186 DE. WILLS AND ME. SMITH OX THE LOWEE [vol. Ixxviii, 



Elles & Wood, D. {Mesograptus) multidens Elles & Wood, 

 T>. {AmpJexograptus) perexcavahis (?) Lapworth, D. ( Glypfo- 

 graptus) teretiuscuhis (Hisinger), D. (G.) teretiusculus var. 

 e ug hjp Inis L ap worth . 



Dr. Groom & Mr. Lake further record Dicellograptus eJegans 

 Carruthers. This latter form sug-gests a horizon about the zone of 

 Pleurograp)fiis linearis, which would agree with the position of the 

 rocks above the Brjn Beds that show a typical Caradocian fauna. 

 The other identifications suggest a much lower horizon, possibly 

 even in the Llandeilo, which goes counter to all the stratigraphical 

 relations that can be shown in the case of the Blaen-y-cwm Beds. 

 In view of this discrepancy, it is interesting to note that, at 

 Pen-v-o-arnedd on the south side of the Berwyns, one of us. 

 together with Mr. W. B. K. King, found well-preserved graptolites 

 in black shales, occupying the same stratigraphical position. ^ The 

 fossil assemblage occurring there has been claimed by Miss Elles 

 as proving the presence of the Diplograptus-pristis Zone of 

 Sweden, hitherto unrecognized in this country. That zone is ap- 

 proximately equivalent to the zone of Pleurograpius linearis. 

 Therefore, until really well-preserved forms are forthcoming from 

 the Blaen-y-cwm Beds, we are inclined to attach more importance 

 to the evidence, stratigraphical and palseontological, which points to 

 the TleuvograpUiS-linearis Zone, than to identifications, based on 

 admittedly inadequate material, suggestive of a far lower horizon. 



Ashgillian Series south of the Llangollen Syncliiiorium. 



(7) Dolhir Beds. — The general lithological characters of the 

 Dolhir Beds have already been indicated in Table I, p. 180, and the 

 variation in thickness is shown in Table II, p. 189, and fig. 3, p. 185. 



(«) Lower or Ty'n-y-twmpath Beds.^ — A group of grey 

 slates, often with difficulty separable from the normal micaceous 

 Dolhir Slates, comes in above the Blaen-y-cvnn Beds, near Gelli, 

 and thickens westwards, where it is frequently characterized by a 

 speckly blotching of the rocks. It is usually far less fossiliferous 

 than the Dolhir Beds proper, although locally it contains a rich 

 fauna of trilobites. 



The upper and lower limits of this group merge into the Dolhir 

 and Blaen-y-cwm Beds respectively, by insensible gradations, and 

 it has not "^ been possible to draw satisfactory boundaries to the 

 formation on the map. It is also uncertain whether the Ty'n-y- 

 twnipath Beds are the equivalent in the west of the Dolhir Lime- 

 stone, which, with its associated shales, forms the base of the 

 Dolhir Beds near Grlyn-Ceiriog. 



1 ' Summary of Progress for 1919 ' Geol. Surv. 1920, pp. 4, 5. 



2 This group takes its name from a large farm, lying immediately ^vest of 

 the small homestead indicated as Bone, about three-quarters of a mile east 

 of Nantyr. Nant-Ty'n-twmpath, referred to in the sequel, is the valley that 

 descends from the north to the farm. 



