Vol.65.] SUCCESSION ABOUND PLTNLIMON AND PONT ERWTD. 497 



sedgivicM, not a single species is common, although the form referred 

 to M. runcinatus in the lower band is certainly allied to the higher 

 form ; the palseontological break seems complete. This is the more 

 surprising, as the succession revealed in the quarry and in the 

 bank of the stream below shows a gradual lithological transition 

 from one set of beds to the other. The gritty material appears at 

 first as thin stripes in the massive flags ; these increase in thickness 

 at the expense of the muddy sediment, until they reach about 

 ■3 inches, the intervening mudstones being then of about the same 

 thickness. In view, however, of the palseontological evidence, 

 I have decided to separate off the strata with Monograptus 

 turriculatus from the underlying beds, and to include them in a 

 new and higher group. 



(6) Nant Meirch (see fig. 10, p. 498).— At the mouth of the 

 ravine which this stream has cut through the pale mudstone-group, 

 the Castell Fault is exposed in section. On its north-west or 

 upthrow side, a few feet of dark-blue shales with thin siliceous 

 flags yielded Monograptus atavus and Climacograptus tornquisti 

 (F. 30) : this, taken in conjunction with the lithological character 

 of the beds and the mode of preservation of the graptolites, 

 suggests that they belong to the upper part of the atavus-zone, 

 Crushed against the fault on the downthrow side is a band of dark, 

 fissile, almost papery shales weathering to an ashen white (F. 31). 

 The same band is visible also in an old leat on the opposite 

 side of the ravine, where it is sharply folded two or three times. 

 The graptolites obtained from it identify it as one of the 

 triangulatus-ha,nds of the Ptheidol Gorge. At this point the throw 

 of the Castell Fault is therefore 80 to 100 yards. The graptolites 

 were Monograptus atavus, ill. cf. difformis (v.c), M. revolutus, 

 M. triangidatus, Climacograptus rectangular is, and CI. tornquisti. 



The shale-band is succeeded by blue mudstones of a rather 

 pale colour, and 25 to 30 feet above it is a band of dark-blue 

 mudstones with a pale-green flag in the centre (F. 32). The 

 following graptolites collected from it prove it to be the leptotheca- 

 band ; Monograptus cf. argenteus, M. gregarius, M. leptotheca, 

 M. cf. triangidatus, Olyptograptus sinuatus, and Climacograptus 

 hugliesi. 



The remainder of the section up to the head of the ravine shows 

 a succession of bluish-green flags, with frequent bands of dark-blue 

 iron-stained shales from most of which graptolites were obtained. 

 The best exposures are along two water-leats — one about midway 

 up the sides of the ravine and the other near the top. These are 

 indicated by fringed lines in the sketch-plan (fig. 10). 



As this is the only section known to me which exposes the 

 whole of the Castell Group, I tabulate it here in full. It is possible, 

 however, that a certain amount of concealed faulting may affect 

 some of the thicknesses in the lower part, although it cannot have 



2m 2 



