﻿594 THE BALA AND LLAXDOVERT BOCKS OF GLYN CEIEIOG. [Ts'ov. 1 908, 



Pandj' Series are contemporaueous aslies ; but, so far as I can see, no evidence is 

 brought forward except their fragmental condition. In the Craig- j'-Pandy "Ash," 

 included angular fragments of slate and limestone are common in the sheared upper 

 margin. The overlying slates and limestones are also sheared, and it is bighl}' 

 probable that the limestone-fragments were introduced from the adjacent beds. 



'The central portion of the "Ash" is compact, and shows beautiful columnar 

 structure, both at Cae Deicws and Craig-y-Pandy, Hitherto, I have favoured the 

 idea that the igneous bands might be intrusive, and were injected into the slates 

 along planes of shearing when the rocks were folded. As a working hypothesis it 

 has been useful, but I have long recognized that no definite results can be obtained 

 without careful and detailed mapping of the sedimentaries. 



' I am jointly responsible for the statement referred to by the Authors, that the 

 slates in contact with the igneous bands show signs of thermometamorphism. It 

 is true that thej- are usuallj- of a harder and more compact nature near the margins; 

 but, speaking for m3-self, I claim nothing more, and certainlj' do not compare the 

 alteration with that which has produced spotted slates in connexion with the 

 Hendre intrusion.' 



Mr. 0. T. Jokes said that he wished to congratulate the Authors 

 on their excellent account of a very interesting and difficult area. 

 As compared with the sequence in South Wales, the Dolhir Beds 

 could he matched exactly with the Eedhill and Slade suhdivision 

 and the Sholeshook Limestone. Below that all correspondence 

 between the districts ceased, for whereas in the South "Welsh area 

 the Lower Bala Beds contained graptolites almost exclusively and 

 brachiopods were rare, in the area described in this paper the 

 reverse was the case. It was certainly difficult to correlate the 

 shelly beds and ashes of the Ceiriog district with the black 

 graptolitic shales of South Wales, which were devoid of ashy 

 material. 



With regard to the Dolhir Fault, although the Authors had 

 demonstrated that a certain amount of movement had taken place 

 along that line, yet it was a curious fact that a break of a similar 

 kind occurred at nearly the same level in South Wales — where it 

 could be definitely proved to be an unconformity, with overlap of the 

 Upper Bala upon the Lower Bala and Llandeilo Series. Nowhere 

 along the line was anything of the nature of a beach-deposit found. 

 The relation of the Llandovery to the Upper Bala was always a 

 difficult question. The Gljn Grit corresponded in position with 

 the Llandovery basal grit and conglomerate of South Wales, which 

 was liable to precisely the same variation in thickness. The Lower 

 Llandovery always showed close faunal affinities with the Upper 

 Bala Series, but the speaker thought that more stress should be laid 

 upon the incoming of new forms than upon the survival of older 

 forms in delimiting the two groups. 



He was glad to find that Pentamerus undatus occupied a 

 relatively high position in the Llandovery of North Wales, as in 

 South Wales. 



Dr. H. J. Johnstoist-Lavis enquired as to the definition of the 

 following terms used by the Authors: — 'fine tuff",' 'coarse ash.' 

 It seemed to him that the word ' ash ' ought to be completely 

 deleted from geological language. 



Dr. Strahan, in reference to an explanation given of the structure 

 of a part of the region, mentioned that he had had experience of a 

 fault the downthrow of which was always on the same side ; it 



