myself visited Cae Côch, and as a result of what we saw I spent seven days in the following month in mapping geologically the area around the mine. In this work I received all possible assistance from Mr. E. J. Morris, F.G.S., the Manager of the mine, to whose energy the successful development of the deposit is due. To him I wish to return my thanks, and also to Dr. H. H. Thomas for petrological notes and to Mr. John Pringle for naming the fossils. The Geological Survey Map (Old Series 78 S.E.) published in 1852, shows a fault, or thrust, at the position of the ore-body, separating Bala Beds above from Contemporaneous Felspathic Trap (engraved F) below. The presence of pyrites is not recorded, nor is it mentioned in the accompanying memoir, probably because it is not seen at the outcrop owing to weathering. At the 4th milestone from Llanrwst the map shows a boundary-line, apparently a fault or thrust, bringing in the same ashes on the north, against Bala Beds on the south. In the memoir by A. C. Ramsay, 1 we read that north of the 4th milestone from Llanrwst volcanic ashes form all the high ground for about a mile in width, as far as Llanbedr.