^26 MR. D. C. EVANS ON THE OBDOVICTAN [Nov. I906, 



of trilobites. They are also seen in Pantglas Quarry, in the 

 Dewi-f awr Valley. As they are followed westward they become 

 modified in character, being still ashy, but much less compact, so 

 that at Llanboidy they have become soft, easily-dressed stone. 

 They have been quarried for building at Hafod and Pengacrfach, 

 and are well exposed in an old quarry in a garden at Maesgwynne. 

 At Pensarn, west of that locality, they have been extensively 

 quarried, supplying building-stone for the country round. They 

 are very fossiliferous here, yielding Asaplius tyrannus, Murch., 

 Ogygia Buchii,BT(mgn.,I{omalonotus, Trinucleus, and graptolites.etc. 

 They have also been quarried in the wood south of Fro wen. At 

 Llanboidy and westward they are immediately overlain by black 

 shales crowded with Diplograptus, associated with trilobites — chiefly 

 Trinucleus Lloydii, Murch., Tr.favus, Salt., Ogygia Bucliii, Brongn., 

 also a few specimens of Orthis, and large numbers of Lingula 

 granulata, Phill. 



In the quarry north of Canerw, already referred to, these beds 

 have been brought up against higher beds by a thrust from the 

 north. In a quarry north of Trehir (at Cefn-trehir) the ash is 

 quarried for road-metal ; but its relation to other beds here is 

 far from clear, owing to some obscure faulting in the immediate 

 vicinity. 



Along the northernmost outcrop there are no quarries, and the 

 presence of the beds, as well as their character, has to be inferred 

 from road- and ditch-exposures. Prom what evidence there is, it 

 is premised that the ash thins out and the shales become less ashy 

 and more argillaceous as they pass northward. The Llanglydwen- 

 Lane section, north of the bridge, proves this, as no hard beds 

 occur there at all, though the section is otherwise complete and 

 typical. 



(e) Llandeilo Group : Dicranograptus-~Beds. 



These rocks occupy a considerable portion of the area, both north 

 and south of the anticline. Exposures are numerous, and usually 

 the beds are highly fossiliferous. A very noticeable feature in the 

 character of the beds is, that they contain much iron-pyrites. This 

 is so abundant in some localities, that springs issuing from the 

 black shales assume the appearance and character of those found in 

 the vicinity of coal- and iron-mines. 



(1) South of the Anticline. — The passage upwards from the 

 Asaphus-tyr annus Beds is nowhere to be observed on the south of 

 the anticline, but the Dicranograptus -Beds themselves are seen 

 passing up in several localities. 



The beds are well exposed in the neighbourhood of St. Clear's, and 

 a good section is seen immediately north of the bridge across the 

 Tave, and in close proximity to the east-and-west fault mentioned 

 above (p. 614). The shales are here intensely black and somewhat 



