Yol. 56.] THE SILURIAN SEQUENCE OF RHAYADER. 89 



are shaly representatives of the Gwastaden basement-beds. Further, 

 we have already seen that the grit- group thins out eastward, 

 and it seems but natural to suppose that, concurrently with an 

 attenuation of the whole of the division, the individual grit-beds 

 also should become attenuated. Fossils are extremely rare. Badly- 

 preserved specimens of the genus Climacograptus, however, may be 

 detected occasionally in the interbedded shales. The thickness 

 of the whole of the basement-grits here is about 100 feet. 



Carrying on the traverse down the slope of the hill, little or no 

 rock makes its appearance on the exact line of section, but at the 

 sides of the path from Upper Bwlch to Little Castle impure grits 

 and speckled soft brown flags and shales are fairly continuously 

 exposed. In the absence of fossils, however, it is difficult to 

 make out the divisions recognized in the typical sections. From 

 some calcareous flags and shales in the old quarry immediately 

 above Upper Bwlch I have extracted 



Tentaculites anglicus (?) Salt. 

 Atrypa marginalis (?) Daim. 

 Ortkis elegantula. 

 testudinaria. 



Leptana transversalis. 



rkomboidalis, Wilck. 



Petraia elongata (?) Phill. 

 Climacograptus sp. 



The exact horizon of these beds is uncertain ; the foregoing 

 fossils, however, agree so well with those obtained from the 

 Gwastaden streams that there is little doubt but that the rocks 

 belong to the Dyffryn Flags (Ab). 



From the foot of the hill the section follows a small stream 

 draining into Rhyd-hir Brook. The whole of the low-lying ground 

 in this area is thickly covered with Drift ; and the stream shows 

 no rock-exposures until we are within 100 yards or so of the 

 ravine at the bottom of the valley. Here a thickness of about 

 50 feet of scarlet and orange-weathered flags and shales of the 

 Monograptus-tenuis Zone (Ac x ) has been washed bare. The 

 nags reach about 2 inches in thickness, and are blue or grey in 

 colour. The blue shales are blue-banded, weathering occasionally 

 to ashy-white. 



I have doubtfully recognized Monograptus tenuis in these shales ; 

 but the graptolites are badly preserved, and cannot be identified 

 with certainty. The dip varies from 90° at the southern end of 

 the exposure to 29° north-westward at the northern end. The strike- 

 fault, mentioned as passing through the Esgair-rhiw section (p. 86), 

 crosses the stream immediately above the exposure ; and it is prob- 

 able that the proximity of this fault is responsible for the beds being 

 tilted up at so great an angle. 



In Ehyd-hir Brook itself, south of the road-bridge, occurs an 

 exposure of the overlying group. About 150 feet below the bridge 

 a few feet of the Zone of Monograptus fimbriatus (Ac 3 ) is 

 shown. The shales yield Monograptus triangulates, M. communis, 

 Climacograptus rectangularis, and Orthoceras sp. 



In the same line of strike, about 300 yards to the westward, 

 the overlying Calcareous-Nodule Group (AdJ may be found in 



