632 ME. D. C. EVANS ON THE OKDOVICIAN [Nov. 1906, 



limestone. Fossils also occur, but it is very difficult to secure a 

 good specimen., owing to the extreme friability of the rock. 



At Dyffryn, a quarter of a mile to the north, however, the beds 

 are much stronger, and have yielded limestone for lime-burning, a 

 portion of the old kiln being still in evidence. The section exposed 

 in this old quarry is very instructive, as showing the true relation 

 of the limestones to the beds above and below. The several 

 alternations of calcareous bands with buff-yellow mudstones and 

 shales prove conclusively that the limestones belong to the beds 

 below, rather than to the slaty black beds above. 



West of this section the limestones are traceable up the dingle 

 towards Sarnau, but they completely disappear before that point is 

 reached. 



(y) The black slaty beds. — These form a belt of varying 

 width along the northern margin of the ground. They are highly 

 cleaved and intensely black, except that in some localities they 

 contain a good deal of what appear at first sight to be mica-flakes. 

 On closer examination, however, the regular triangular form of 

 these tiny bodies points to their organic origin, and it is suggested 

 that they are graptolite 'embryos,' or what afterwards became 

 siculaB. 



Although the beds are, as a rule, highly fossiliferous, they have 

 yielded little besides graptolites, and these, owing to cleavage, very 

 much distorted. Iron-pyrites is also very abundant. There are 

 abundant exposures all along their outcrop, and quarries have been 

 opened in them at frequent intervals — occasionally for building- 

 material, but much oftener for drain-rubble. 



They enter the district at Ehyd-aber-wern near Melin-ricket, and 

 are frequently seen up the dingle in the direction of Bwlch, Sarnau, 

 Penrheol, and Mydrim, at all of which places they are exposed in 

 quarries and otherwise. They are well seen on Llangarthgynin 

 Hill, near Trip Bridge, and along the road leading to Caerlleon Farm. 

 They are affected by much faulting north of Penlan, and their 

 junction with the brown-grey mudstones above has been thrown 

 northwards from near Trip Bridge to a point some distance north of 

 Melin-dol-goediog. 



They are much in evidence again at Melin-nant-yr-eglwys, where 

 they are considerably cut up by faults. Fossils are very abundant, 

 but distorted, in Melin-nant-yr-eglwys Quarry. The following 

 occur : — 



Dicranograptus rectus, Hopk. 

 Dicranograptus Nicholsoni (!) Hopk. 

 Biplograptus foliaceus, Murch. 



Climacograptus cf. tubuliferus, Lapw. 

 Climacograptus sp. 

 Glossograptus sp. 



The black beds frequently come to the surface on both sides of the 

 Fenni, as far as Llechglawdd, near which place they are abundantly 

 exposed, but pass up into higher beds — the brown-grey mudstones — 

 in the immediate neighbourhood. 



At Cwm-felin-mynach they are so much cleaved that both bedding 

 and fossils are obliterated. From Cwm-felin they are traceable up 



