492 MR. 0. T. JONES ON THE HAKTEELL-VALENTIAN [JSTov. I909, 



difficult of access, owing to the precipitous character of the walls 

 of the gorge. 



This concludes our examination of the Eheidol Group in the 

 region around and to the east of Pont Erwyd ; there remains only 

 the northern belt along the west side of the Eheidol Valley. 



The general sequence in that area tallies so exactly with that 

 observed in the other regions, that it is unnecessary to describe all 

 the sections, and I shall only refer to one which exhibits some slight 

 differences from those already described. This is found in and near 

 a small stream which flows past the farm of Gwenffrwd-uchaf, 

 about 2 miles north-west of Pont Erwyd. Opposite the southern- 

 most farm-building is an exposure of dark-blue shales and massive 

 mudstones which weather deeply in patches to a buff colour with 

 limonitic stains (F. 21). They yielded Climacograptus medius in 

 some abundance, together with Orthograptus vesiculosus and a form 

 which suggests Dimorphograptus extenuatus ; but the majority of 

 the fossils are indifferently preserved. I have not met with this 

 assemblage anywhere else within the district ; the lithological cha- 

 racter of the beds and their mode of weathering recall the higher 

 part of the Eisteddfa Group in Naut Fuches-gau. They probably lie 

 at the top of the acuminatus zone or at the base of the atavus zone. 

 The section in ascending order is continued up stream in soft dark 

 shales with grey stripes from an eighth to a quarter of an inch thick; 

 they yielded no fossils, but are not unlike the beds of the atavus- 

 zone. The pale mudstone group commences about 200 feet above 

 the beds near the farm, and immediately underneath them I obtained 

 Monograptus Jimbriatus. If the reference of the fossiliferous beds 

 to the atavus zone is correct, then either the succeeding strata must 

 be much attenuated in this district, or (as is more probable) some 

 beds are faulted out ; the rocks beneath the pale mudstone group 

 are greatly disturbed. 



An interesting exposure of the leptotlieca band occurs at the 

 entrance to an old quarry near the stream, about 300 yards north- 

 east of Gwenffrwd-uchaf Farm. It is almost the lowest bed in the 

 quarry, and is exceedingly rich in graptolites, among which Mono- 

 graptus cf. argenteus is especially abundant (F. 22). That form 

 seems, however, to have a very restricted range in the band : for it 

 is extremely difficult to find specimens unless the exact layer is 

 hit upon, when they can be obtained in plenty. The full list from 

 this locality is as follows : — Rastrites approximatus, Monograptvs 

 cf. argenteus, M. argutus, II. gregarius (c), II. leptotlieca, II. cf. 

 millipedia, II. cf. mirus, Orthograptus vesiculosus var. penna, 

 Glyptograptus sinuatus, 61. tamariscus, Glimacograptus hughesi. 



It is an interesting fact that the pale-green flag noted in several 

 other localities is found here also, occupying its usual position near 

 the centre of the graptolitic shales. 



