I. Introduction. The coast of County Dublin is well known for the variety and interest of the rocks exposed along it, but there is probably no part of it more interesting than that in the neighbourhood of the village of Portraine, some 14 miles north of Dublin itself. Here occurs an inlier of pre-Devonian rocks which prove the occurrence in the district, during Bala times, of much volcanic action and of reef-building corals. The interest attaching to the rocks and the difficulty of interpreting their meaning are increased by the great amount of disturbance which has gone on and has given rise to extensive developments of conglomerate-like beds. Volcanic rocks, which are certainly of the same age as those at Portraine, and coarse conglomerates, occur on Lambay Island, some 3 miles out to sea, and we hope to give some account of these at no distant date. We were led to examine the beds at Portraine from a wish to compare them with the Bala Beds occurring in the neighbourhood of Kildare, which we described last year. The Portraine area has, perhaps, hardly received so much attention from geologists as it deserves. The first description is that given by H. B. Medlicott; a brief account is given by G. V. Du Noyer, in the Memoirs of the Irish Geological Survey, the memoir including a list of some fifty fossils determined by W. H. Baily; later Prof. Cole writes of the area in his articles on ‘County Dublin, Past and Present’