114 Mli. F. DIXEY AND PUOF. T. F. SIELY ON THE [vol. Ixxili, 



immecliatel}" north of that examined by Prof. Delepine near Llan- 

 hany, and also in the outcrop farther west ; but D., has not been 

 recognized anywhere within the district here described. 



In our description and interpretation of the Hthological succes- 

 sion, we are glided very largely b}' Mr. Dixon's luminous researches 

 on the Carboniferous Limestone of Gower. The joint paper by 

 him and the late Dr. A. Vaughan, dealing with that arj^a, will be 

 referred to as the ' G-ower paper.' ^ Several lithological terms 

 applied by Mr. Dixon to special rock-ty^jes, and defined in the 

 Gower paper, will be employed. 



It would be difficult to exaggerate our indebtedness to the 

 publications of the Geological Survey : to the maps,=^ for providing 

 an invaluable basis for our zonal mapping, and to the memoirs 

 for mueli information and guidance. We wish to express our 

 thanks to Dr. A. Strahan, Director of II. M. Geological Survey, for 

 permission to examine specimens in the Survey collection, and for 

 facilitating access to the original field-maps of the district. 



II. Structuee iND Physical Features or the Outcrop. 



On the east, the deep gorge through which the Ebbw River 

 escapes from the coalfield at Pisca marks one extremity of the area 

 dealt with in th"s paper. At that point the outcrop of the Carbon- 

 iferous Limestonj, followed from the Avest, has just swung into a 

 direction a little east of north, to persist in a much attenuated 

 form along the eastern margin of the coalfield. In the west, near 

 Puthin, St. Mary Hill, and Penlline,^ the Carboniferous Limestone 

 disappears beneath a cover of Keuper and Lias, to rea])])ear south 

 and west of Bridgend, and outside the area now under considera- 

 tion, in extensive outcro])s around St. Bride's Major and Porthcawl. 



Tlie Main Limestone forms an escarpment ridge wliich, although 

 much more pronounced in the eastern than in the Avestern part of 

 the outcrop, remains well-developed as far west as Llansannor. 

 Three rivers flowing from the coalfield breach this ridge. Named 

 in order from east to west, these are : the PhA^mney, which breaks 

 through in a deep, narrow valley at Machen, only 2 miles distant 

 from the Ebbw Piver at Risca ; the Taff, which floAvs in a notably 

 steep-sided, narrow gorge (PI. XIII, fig. 1) between Taff's Well 

 and Tongwynlais ; and the Ely, which crosses the limestone in a 

 much wider, shallower valley at Miskin. Farther west, the river 

 Da wen rises on the limestone near Llanharry, and Hows south- 

 wards between Llansannor and Penlline to Cowbridge. These 

 rivers traverse the country with a complete disregard of geological 



^ ' The Carboniferous Succession in Gower (Glamorganshire) " Q. J. G. S. 

 vol. Ixvii (1911) p. 477. 



- Particularly the 6-inch maps with geological lines, published in the case 

 of all sheets which include outcrops of Coal Measures. 



•^ The spelling adopted is that of the latest edition of the 1-inch Ordnance 

 Survey map. 



