Vol. 56.] SHALES OF THE WELSH BORDERLAND. 377 



the lowest beds exposed in the quarry at Coed Mawr (see Tnble I, 

 col. G 1 , p. 378). This quarry has been worked across the strike of 

 the beds, and the rocks are seen to be dipping at 25°, parallel to the 

 direction of the main road ; the beds with Oyrtograptus symmetricus 

 form the floor of the quarry, and are seen on the dip-face. 



A noteworthy feature in the fauna is the abundance of Mono- 

 graptus dubius, Suess, while M. priodon (Bronn), so common in the 

 earlier horizons, is rare. In the north-eastern part of the district 

 the zone is not more than 50 feet thick, but where the flagstones 

 are developed it attains a thickness of fully 200 feet. 



(4) Zone of Cyrtograptus Linnarssoni, Lapw. — The 

 soft shales which form the base of the quarry at Coed Mawr, 

 and are crowded with C. symmetricus, are succeeded by a series 

 of hard calcareous flagstones with limestone-concretions, which are 

 exposed in vertical section for a thickness of 20 feet. Fossils may 

 be found throughout this thickness of rock by diligent searching, 

 but are most abundant in a band 3 feet above the C. -symmetricus 

 zone, and again 10 feet higher up ; the same graptolites are found 

 in these two bands, but this fauna is quite distinct from that of 

 the softer shales forming the floor of the quarry. The hard flag- 

 stones are deeply weathered at the summit of the quarry ; they 

 contain the graptolites enumerated in Table I, col. G 2 (p. 378). 

 These beds represent, then, the base of the O.- Linnarssoni zone. 



The only other exposure of this zone is the excellent section seen on 

 the Rhayader road, north-east of Builth Road Station. The road 

 cuts the beds obliquely across their strike ; the beds at the extreme 

 southern limit should probably be included in the C .-symmetricus 

 zone, since they have yielded that zone-fossil, but these are at 

 once succeeded by higher beds, with a fauna in all respects similar 

 to that found in Coed Mawr Quarry. The beds consist of hard 

 calcareous shales, with large limestone-concretions ; they have 

 yielded the forms enumerated in Table I, col. H (p. 378). Orthoceras 

 subundMlatum, O. primcevum, O. striato-punctatum var. originate, 

 Barr., and Cardiola interrupta are also abundant. The rarity of 

 Monograptus priodon and the incoming of M. Flemingii charac- 

 terize this horizon. The C .-Linnarssoni zone appears to be about 

 200 feet thick. 



It is succeeded by a considerable thickness of coarse unfossiliferous 

 flagstones ; these are exposed at the end of the Rhayader road- 

 section in an old quarry ; for some little distance along a new road 

 leading to Pencerrig ; and in a field north of Castle Crab. They are 

 also exposed on the south-west, along the road to Llanafan-fawr, 

 just above where it crosses the railway-line, and in the railway- 

 cutting itself. This band of flagstones is of great interest, for as it 

 bends round south-eastward it overlaps the lower members of the 

 series, so that eventually only a small part of the Monograptus- 

 riccartonensis zone and the Cyrtograptus-Murchisoni zone come out 

 below it. 



