Ixvi PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



the southern part of the PrincipaHty, to the structure of which also 

 I shall, in the sequel, have to direct your attention. 



This abnormal form of the geological surface, defined by our ideal 

 stratum, is accompanied, as might be expected, by irregular faults 

 in this part of the district. Towards the eastern extremity, however, 

 of this local E. and W. anticlinal, a distinct tendency to E. and W. 

 faults is easily traceable. 



If we produce the line of the Merioneth anticlinal to the S.W., it 

 just meets the south-western extremity of South Wales near to St. 

 David's, where rocks belonging unequivocally to the older non-fossi- 

 liferous rocks of the Cambrian series again show themselves at the 

 surface. It seems impossible to regard this phsenomenon otherwise 

 than as indicating the probable continuation of the above anticlinal 

 from Barmouth to St. David's along the bed of Cardigan Bay. Again, 

 along the margin of the Old Red Sandstone from Builth to Llandovery 

 and thence along the valley of the Towy, we have a strongly-marked 

 anticlinal ridge, the direction of which between Builth and Llandeilo 

 is exactly parallel to the ISIerioneth line, but becomes more westerly 

 in proceeding by Carmarthen. At the Longmynd we again find 

 non-fossiliferous beds, which Professor Ramsay* considers as iden- 

 tical, in all probability, with the old purple, green, and grey sand- 

 stones of Barmouth ; and superincumbent upon them, to the north 

 of Bishopscastle, we have other beds which he regards as the re- 

 presentatives on a small scale of the igneous rocks and slates of 

 Merionethshire. Also immediately to the north of Builth, beds 

 similar to these latter show themselves, and the line joining these 

 localities, near Builth and Bishopscastle respectively, coincides ap- 

 proximately with the strike of these beds at those places f, is nearly 

 parallel to the Merioneth line, and nearly coincident in direction 

 vrith the anticlinal just mentioned between Builth and Llandeilo. 

 Hence the whole line from Llandeilo to the Longmynd, very nearly 

 parallel to the Merioneth hue, may be regarded as the south-eastern 

 boundary of the great general synclinal trough which proceeds 

 southwards from the borders of Denbighshire to Pembrokeshire, as 

 the Merioneth line prolonged to St. David's may be considered its 

 parallel and opposite boundary. South-west of Llandeilo, the 

 boundary trends more and more from its south-westerly course till, 

 on the southern boundary of the trough, it assumes a direction 

 almost exactly east and west. 



Sections running from Aberystwith, or points south of that place 

 on the shore of Cardigan Bay, in a S.E. direction, and meeting the 

 Old Red Sandstone S.W. of Builth, exhibit many undulations, the 

 axes of which run nearly N.E. and S.W., the most important, 

 perhaps, being that which follows, as an anticlinal ridge, the course 

 of the River Teifi, as the similar ridge already described on the 



* Quart. Joum. Geol. Soc. vol. iv. p. 295. 



t This must be understood as stated in very general terms. The strike of the 

 Longmynd beds deviates from the direction spoken of in the text, by a consider- 

 able number of degrees. 



