Vol. 68.] GEOLOGIC.IL STRUCTUKE OF CENTRAL WALES. 335 



were observed near the junction. These rocks are followed north- 

 westwards along the deep and picturesque valleys of the Pysgottwr, 

 Doethie, etc., by a great thickness of pale mudstones and shales of 

 Gala type, which appear to form an ascending series towards the 

 Towy watershed. I have traced a similar succession for several 

 miles to the south-west, past Cilycwm and Bwlch Trebannau, where 

 the red-stained rocks overlie the fossiliferous conglomerates de- 

 scribed by Sedgwick {loc. supra cit.). They were observed also by 

 Miss Drew & Miss Slater in the Llansawel area, and were traced 

 along the strike for some distance to the soutk-west, there fol- 

 lowing in the main the Cothi Valley. There is little doubt, conse- 

 quently, that Lower Birkhill rocks form a continuous outcrop from 

 Ehayader to beyond Llansawel. Their base-line probably occurs 

 some distance to the east, following in all likelihood a belt marked 

 on the maps by frequent outcrops of grits. In some cases these 

 grits have proved to be of Bala age (for instance, Taliaris), in others 

 (Llansawel) they form the base of the Silurian. Without further 

 -investigation, therefore, it is impossible to say at what precise point 

 the boundary should be drawn, but that it lies within this arenaceous 

 belt is clear. Along this belt, farther south-west, lie the massive grits 

 of Conwil, which appear to have yielded shelly fossils to Ramsay and 

 others, and were assigned to the Lower Llandovery. They bear a 

 close resemblance lithologically to those of Moelfre, near Llansawel 

 (base of Birkhill), and the sandstones forming the base of the Lower 

 Llandovery south of Llandovery. This range of grits in its west- 

 ward prolongation passes some distance to the north of Mydrim, 

 where the succession has been described by Mr. D. C. Evans,^ and 

 in the Geological Survey Memoir on the District around Carmarthen. 

 The rocks of that district lie on the northern limb of a great 

 anticline which passes near St. Clears and Whitland, and the gritty 

 •strata occupy either a high horizon in the blue mudstones (Upper 

 Bala ?) which succeed the Dicranogntptus Shales, or lie immediately 

 above them at the base of the Silurian. "What leads to some doubt 

 as to their precise stratigraphical position, is the fact that the Slade 

 and Redhill Beds (or blue mudstone group) along the Eastern 

 Gleddau contain beds of grit in the upper part similar to those at 

 the base of the Silurian. 



In the southern limb of the St. Clears anticline the first important 

 grit horizon lies at the base of the Silurian, and above the blue mud- 

 stone group. Whatever the precise age of the Conwil Grits, the 

 stratigraphical error of regarding them as the base of the Silurian 

 is comparatively unimportant, so far as regards the structure. 



Along the strike of these grits, farther west, Dicranograptus- 

 Shales and other Ordovician rocks were traced by Mr. D. C. Evans, 

 and similar rocks probably occur almost continuously between 

 Mydrim and Llanfirnach or Crymmych Arms (see PI. XXXIV). 

 The grit outcrop must, therefore, come to an end not far to the north 

 of Mydrim. 



1 Q. J. G. S. vol. Ixii (1906) p. 597. 



