514 MR. 0. T. JONES ON THE HAItTFELL-VALENTIAN [Nov. 1909, 



involutus (c), M. jaculum, M. nodifer, M. nuclus var. variabilis 

 (v.c.) and another variety, M. runcinatus, M. turriculaius, Olypto- 

 graptus cf. tamariscus, Petalograp>tus palmeus, P. (? ) sp., and Clima- 

 cograptus scalaris. The beds dip E. 25° S. at 70°, but a few yards 

 farther west they turn over in a sharp anticline. 



Between this locality and The Arch, 2 miles from Devil's Bridge, 

 graptolites were obtained from red-stained shales at several points 

 by the roadside. Thus, about 200 yards down the road from the 

 Arch (P. 55), Mr. Pringle and I found Monograptus intermedins, 

 M. lobiferus (?), M. nodifer, M. cf. nudus, M. nudus var. variabilis 

 (v.c), and Climacograptus scalaris (1). 



Beyond The Arch, where the road begins to descend towards the 

 valley of Nant Peiran, the rocks consist mainly of blue mudstones 

 with occasionally a few gritty bands. Lumps of flaggy rottenstone 

 are conspicuous among the debris for some distance on both sides 

 of The Arch. 



Near Pwll Peiran Bridge is a quarry by the roadside, in hard 

 blue-grey, rather massive mudstones ; but no fossils were discovered 

 in tbem. The gritty bands disappeared somewhere between this 

 point and The Arch. 



There are very few exposures along the road to the east of this, 

 until the steep descent towards Cwm Ystwyth School is reached, 

 where there is a good section in smooth, dark-blue, thickly-bedded 

 mudstones, weathering in rusty-brown shades and frequently 

 showing concentric orange-red and vermilion stains. Bands of 

 soft shales occur among them, but all are astonishingly contorted 

 and so intensely cleaved that it was almost a hopeless task to search 

 for graptolites in them. However, their marked lithological cha- 

 racters and mode of weathering render their identification with the 

 blue-black mudstones at the head of the Myherin and Bhuddnanfc 

 Yalleys almost a certainty. 



About half-a-mile along the road, just beyond the Post Office of 

 Pentre Briwnant, is a small exposure of highly- cleaved smooth 

 shales, which led me to expect the grit-group hereabouts. In this, 

 however, I was disappointed ; for the next exposure, less than 

 half-a-mile to the eastward, was of blue-grey mudstones, probably 

 below or about the base of the dark-blue mudstone-group. I 

 followed the Ystwyth Yalley for nearly a mile beyond the Cwm- 

 Ystwyth Lead-works in the hope of seeing the grits, and perhaps of 

 finding higher beds than are exposed in the Rhuddnant section, but 

 only found blue-grey and dark-grey mudstones like those towards 

 the upper part of the Myherin Yalley, and therefore a considerable 

 distance below the base of the grit-group. 



The district around Pentre Briwnant is, however, extremely 

 faulted, as is indicated by the complicated network of metalliferous 

 veins which have been worked there for several centuries. 



A better idea of the sequence in the higher part of the Ystwyth 

 Group may be obtained by leaving the road at The Arch, and 

 following a cart-track over the hills to the north. Por about three- 





