Vol. 65.] SUCCESSION AROUND PLYNLIMON AND PONT EEWID. 491 



which, as will be shown on p. 503, form part of the overlying 

 {Jastell Group. The section here closely resembles the upper part 

 of the one that has already boen described in detail. There are, 

 however, two bands of calcareous nodules, including about 5 feet of 

 black shales, near the base of the shale-group. Above these are 

 two prominent shale-beds separated by massive flags of a paler 

 colour. These two contain the characteristic graptolites of the 

 triangulatus bands. A few feet above the upper is a thin band of 

 flaggy shales, containing abundance of Mesograptus magnus and 

 Monograptus communis var. The succeeding flags, which are paler in 

 colour, are seen to a thickness of about 20 feet, when the section is 

 concealed for a space by debris and fallen blocks. This deficiency 

 is made up for by the exposures along the old leat, which has been 

 cut in the side of the hill some distance above the river-level. There 

 the leptotheca band with the pale-green flag at the centre yielded 

 its characteristic fossils ; moreover, the magnus band (F. 19), with 

 liastrites approximates, Monograptus argutus, M. communis var., 

 M. gregarius, Mesograptus magnus, Petalograptus cf. minor, and 

 Glimacograptus rectangularis, occurs at about the same distance 

 from the leptotheca band as in the main section. Below that again 

 is the higher of the triangulatus bands ; the lower band was not 

 identified with certainty in the leat, but as the beds roll over 

 hereabouts, probably the section does not descend to that level. 



If now we return to the point where we commenced our exami- 

 nation of the gorge, and proceed up the river from the highest 

 grit band exposed there, we find precisely the same general sequence 

 of rock-types ; but, owing to the prevailing cleavage and to the 

 frequent belts of sharp folding, fossils are more difficult to obtain, 

 and the thicknesses cannot be accurately determined. It is unneces- 

 sary to do more for this section, than to mention a few important 

 points. 



About 100 yards below the point -where the Llewernog falls into 

 the Rbeidol, near the Hotel, a thin bed of shale (F. 20) yielded 

 abundant well-preserved specimens of Monograptus atavus(?), M. 

 rheiclolensis, together with M. sandersoni, Orthograptus mutabilis, 

 Ghjptograptus tamariscus (?), Glimacograptus hughesi, CI. scalaris 

 var. normalis (?), and CI. tbrnquisti. About 50 feet higher is a layer 

 of calcareous nodules which introduces the black-shale group, at the 

 top of which lies the leptotheca zone followed by the pale mudstones. 

 The sides of the gorge are here precipitous and inaccessible, but 

 the characteristic fossils of the magnus and leptotheca bands were 

 collected from some cleaved beds on the top of the cliff. In the 

 latter Monograptus cf. argenteus was found in considerable numbers ; 

 the green flag also occupied its usual position at the centre. 



The loop of the gorge which lies nearest the main road affords 

 another confirmatory section. The calcareous nodules are seen 

 there also, and some distance below them were found good speci- 

 mens of Monograptus rheiclolensis. These rocks are, however, 



