Yol. 56.] THE STLTJEIAN SEQUENCE OF EHAYADEE. 123 



The most easterly exposure that I propose to examine on the 

 fringe of these pale shales lies about a mile north-east of Cwm 

 Barn. It may be reached by following the road to Abey-Cwmhir 

 up the Drift-covered valley of the Dulas. One or two small sections 

 may be noticed on the hillsides, and in the stream to the north 

 of Cwm-gwr, but the above-mentioned exposure offers more interest. 

 It may be found in a small quarry on the east side of the Bwlch- 

 sarnau road, about 500 yards above the bridge. The rocks 

 are mainly soft, pale grey shales, interbanded with grits, and 

 weathering to a brilliant orange colour. The shales are quite free 

 from cleavage, and graptolites, which are abundant in certain 

 seams, may be readily identified. They are well preserved, and 

 consist of the following species : — 



Monograptus priodon. 



Holmi, Perner. 



Marri (?) Perner. 



■ Sedgwickii. 



Monograptus galaensis, Lapw. 



?iudus, Lapw. 



runcinatus. 



Betiolites obesus, Lapw. 



The beds seem to be folded on both sides of the valley. On the 

 west side the dip is north-westerly, while in the above quarry the 

 beds pitch at 55° east-south-eastward. 



This, then, completes our examination of the southern border 

 of the Rhayader Group. One remarkable peculiarity may be 

 noticed — the manner in which the fossil species die out from west 

 to east, giving place to new forms. From this it will be inferred 

 that in following the Bhayader fringe we are not tracing the same 

 bed or horizon, but ascending the series. A fault separating the 

 Gwastaden and Rhayader Groups might explain this, but the 

 basal line of the pale shales is not of the form that usually results 

 from a dislocation — it is too irregular and undulating. It seems, 

 therefore, natural to infer, along this line, the same set of conditions 

 as that which we have seen prevail at the base of the Caban 

 Group ; that is, an ascent in the group when the line of junction 

 between it and the underlying series is followed from west to east, 

 — the direct result of overlap produced by a sinking of the sea- 

 floor during deposition. It seems also more than likely, that, in 

 order to produce this particular overlap, the denuded Gwastaden 

 surface was slightly tilted up to the eastward, as shown in fig. 19 

 (p. 122). This is practically the same section as that figured on p. 116 

 (fig. 17). The Bhayader Pale Shales, however, are plotted in an 

 horizontal position, to reproduce the sea-floor conditions of Bhayader 

 times. An arbitrary thickness of rock between the Llanfadog and 

 Cwm Barn beds is assumed. It would seem, then, in order to bring 

 about the existing conditions, that the Gwastaden beds were elevated 

 to the eastward, previous to the deposition of the Bhayader Pale 

 Shales. The vertical heights in the section are greatly exaggerated: 

 in consequence of which the easterly tilt appears very much greater 

 than is likely to be the case. The real amount of elevation is 



