﻿570 DPv. T. GKOOM AND MR. P. LAKE ON THE [NoY. I908, 



where they underlie beds containing a typical Dolhir fauna. Grapto- 

 lites are abundant, but are too imperfectly preserved to be identified. 

 Similar slates, occupying a similar position, are exposed in the 

 stream by Ty'n-y-celyn. They are again seen in the small gully 

 which lies about 250 yards north-east of Ty-isaf, and in the 

 side of the footpath close to this gully we obtained Dicellograptus 

 elegans, CUmacograptus,^ a few small brachiopods, and the thorax 

 and tail of a small Ampi/x or Trinudeus. 



Similar dark slates with imperfect graptolites extend, below 

 the true Dolhir Beds, for some little distance to the east of this 

 last-mentioned locality ; but they have entirely disappeared before 

 we reach the valley of the Ceiriog. 



In Nant Tyn-y-twmpath the graptolitic slates are underlain by 

 pale slates containing a very peculiar fauna. These slates are 

 distinct, both lithologically and palceontologically, from any other 

 rocks in the area, and they evidently belong to a part of the 

 succession that is elsewhere cut out by the Dolhir Pault. Until 

 we have more closely examined the country which lies on the 

 west, we hesitate to elevate them into a separate group, and content 

 ourselves for the present with tabulating the list of fossils obtained 

 from them : — 



Leptcena rhomhoidaUs, Wilck. 1 Rafinesquina expansa, Sow. 

 Orthis Actonim (I) Sow. Triplesia spiriferoides (?) M'Coy. 



Ortkis elcgantula (?) Dalm. Echinosphcerites arachnoidea, Forbes. 



Plectarahonites sericea (?) Sow. | 



Also several other forms that we have failed to identifj-. 



(7) Dolhir Beds. — In Nant Tyn-y-twmpath and in the 

 Ty'n-y-celyn stream the Graptolitic Slates are succeeded by the 

 Dolhir Beds, which cover a broad strip of country extending east- 

 wards as far as Pentre-cilgwyn, where they end against the great 

 Cae-mawr Fault. On the other side of this they are nearly cut 

 out by the Dolhir Fault. 



Immediately above the black graptolitic beds the Dolhir Series 

 consists of slates which are fragile and shivery in texture. They con- 

 tain numerous fossils, and are well shown in the streams already 

 mentioned; on the ridge north of Gelli; and on the ridge that runs 

 from Aber-chwil to Plas Einion. Similar beds form the lowest 

 portion of the series that is visible in the valley of the Ceiriog ; and 

 there they are accompanied by a band of limestone (30 feet or 

 so thick), which has been quarried above Dolhir and also on the 

 other side of the valley east of Plas Einion. West of the Ceiriog 

 Valley this limestone is nowhere actually visible, but eastward 

 it occurs at Cefngoed, where a small quarry has been opened in 

 it ; it is also seen in the road at Bedwlwyn. 



The beds above the limestone consist chiefly of firmer slates ; but 

 towards the top, in the Ceiriog Valley, they become full of calcareous 



1 We are indebted to Miss G. L. Elles, D.Sc, for identifying these and our 

 •other specimens of graptolites. 



