648 MISS E. M. R. WOOD ON THE [Nov. 1906, 



Central Walos. — Turning- now to Central Wales, we find that 

 the whole of the country lying west of the outcrop of the ' Tarannon- 

 Shale ' hand, as far as the coast of Caerdigau Bay, is shown upon 

 the Survey-maps to he occupied by strata coloured as of Lower 

 Llandovery and Bala age. The rocks of this great range of country 

 were first described in outline by Sedgwick ! (1847), and wero 

 divided by him into three groups : — the Aberystwyth Beds on the 

 west, the Plynlimmon Grits in the centre, and the Rhayader Slates 

 on the east ; but no boundaries were laid down between these 

 three divisions, and no fossils were recorded from them. 



The first important piece of palseontological work in theso beds of 

 Central Wales was done by Walter Keeping a in 1880. He adopted 

 in the main the three great geographical divisions of Sedgwick, 

 but divided the Plynlimmon Grits into two groups — a lower or 

 Metalliferous Slate group and a higher, the Plynlimmon Grits. In 

 the Aberystwyth Grits and Metalliferous Slates he discovered 

 numerous graptolites, which were identified by Prof. Lapworth and 

 shown to be characteristic of the Upper Birkhill Beds of the South 

 of Scotland. Keeping, therefore, was of the opinion that nearly 

 the whole of Central Wales was made up of rocks of Upper 

 Llandovery age, much folded and contorted, and he suggested that 

 the Plynlimmon Grits and underlying Rhayader Pale Shales which 

 overlay the Metalliferous Slates at Rhayader should be assigned 

 to tho Tarannon horizon. 



In the course of his researches in Central Wales, Keeping visited 

 the Llanbrynmair-Tarannon district and briefly described the 

 succession of the beds there exhibited. He recognized three main 

 groups : — 



(1) The Denbighshire Grits at the lop. 



(2) The Tarannon Shales, in which lie discovered somo 'obscure fragments 



of graptolites.' 

 (o) The Metalliferous Group, fully developed in the Pennant Valley and 

 containing characteristic Upper Birkhill fossils. 



He observed that, in this district, the ' Tarannon Shale ' is 

 separated from the Metalliferous Group by a zone of grits (Lower 

 Llandovery Grits of the Geological Survey), which he found difficulty 

 in correlating with the other main grit-masses of Central Wales, 

 namely, the Aberystwyth and Plynlimmon Grits. He regarded 

 them as a local and arenaceous upper part of his Metalliferous 

 Group, and ho agreed with the Survey in considering that an un- 

 conformity existed between them and the Tarannon. 



The most important recent work on the beds of Central Wales is 

 that by Dr. Herbert Lapworth,' 1 who in 1899 discovered in the neigh- 

 bourhood of Bhayader, following on local Bala rocks, three; distinct 

 stratigraphical groups — (I) tho Gwastaden, which he assigned to 

 the Lower Llandovery, (2) the Caban to the Upper Llandovery, and 

 (IJ) the Rhayader Pah; Shales to the Gala-Tarannon. His Lower 



1 Quart. Journ. Geol. Soo. vol. iii (1847) pp. 152-53. 



2 Ibid. vol. xxwii (I SSI) pp. 1 U.etc. 



3 Ibid. vol. lvi (l'JOO) pp. 67, etc. 



