Vol. 56.] SILT7EIAN SEQUENCE OE KHATADER. 87 



rapidly rises ; and along the crest, both above and below the road, 

 the rocks of the Zone of Monograptus convolutus (-Ad 2 ) form 

 a prominent ridge across the fields. They consist of blue-banded 

 hard bine mudstones and shales with brown sandy bands, and 

 contain the following fossils : — Monograptus convolutus, M. argutus, 

 M. crenularis (7), M . communis (?), Climacograptus normalis, and 

 Diplograptus sp. 



Beyond the ridge to the northward the ground falls towards 

 Rhyd-hir Brook, where a shallow glen has been cut through the 

 basement-beds of the Rhayader Pale Shales((7). These rocks 

 are highly cleaved in the banks of the stream, and graptolites are 

 extracted only with difficulty, but among others I have identified 

 Monograptus crassus, — a sure indication that we are somewhere 

 near the same horizon as that of the Tannery beds. 



(b) Supplementary Sections through Castle Hill and the 

 Eastern Portions of the District. 



(1) Castle Hill. 



The three previous sections may be fairly said to cover the 

 central portions of the Rhayader District. They show that the 

 general succession on the flanks of Gwastaden Hill and in the floor 

 of the Rhayader Valley is, to all intents and purposes, constant 

 from one end of the range to the other. At the eastern extremity 

 of Gwastaden the hill-slopes dive suddenly down into the valley 

 of the Black Brook, which may be taken as marking the line 

 that separates the central and eastern portions of the area under 

 description. The sluggish Black Brook winds through a Drift- 

 covered valley, in which no rock is visible. In the line of strike 

 of Gwastaden, however, rises on the north side of the brook a 

 small wooded knoll about \ mile in length, to which the name 

 Castle Hill is given. Through the crest of this the traverse 

 figured on p. 88 is taken (fig. 4). 



The exposures on the summit are poor ; and only a few ribs of 

 rock can be seen projecting through the short turf on the hillside 

 immediately south of the cairn. The rock consists of impure, thin 

 grey grits and soft shales. The grits seldom exceed 6 inches in 

 thickness, and are curiously striped with wavy black and brown 

 bands. The shales are soft ; sometimes flaggy, and all highly 

 cleaved. They are light brown, and present a peculiar speckled ap- 

 pearance. The beds weather brown as a rule, but are often coated 

 with iron-oxide. So different is this rock-group from anything 

 that has been seen in the Rhayader Valley, that one might doubt 

 whether these strata belong to the Gwastaden Group ; but as they 

 lie in the same line of strike as the Cerig Gwynion Grits, have the 

 same north-westerly dip, and, moreover, are underlain by the same 

 cleaved Blue-black Shale that is always to be found in a correspond- 

 ing position on Gwastaden Hill, one may safely assume that they 



