Vol. 65.] SUCCESSION ABOUND PLYNLIMON 4ND PONT EKWYD. 507 



the nature of the folding which they have suffered. Thrown as 

 they are into a rapid succession of rather shallow folds with a low 

 southerly pitch, these rocks occupy the surface of the country for 

 many square miles, and it becomes a matter of some difficulty to 

 ascertain the relations of the various rock- types which this stage 

 comprises. 



In dealing with the Plynlimon and Pont Erwyd Stages, we 

 saw that the outcrops of the different groups were arranged in a 

 series of broad V's pointing southwards, so that in a general sense 

 newer rocks succeed the older in traverses from north-west to 

 south-east, from north-east to south-west, or from north to south ; 

 and one may reasonably assume that a similar order prevails among 

 the rocks of the highest stage. In order, therefore, to gain an 

 acquaintance with the disposition and characters of the later 

 rocks, the results of two or three traverses across the hilly 

 district lying east of the Pheidol Valley will be described. 



The first traverse (fig. 12, p. 508) was taken from the main road 

 about half a mile north of Yspytty Cyufyn, in an easterly direction 

 to the head of the Myherin Valley. On the east side of the road is a 

 steep grassy and gorse-covered slope, capped by a scarp of bare rock 

 dipping eastwards at 45°. The debris from the scarp accumulates 

 at its foot, and also tails down the slope, reaching in places nearly 

 to the road. The strata consist of alternations of pale-grey flags, 

 and numerous 1 to 3-inch bands of grit with curved black lamina?, 

 in this respect resembling the basal beds of the Ystwyth Stage in 

 the Castell Valley. It is possible, however, that the rocks under 

 consideration lie a short distance above the base. The debris of 

 the grits (F. 44) yielded Monograptus becki, M. cf. dextrorsus, 

 M. runcinatus, and M. sp. with a tenuis-like curve, together with 

 Dictyonema. 



In addition to the above, Mr. John Pringle and I discovered 

 several specimens of what have been pronounced by Dr. G. J. Hinde, 

 F.K.S., to be the jaws of annelids. He reports as follows : — 



'The specimens from the Tarannon of Cardiganshire . . . are the jaws of 

 Annelids of the same character as those which I worked at in 1879 and 

 subsequently from Canada and the Isle of Gotland, and also from the Wenlock 

 and Ludlow of the West of England ; see Quart. Journ. Geol. Soc. vol. xxxv 

 (1879) p. 370, pis. xviii-xx & -vol. xxxvi (1880) p. 368, pi. xiv; also Bihang 

 till K. Svenska Vet.-Akad. Handl. vol. vii, No. 5 (1882) p. 3 & pis. i-iii. 



' I can distinguish two forms In one there is a prominent anterior 



hook, followed by a series of small pointed denticles extending along the upper 

 margin of the jaw. In the second one there is a stout hook, followed, after an 

 interval, by three minute pointed denticles and then by a series of fiat-topped 

 denticles. They are, in matters of detail, different from any of the forms 

 described in the papers referred to above.' 



"West of the road rocks of a like lithological type are exposed, 

 dipping westwards, and, judging by the form of the ground, 

 I am disposed to think that an anticlinal axis must bring 

 up a considerable area of the Castell Group on the south side of 

 the Castell Fault. 



