﻿Vol. 6 1.] ARENIG FAWR AND MOEL LLYFNANT. 637 



to Mr. H. H. Thomas, M.A., F.G.S., and other officers of the 

 Geological Survey, at Jermyn Street, by whom the colour-model of 

 the map was prepared. 



EXPLANATION OF PLATE XLI. 



Geological map of Arenig Fawr and Moel Llyfnant, on the scale of 3 inches to 

 the mile. The topography is based od the Ordnance-Survey map, and 

 consequently the mutation in the spelling of certain place-names, etc. 

 differs occasionally from that adopted in the text of this paper. 



Discussion. 



The President said he was glad that the Author had turned 

 his attention to Sedgwick's area of typical Arenig rocks. The 

 founders of our systems had been obliged to draw the lines between 

 different series of those systems somewhat vaguely, and it was desirable, 

 in the light of our increased knowledge of fossil-horizons, to ascertain 

 what fossil-horizons were represented in these typical districts. 

 The Author had availed himself of the information which Prof. 

 Hughes and Mr. G. J. Williams had generously placed at his 

 disposal, and was able, as the result of this and of his own work, to 

 give us important information concerning the Lingula-F\a,gs and 

 Tremadoc Slates, as well as the Arenig rocks. 



With regard to the line between the Lingula-Flngs and the 

 Tremadoc Slates, if the Dict//onema-Bed were taken as the horizon 

 of separation, the Psilocephalus-Beds of Tremadoc, once placed in 

 the Tremadoc-Slate Series, must be assigned to the Lingula-Fl&gs. 

 It seemed desirable that this should be done. 



The occurrence of the Shineton fauna in Wales, originally proved 

 by Williams in the Arenig district, had also been detected by the 

 Author in that of Tremadoc, and he had largely added to our know- 

 ledge of those beds, which occurred below the well-known Angelina- 

 Beds. The term ' Shumardia-Beds ' used by the Author seemed 

 appropriate. 



Passing to the Arenig rocks of Arenig, the Author mentioned the 

 occurrence of Loganograptus in his Extensus-Beds. It would be 

 interesting to know whether the rich assemblage of Loganograpti 

 and Dichograpti found in Lakeland, Scandinavia, Canada, and 

 Australia would be eventually discovered in these beds, in order to 

 ascertain the nature of the earliest fauna of these Arenig rocks. 



The poverty of the fauna of the Daerfawr Shales was a matter 

 for regret. If these shales were referable to the Murchisoni- 

 horizon, the shales and the great mass of Upper Ashes were not of 

 Arenig, but of Llandeilo age. Put there was still a difficulty. 

 Where were the Upper Llandeilo Beds ? He (the speaker) had a 

 dim recollection of having seen a specimen of Ogygia dilatata 

 collected by Williams from shales above the ashes, but would not 

 speak with certainty. But, so far as the tract described in the 

 paper was concerned, the Author gave reasons for supposing a 



