Vol. 68.] GEOLOGICAL STETJCTTJRE OP CENTKAL AVALES. 331 



Several of the principal axes of folding were detected by Keeping ; 

 but, as he had unfortunately misinterpreted the succession, the 

 anticlinal axes were regarded as synclinal and vice versa. As the 

 result of the discovery of graptolites near Newport and Cardigan ^ 

 he was able to prove that the gritty beds of those districts in- 

 dicated on the Geological Survey map as Lower Llandovery (h^) 

 should be assigned to the Bala. 



This discovery was confirmed many years later by Dr. C. A. 

 Matley,^ who examined the coastal tract to the north of Newport. 



The investigation of the Rhayader district by Dr. H. Lapworth^ led 

 to the discovery of lower rocks than had been suspected by Keeping, 

 for Lower Birkhill graptolites were found in abundance, and there 

 was strong probability that the strata underlying the fossiliferous 

 rocks were of some Bala age, although no palasontological evidence 

 was forthcoming to support this conclusion.* 



This area, however, afforded no sufficient clues for the solution 

 of the structure in the region to the west, inasmuch as it was 

 difficult to correlate the rock-groups described by Keeping with 

 those established on a firmer basis in the Rhayader area, and it 

 became therefore necessary to re-examine Central Wales in the 

 light of these discoveries. 



More recently Miss Drew & Miss Slater ' proved the extension 

 of the rocks of the Rhayader district along the strike south-west- 

 wards, at least as far as the district of Llansawel, although certain 

 differences in detail were observed. 



The most important results, in their bearing upon the rock- 

 succession of Central Wales, were obtained by Mrs. Shakespear ® 

 during the- investigation of the Tarannou area. It was then shown 

 that the Birkhill rocks were of almost insignificant thickness, as 

 compared with the equivalents of the Gala Group of the South of Scot- 

 land (Tarannon Group of Mrs. Shakespear). It would appear from 

 this that the representatives of the great thicknesses of rock which 

 Keeping discovered in Central Wales should be looked for chiefly 

 among the Gala rocks and not among the Upper Birkhill. 



Three years later the above results were confirmed in the Plyn- 

 limon district" in regard to the general rock-sequence, though the 

 succession there exposed does not include the upper portion of the 

 Gala Group. The presence of graptolitic beds of Upper Bala or 

 Upper Hartfell age was also demonstrated, and an enormouB 

 thickness of barren beds was found to intervene between them and 

 the base of the Birkhill Group. As these barren strata resemble 



1 Geol. Mag. dec. 2, vol. ix (1882) p. 519. 



2 Proe. Birmingham Nat. Hist. & Phil. Soc. vol. x, pt. ii (1897). 



3 Q. J. G. S. vol. Ivi (1900) p. 67. 



4 Dlplograptiis foliaceus was recorded by Dr. H. Lapworth from the rocks 

 some 4 or 5 miles east of Rhayader, indicating that in that direction the strata 

 may even be of Llandeilo age. See ' The Silurian Sequence of Ehayader,' 

 <^. J. G. S. vol. Ivi (1900) p. 96. 



^ Ibid. vol. Ixvi (1910) p. 402. 

 « Ibid. vol. Ixii (1906) p. 644. 

 ^ Ibid. vol. Ixv (1909) p. 463. 



