I. Introduction. The outcrop of Carboniferous Limestone which is the subject of this communication extends from the valley of the Ewenny River about 3 miles east of Bridgend (Glamorgan) to the valley of the Ebbw River at Risca (Monmouth), a distance of about 19 miles from west-south-west to east-north-east. 1 It lies on the south-eastern margin of the South Wales coal-basin, and belongs in its western portion to the northern limb of the Cardiff-Cowbridge anticline (see map, fig. 1, p. 112). The area within which this outcrop is comprised was mapped by the officers of H.M. Geological Survey (Dr. A. Strahan, Mr. R. H. Tiddeman, and Mr. T. C. Cantrill), in the course of the re-survey of the South Wales Coalfield, and the results were embodied in the official maps and memoirs. 2 The present paper supplements the description of the Carboniferous Limestone Series given in those publications, in the light of recent researches on the Carboniferous rocks of this country and Belgium by the late Dr. A. Vaughan and others. The nature and significance of the broad variations shown by the Carboniferous Limestone Series in the extensive South Wales outcrops have been admirably defined by Dr. Strahan in the following words:— ‘The Carboniferous Limestone Series presented a succession of stages in development. The fullest development was exhibited in the southernmost occurrences, as regarded both the sequence and the thickness of zones. A second stage, showing an incomplete sequence and considerable attenuation, with indications of near-shore origin, was presented along parts of the