318 PROF. A. H. COX AND MR. A. K. WELLS ON THE [vol. Ixxvi, 



thicker than along the Cader-Idris-Arenig line. As in the other 

 localities mentioned, the succession is essentially arenaceous below 

 and argillaceous above. Lithologically similar beds occur at 

 approximately the same horizons in South Wales and at Arthog ; 

 but the differences in thickness are very marked, and the actual 

 details do not correspond exactly. In North Pembrokeshire, the 

 Trefgarn Volcanic Series 1 possibly represents the Lower Acid 

 Volcanic Series ; but the paheontological evidence is as yet incom- 

 plete, and the two series are chemically dissimilar: that at Arthog 

 being essentially rhyolitic, while the North Pembrokeshire volcanics 

 are keratopbyres ~ of intermediate composition. 



The junction of the Arenig and Llanvirnian Series in South 

 Wales, as in the other districts considered, is among shales or 

 slates, and is not marked by any great lithological change. The 

 * China-stone ' Ashes near the base of the Lower Llanvirnian at 

 Arthog have their exact equivalents at the corresponding horizon 

 in South Wales (for references, see p. 272) ; while the ashy grits of 

 Bryn Brith appear to be at about the same horizon as the ashy 

 sandstones which are intercalated in the lower half of the JBiJichis 

 Slates over parts of Carmarthenshire. 3 



The JBifidus Slates are overlain by volcanic rocks in both dis- 

 tricts, the Cefn Hir Ashes and Lower Basic Series in the Cader- 

 Idris district, the rhyolitic rocks of Abereiddy 4 and Ramsey 

 Island in Pembrokeshire. There is no doubt that volcanic activity 

 broke out simultaneously in the Arenig, Cader-Idris, Shropshire, 

 and Pembrokeshire districts ; the horizon at which the volcanic 

 rocks begin is in all cases richly fossiliferous. Yet the lavas in 

 North Wales and Shropshire are basic to intermediate in com- 

 position — spilites and andesites ; while those of Pembrokeshire 

 (Abereickly and Ramsey Island) are rhyolitic. 



VI. Summary and Conclusions. 



South of the Harlech Dome, between the Mawddach Estuary and 

 the northern flanks of Cader Idris, a succession has been established 

 -extending from Upper Cambrian (Lincjula Flags) to beds of 

 Llandeilo age. 



The Lingula Flags — Ffestiniog and Dolgelley Beds — show a 

 typical development, and are succeeded by Tremadoc Slates. The 

 Tremadoc Slates contain no volcanic rocks of any kind. On the 

 contrary, their development corresponds closely with that seen 

 north of the Harlech Dome in the Arenig and Tremadoc areas, 

 and most of the subdivisions that have been distinguished north 

 of the Dome can be recognized again in the south. 



1 H. H. Thomas, in c The Country round Haverfordwest : Geology of the 

 South Wales Coalfield, pt. xi' Mem. Geol. Surv. 1914, p. 21. 



2 Ibid. ; also A. H. Cox, Q. J. G-. S. vol. lxxi (1915) p. 314. 



:i T. C. Cantrill & H. H. Thomas, ' The Country round Carmarthen : Geology 

 of the South Wales Coalfield, pt. x ' Mem. Geol. Surv. 1909, p. 25. 

 4 A. H. Cox, op. supra cit. p. 302. 



