Yol. 65.] SUCCESSION ABOUND PLYNLIMON AND PONT ERWXD. 511 



group of mudstones and shales which lie to the west. This is 

 illustrated very clearly in the Ehuddnant section described below. 



There is an admirable section through the upper part of the 

 mudstone-group in the ravine of Nant-y-Crwth, east of Blaen 

 Myherin. The rocks are soft, thickly-bedded, and of a dark-blue 

 colour, frequently exhibiting on weathering concentric rings of 

 orange-red and vermilion ; this is a characteristic feature of these 

 beds wherever they are well exposed. The shale-bands interbedded 

 with the mudstones weather in somewhat brighter tints ; most 

 of the graptolites occur in them. About 150 yards east of the 

 point where the section ends on the brow of the hill, coarse speckly 

 grits are exposed on the moorland, and are associated with dark- 

 blue smooth shales: among these latter are intercalated numerous 

 greyish-white siliceous bands, from half an inch to 2 inches thick 

 and of a fine texture. A single specimen of Monograptus turri- 

 culatus was found in a small exposure about 100 yards west of the 

 grit-outcrop, and therefore not very far below it. 



East of the grits lies an extensive moorland tract, traversed in a 

 north-and-south direction by ridges and hollows. A few exposures 

 of coarse grey grits, added to the debris of similar material lying 

 about, suggest that such rocks may extend for a considerable 

 distance eastwards ; but, from the stratigraphical point of view, it 

 is useless to pursue the section farther in that direction. 



This traverse has shown that there is a sequence of distinct rock- 

 types, arranged in ascending order from west to east. On the west 

 are the pale greenish flaggy mudstones with frequent grit-bands, 

 which extend uninterruptedly from Yspytty Cynfyn to the Myherin 

 Valley ; then follow along that valley higher strata of a somewhat 

 similar colour, but in which the mudstone-type is dominant and 

 the gritty bands are thin or absent. These form the greater part 

 of the valley ; but towards its upper end they become more shaly, 

 and are distinguished also by their soft smooth texture, dark-blue 

 colour, and their property of weathering in brightly-tinted con- 

 centric patches. Finally, on the high moorland which dominates 

 the valley on the east there occurs a very distinct lithological type, 

 consisting of coarse speckly grits associated with blue shales that 

 contain greyish-white siliceous bands. The section (fig. 12, p. 508) 

 is intended to illustrate this sequence, and to represent to a certain 

 extent the manner in which the rocks are folded ; it does not lay 

 claim to more than approximate accuracy. 



Although the strata are thrown into numerous undulations, there 

 is no difficulty in seeing, when on the ground, that the changes 

 described above take place in a series of considerable thickness. 

 They cannot be explained on the assumption of repetition by 

 folding and faulting of a small thickness of variable deposits. 



The group of regularly alternating mudstones and grits forming 

 the lower part of the stage can be recognized over a great extent 

 Q. J. G. S. No. 260. 2 n 



