Vol. 65.] SUCCESSION AKOTJND PLYNLIUON AND PONT ERWYD. 513 



in beds from 2 to 4 inches thick, which alternate regularly with 

 about an equal thickness of shales. A few pieces of a flaggy brown 

 rottenstone occur among the debris, but I found none in situ. At 

 this point the section ends. 



The shaly group with grits yielded very few fossils, although these 

 were diligently searched for on more than one occasion. The beds 

 at the tarn (F. 52) were found to contain Monograptus holmi, 

 M. marri, and M. priodon ; and, in the neighbourhood of a coarse 

 grit about 100 yards down stream, Monograptus marri (?) and 

 M. turriculatus were obtained. 



The few scattered exposures, mainly of grit, on the moorland 

 east of the tarn are of no value, beyond proving that the group 

 extends for a considerable distance in that direction. 



I propose to describe briefly a third traverse eastwards, com- 

 mencing at Devil's Bridge and following for some distance 

 the road to Rhayader, for the reason that an extensive suite of 

 graptolites was obtained from the lower group of mudstones with 

 thin grits, thus making up for a deficiency in the traverses across 

 the hilly moorland-country on the north. 



In the description of the Rheidol gorge, below Parson's Bridge, 

 attention was called to the numerous undulations which have 

 affected the strata ; from this circumstance, and from the low pitch 

 of the folds, it results that no great thickness of rocks intervenes 

 between Devil's Bridge and Parson's Bridge, where the group 

 under consideration commences. The fossils obtained from various 

 places along this part of the gorge (see p. 503) agree closely with 

 those extracted from the base of the group in the Castell Valley 

 and elsewhere. 



The rocks of Devil's Bridge were diligently searched by Walter 

 Keeping, who was rewarded by the discovery of numerous forms 

 of Cladophora, subsequently described by Prof. Charles Lapworth. 

 He also obtained Monograptus turriculatus near the same spot. 

 I was not successful in discovering the locality from which he 

 obtained it, nor could I find any of the forms of Cladophora 

 recorded by him from the quarry near the bridge. The rocks 

 are of the pale-greenish mudstone type, with frequent laminated 

 grits up to 2 or 3 inches thick ; they extend, with but little 

 variation, for about 2 miles along the Rhayader road. 



Graptolites were found by the side of the road due south of 

 Bodcoll Mill, about two thirds of a mile from the Hafod Arms 

 Hotel (P. 53). They comprised the following species : — Mono- 

 graptus becJci(?), M. densus, M. marri (?), M. nodifer, M. nudus 

 var. variabilis (?), and M. turriculatus. 



About 500 yards up the road is a quarry in tough, flaggy, 

 laminated grits, which are interbedded with thin bands of smooth 

 blue shales (P. 54). One of these yielded to Mr. John Pringle 

 and myself a large number of excellently-preserved graptolites of 

 the following species : — Monograptus becki (c), M. crassus, M. cf. 



2n2 



