Vol. 65.] SUCCESSION AROUND PLYNLIMON AND PONT ERWYD. 529 



from the M. sedgwicki grits (or Gafallt Beds) of Rhayader agree 

 closely with those of the M. sedgwicki zone in the Fuches-wen 

 section and elsewhere, the rarity or absence of M. tenuis pointing 

 perhaps to an horizon somewhat above the base of the zone. 

 Probably, therefore, the underlying Conglomerate Group corresponds 

 to the lower part of the M. sedgwicki zone or may, perhaps, have 

 commenced at a slightly lower horizon. It is interesting to observe 

 that, while the Caban Group has an average thickness of about 

 1100 feet, the equivalent deposits in North Cardiganshire have a 

 thickness of about 110 feet, or only one-tenth as great. 



In the Rhayader district, while no subdivision of the Rhayader 

 Pale Slates was attempted, the graptolites obtained from them 

 agree closely with those of the Ystwyth Stage. 



The Pont Erwyd sequence presents a striking similarity to that 

 of the recently described Tarannon district. The most important 

 exception is the absence of the M. cyphus zone in that district, as 

 observed by Mrs. Shakespear. It is quite possible that, in the 

 disturbed tracts along which the lower beds occur, that zone may 

 have been faulted out. The zones of Dimorphograptus swanstoni 

 and Monograptus jimbriatus are undoubtedly equivalent to the zones 

 of M. atavus and M. communis respectively in the Pont Erwyd 

 district ; but, as at Rhayader, the leptotheca band has been included 

 with the zone of M. convolutus. The higher zones of the Dolgadfan 

 and Twymyn Groups are the equivalents of the Castell Group, 

 agreeing therewith in fauna, lithology, and thickness. The band 

 with M. ' discretus' ( = tenuis) and M. sedgwicki matches exactly 

 the one near the Fuches-wen quarry. 



The zone of Bastrites maximus was included by Mrs. Shakespear 

 with the Brynmair Group of the Tarannon Series : its fauna shows 

 affinities with the underlying and with the overlying beds. This 

 is also the case in the Pont Erwyd district; but there the litho- 

 logical character of the beds containing B. maximus unites them 

 more closely with the sedgwicki zone than with the overlying 

 Devil's Bridge Group. 



The Devil's Bridge Group coincides in the main with the Bryn- 

 mair Group, although the limits chosen for the two may not be 

 identical. The Cardiganshire beds seem to be more sandy than 

 those in the Tarannon district, and perhaps somewhat thicker ; but 

 the presence of Monograptus turriculatus, M. becki, M. runcinatus, 

 and Bastrites linncei, and the absence or rarity of forms such as 

 Monograptus marri, M. exiguus, and M. galaensis in both areas, 

 are strong points of resemblance. 



The Myherin Group, again, may be matched in the Gelli Group 

 of Tarannon, both being characterized by the prevalence of the last- 

 named graptolites. Also there is a suggestion in the latter district 

 of a twofold lithological division, corresponding with the Dolwen 

 Mudstones and the Blaen Myherin Mudstones. The most remark- 

 able difference is the absence in the Pont Erwyd district of forms 

 like Monograptus crispus, M. discus, and M. subconicus, which are 



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