502 MR. 0. T. JONES ON THE HARTFELL-VALENTIAN [XoY. I909, 



are considerably weathered and the divisional planes of bedding 

 and cleavage have been picked out, which gives to the whole group 

 a more shaly aspect than it exhibits in the stream-sections. The 

 softer beds crumble away, forming a shaly scree ; while the harder 

 bands staud out as ledges, by means of which an individual band 

 can be followed easily by the eye for a considerable distance along 

 the face of the scarp, thus enabling the investigator to supplement 

 deficiencies in one part of the slope by observations in another part 

 some distance away. It is unnecessary, however, to describe this 

 section in detail. In a shale band about 35 feet above the base (F. 39) 

 Monograptus convolutus was obtained, together with the peculiar 

 Rastrites plileoides, Monograptus decipiens,M. limatulus, M. lobiferus, 

 and M. regularis, an assemblage clearly indicative of the Mono- 

 graptus-convolutus zone. The succeeding alternations of mudstones 

 and shales did not yield many graptolites, but lithologically they 

 recall the zones of Cephalograptus cometa and M. sedgwicJci. A 

 short distance beyond the brow of the escarpment, some pale greenish 

 mudstones with gritty bands form a low ridge : although no fossils 

 were obtained from them, it is almost certain that they belong to 

 the group with Monograptus turriculatus and its associates. 



Rheidol Gorge. — As stated on pp. 488 et seqq., the Eheidol 

 Group is followed south of Bryn-chwith by pale mudstones and flags. 

 At the point where they make their appearance the gorge becomes 

 impassable, but the beds can be examined on the steep hillside to 

 the west. About 200 yards to the south the gorge is accessible 

 once more, but the best sections are in the old leat above, where 

 the pale mudstones contain shaly graptolitic beds. On account of 

 the numerous sharp folds into which the strata are thrown, no 

 great thickness is exposed. 



The shaly portions yielded the characteristic Monograptus 

 convolutus and M. lobiferus in some abundance, together with 

 various other poorly preserved graptolites. The abundance of M. 

 convolutus in these beds suggests comparison with the lower part 

 of the Castell Group, as displayed in the Castell Valley and else- 

 where, which agrees with their stratigraphical position immediately 

 above the leptoiJieca band. 



In this part of its course the Rheidol follows the line of the 

 Castell Fault, which is visible in places in the bed of the river ; 

 but about 300 yards north of the Parson's Bridge it turns suddenly 

 southwards out of the fault. The strata below the bend dip at about 

 45° down the river, and consist of thickly-bedded blue mudstones 

 with a considerable proportion of gritty material which, on weather- 

 ing, imparts to the whole group the superficial appearance of 

 massive grits. For this reason, I was for a considerable time in 

 doubt as to the position of these beds in the sequence, upon which 

 depends the throw of the Castell Fault at this point. There are 

 some shaly bands among the harder beds, but they are so much 

 cleaved that it was only after prolonged search that they yielded 



