620 ME. D. C. EVANS ON THE OEDOVICIAN [NoY. I906, 



(2) North of the Anticline. — The Didymograptus-Murchisoni 

 Beds are, however, well developed on the north of the anticline,. 

 and form a definite and useful stratigraphical landmark. 



Tbe series is made np of three members. The lower member 

 consists of black mudstones which, on weathering, become 

 fawn and cream-coloured, easily distinguishable from the underlying 

 D.-bijidus Beds. Incipient striping is observable in the very lowest 

 beds, but this characteristic becomes more pronounced towards the 

 top. The beds are crowded with ' forked ' graptolites ; but, with 

 one solitary exception, no other kind of fossil has been found. The- 

 black mudstones in Frowen Dingle have yielded one trilobite-tail 

 which probably belonged to an Ogygia. The graptolites include Didy- 

 mograptus Murchisoni, Beck, and D. Murchisoni var. geminus, His. 



The middle member is made up of several beds of blue ash, 

 with interbeddings of ashy shales. Neither ashes nor shales 

 have yielded any fossils. 



The upper member consists ofstriped flags, and thickly-bedded 

 arenaceous mudstones, rather highly cleaved. The fossils, and 

 even the bedding, are sometimes obscured by the cleavage ; but 

 the beds are, as a rule, readily recognized from their characteristic 

 texture and striping. Under normal conditions fossils are abun- 

 dant and typical, and include : — Dklymograptus Murchisoni, Beck ; 

 D. Murchisoni var. geminus, His. ; D. cf. indentus, Hall ; D. 

 foliaceus, Murch. ; D. cf. dentatus, Brongn. ; Climacograptus ccelatus* 

 Lapw. ; Siphonotreta micula, M'Coy ; Lingida cf. attenuata, Sow., etc* 

 These beds enter the district at Melin-ricket, where they are 

 well exposed near the mill. The upper and middle members are 

 clearly shown in a quarry, where the beds are vertical. The lower 

 division is seen in a garden adjoining the quarry on the south, 

 where they pass down rather abruptly into Didymograptus-bifidus 

 Beds with a fair abundance of typical fossils. 



They are again seen in the wood on the west of the valley, and 

 in a quarry on the brow of the hill above. Only the upper division, 

 however, is exposed here. From this quarry their debris is trace- 

 able to a point a little south-west of Bwlch, where they are cut off 

 by a fault and thrown southwards, again appearing in the dingle 

 north-east of Cefn-crwth. Passing under the house at that place 

 they appear in a quarry in the rickyard, where typical fossils are 

 abundant. They run west along the ridge, and are again well seen 

 in the Dewi-fawr Valley, a third of a mile south of Mydrim. On 

 the eastern side there are several good sections, but presumably 

 only the upper division is again seen. In Pant-yr-hendref Quarry 

 opposite, these beds are merely uncovered by quarrying operations in 

 higher beds. Westward from here again they run along the crest 

 of the ridge, cropping out on the Salem lload south of Oernant 

 and make for the Gynin Valley, but before reaching it they are cut 

 out by a fault which brings D.-bifidus Beds against Dicranograptus- 

 Beds. On the hill south of Castell-gorfod they are again seen in 

 quarries and otherwise, but here and along the ridge westward 



