Vol. 65.] SUCCESSION AROUND PLYNLOION AND PONT EEWYD. 503 



graptolites sufficiently good to fix the age of the group thereby. 

 One of these bands (F. 40), below a disused mine-building 60 yards 

 south of the bend in the river, yielded : — Monograptus convolutus, 

 M.decipiens(l), M. lobiferus, and Climacograptus scalaris (1). The 

 presence of the first is sufficient to assign fairly close limits to 

 the band. Another highly-cleaved bed of ferruginous shales forms 

 the base of a bluff about 70 yards down the river, and is separated 

 from the foregoing by about 80 feet of strata. It strikes across 

 the river and up the eastern bank, where the beds are less cleaved, 

 and yielded the following species (F. 41) : — Monograptus decipiens, 

 M. harpago, M. cf. involutus, M. sedgwiclci, Orihograptus cyperoides, 

 and Climacograpius hughesi. It may, therefore, be referred to the 

 zone of M. sedgwiclci, but probably a few feet above the base of 

 that zone. The hard gritty mudstones higher up the river must 

 consequently be low down in the Castell Group ; and the throw 

 of the Castell Fault at this point is probably not much more than 

 30 yards, being, in fact, but little greater than the thickness of the 

 zone of Monograptus communis. 



The strata succeeding the higher graptolitic band dip down 

 the river as far as the Parson's Bridge. At this point the river- 

 channel becomes exceedingly narrow, being in many places not 

 more than 2 or 3 yards wide, and the bed has been worn into 

 large and deep potholes, which figure in most of the local guide- 

 books. Below the bridge the rocks are sharply folded into fairly 

 symmetrical anticlines and synclines with a low southerly pitch ; 

 and it is interesting to observe the manner in which the river- 

 channel follows one of these structural lines for a considerable 

 distance, then, suddenly breaking out of it, takes up another a few 

 yards away. For over 300 yards south of the bridge the channel 

 is cut along the crest of an anticline ; it then leaves the anticline 

 abruptly, and, after several small folds have been crossed, a well- 

 marked syncline exactly defines the course of the river for the next 

 200 yards. Below this point it sways now to one side, now to the 

 other, of the synclinal axis, but for nearly half a mile does not 

 deviate therefrom by more than a few yards. Then once more 

 breaking away at right angles, the river-channel falls into the line 

 of an anticline and maintains this position for a long distance to 

 the south. It follows, from the relation of the channel to the folds, 

 that no great thickness of rock is exposed along the river for at 

 least a mile below Parson's Bridge. The beds consist of flags and 

 shales, with some thin grits. In several places the shales yielded 

 graptolites, among which the following have been identified :— 

 Monograptus becki, M. involutus (?), M. jaculum (?), M. nodifer (?), 

 M. nudus var. variabilis (v.c), M. runcinatus (c), M. sedgivicJci, 

 M. turricidatus, Gly ptograptus serratus (?), Petalograptus palmeus, 

 and Climacograpius scalaris. 



This fauna agrees essentially with that of the beds following the 

 sedgwiclci zone in other parts of the district, and therefore must be 

 included with them in a higher group ; but it is impossible in this 



