Vol. 65.] SUCCESSION AROUND PLYNLIMON AND PONT ERWYD. 519 



Monograptus galaensis, which has only been found in the latter ; 

 and M, proteus, which occurs rarely in the upper part of the Blaen 

 Myherin Mudstones. 



M. runcinatus, M. crassus, and the form referred to M. involu- 

 tus appear to be restricted to the Devil's Bridge Group, but are 

 represented by allied forms in the upper part of the Castell Group. 



It is clear, therefore, that the gradual lithological changes 

 between the base and the summit of the Ystwyth Stage are accom- 

 panied by gradual though quite distinct changes in the graptolitic 

 fauna — a fact which inspires some confidence in the correctness of 

 the stratigraphical sequence established in this great mass of folded 

 strata. 



Further, it seems certain that, given time and patience, it would 

 be found possible to map out the subdivisions of the Ystwyth Stage 

 in considerably more detail than I have done, and thus elucidate 

 more fully than I have been able to do the structures within the 

 area which they occupy. 



V. The "Western Limb of the Rheidol Anticlinorium. 



I have only made hurried traverses of the ground that lies 

 between Pont Erwyd or Devil's Bridge and Aberystwyth ; but 

 I believe that, at any rate from Devil's Bridge westwards, the 

 rocks everywhere belong to the Ystwyth Stage. In recent years 

 a narrow-gauge railway has been constructed from Aberystwyth to 

 the latter place, necessitating a large number of rock-cuttings along 

 the south side of the Bheidol Valley. For more than a mile to the 

 west of Devil's Bridge the strata exposed along the railway and 

 along the Aberystwyth road can be assigned to the Devil's 

 Bridge Group ; and the abundant graptolite of that group, Mono- 

 graptus nudus var. variabilis, was collected in a cutting near 

 Bhiw Fron Station, where it was associated with M. nudus and 

 M. resurgens (?). 



To the west of this group there is a great development of 

 massive blue-grey and dark -blue mudstones, with frequent shaly 

 bands weathering in bright colours. On account of the excessive 

 folding of the strata, I did not attempt to determine their exact 

 relationships ; but, from the knowledge of the sequence gained 

 to the east of Pont Erwyd and Devil's Bridge, and the general 

 structure of the country, I infer that the succession ascends 

 gradually towards the west. About Capel Bangor, near the Bheidol 

 Valley, some 6 miles west of Pont Erwyd, are several exposures 

 of highly-cleaved blue shales ; and in the railway-cutting on the 

 south are dark-blue mudstones and shales with a massive grey 

 speckly grit — an association of sediments which recalls the lower 

 part of the Rhuddnant Group. 



West of this, on both sides of the Rheidol Valley, the grits 

 referred to the Aberystwyth Grit Group commence. Near their 

 eastern limit they form beds of 2 to 4 inches in thickness, which 

 alternate with dark-blue shales. Such a type is seen near Loves- 



