﻿Vol. 64.] BALA AND LLANDOVERY ROCKS OF GLYN CEIRIOG. 589 



YI. Comparison with other Areas. 



It requires but a brief inspection of the list of fossils from the 

 Dolhir Beds to show that the fauna resembles that of the upper 

 portion of the Bala Series in Pembrokeshire and in the Lake District. 

 "When, however, we compare the lower beds of these districts, the 

 similarity diminishes or even disappears. In the Llandovery Series 

 also the differences are so great, that no direct comparison with 

 the rocks of those areas is possible ; and it is to the shelly facies 

 of the Llandovery, as developed on the Welsh Border, that we 

 must look for the representatives of the Pron-Frys Slates of Glyn 

 Ceiriog. 



In the following pages, accordingly, we compare the Bala Beds 

 with those of Pembrokeshire and of the Lake District, and the 

 Llandovery Beds with those of Llandovery and other areas. 



In South Wales ^ the Bala Series is divided by Dr. Marr and the late 

 T. Eoberts into the following subdivisions, in descending order : — 



Trinucleus-seticornis Beds. 



Slade Beds. 



Eedhill Stage. 



Sholeshook-Limestoae Stage. 

 E,obeston-Wathen Limestone. 

 Bicranograptus-^'hdlQ?, (in part). 



The Dicranograptus-^\iQ\e?i are considered by these authors to 

 represent generally the Glenkiln Group and the lower part of the 

 Hartfell Group of the Moffat area, and this is also the view that 

 seems to be taken by later observers. The boundary between the 

 Llandeilo and Bala Series therefore lies somewhere in the Dicrano- 

 graptus-^\idXe^ ; but its precise position has not yet been determined, 

 nor has it been shown to what horizon the shales extend in an 

 upward direction. 



As these shales contain few fossils excepting graptolites, no 

 direct comparison is possible with any part of the Glyn succession. 

 Messrs. Cantrill «& Thomas have, indeed, found one or two bands of 

 limestone in the Dici'anograptus-^hdles, and from these a few trilo- 

 bites, brachiopods, and other fossils have been obtained ; but the 

 number of species is small, and they do not afford a very satis- 

 factory basis for comparison. The presence of Triiiucleus concen- 

 tricus var. favus suggests that these calcareous bands belong to 

 the Llandeilo Series. 



The precise position of the Bobeston-Wathen Limestone is also 

 somewhat doubtful. It unquestionably belongs to the Bala Series ; 

 but its relations to the Sholeshook Limestone appear to be obscure, 

 for where the one is present the other is almost invariably absent. 



^ Marr & Eoberts, Quart. Journ. Geol. Soc. vol. xli (1885) pp. 476-90 ; 

 D. C. Evans, ibid. vol. Ixii (1906) pp. 597-642 ; and T. C. Cantrill & H. H. 

 Thomas, ' Geology of the South Wales Coalfield, pt. vii : The Country around 

 Ammanford' (Sheet 230) Mem. Geol. Surv. 1907, pp. 8-36. 



2r2 



