164 PROF. o. T. JO?^Es OX [vol. Ixxvii^ 



strata ; and this form is represented in the Woodland Beds of 

 Girvan, though not in the Haverfordwest district. I have found 

 no specimen of Pliacops in the Lower Llandovery of the type area. 



There appears to be no representative of the Gasworks Sandstone 

 in that district ; but the highest beds of the Lower Llandovery 

 that are exposed contain PJectamhonites duplicaiiis in great 

 profusion, and these recall a horizon in the Gasworks Mudstones. 

 just below the sandstones. 



The L^pper Llandovery consists of a lower group, mainly of 

 mudstones, and an upper group of sandstones. The fauna of the- 

 mudstones is distinguished by abundance of Barrandella globosUy 

 one of the type-specimens of which was derived from these beds, 

 and a small species of Plectamhonites ; Cafazijga liaswelli and 

 Cyrtia exporrecfa are also not uncommon. These forms indicate the- 

 similarity of the fauna to the Canaston Group of Haverfordwest ; 

 but at Llandovery the}" are associated Avith Peatameriis ohlongus^ 

 The lithology of the rocks is closely comparable. 



In the upper or sandstone division that species occurs in abund- 

 ance, and in this and some other respects the division is comparable 

 with the Rosemarket Stage. The special fauna of the L^zmaston 

 Group of Haverfordwest has not been found so far. Its absence- 

 and that of the uppermost beds of the Haverford Stage suggest 

 a physical break between the L'^pper and the Lower Llandovery of 

 the type area. Such a break is in accordance with the relations 

 of these two groups at the northern and southern limits of the 

 area, where the Upper Llandovery overlaps the Lower. 



No fossils have been found in the ' Tarannon, Shales' ; and the- 

 exact horizon of the lowest Wenlock rocks being unknown, it is. 

 not possible todetermine whether these shales represent the highest 

 horizon of the Yalentian Series. 



YII. NoRTif Wales : Coewex axd Gltx Ceieiog. 



Another district where both graptolitic and shelly fossils have- 

 been obtained from the Yalentian rocks is that along the northern 

 flanks of the Berwyn Hills in the Cor wen and Glyn Ceiriog areas, 

 where they have been described by Dr. T. T. Groom & Mr. P. Lake,^ 

 and still more recently bv Dr. L. J. Wills,- and bv him and 

 Mr. B. Smith.3 



The general succession of the Yalentian and their relation to the 

 adjoining rocks in the two areas, as described by Groom &. Lake,, 

 is as follows :- — - 



CORWEN. G-LTN CeIRIOG. 



Denbighshire Grits. Denbighshire Slates. 



Pale Slates. Ty-di-aw Slates. 

 Graptolite Shales ] 



(gregarius zone). ^ Fron-Frys Slates. 

 Grey Slates. J 



Corwen Grit. Glyn Grit and Limestone. 



Blue Slate -with Bala fossils. Dolhir Beds. 



1 Q. J. G. S. vol. xlix (1893) p. 426 ; and ibid. vol. Ixiv (1908) p. 546. 



^ Proc. Geol. Assoc. toI. xxxi (1920) p. 1. 



3 Abs. Geol. Soc. 1920-21, No. 1064, pp. 34-35. 



