﻿Vol. 6l.] APvENIG FAWK AND MOEL LL1FNAXT. 609 



less easy to extract. Much more instructive are the few pages 

 devoted by Messrs. A. V. Jennings & G. J. Williams r to the Garth 

 Grit, and the recent paper by Miss G. L. Elles 2 on the general 

 palaeontology of the Arenig rocks of Wales. 



Passing now from printed matter, I take this opportunity of 

 acknowledging my indebtedness to Prof. T. McKenny Hughes. 

 F.R.S., and also to Mr. G. J. Williams, F.G.S., H.M. Assistant- 

 Inspector of Metalliferous Mines, of Bangor. Prof. Hughes has, 

 in the course of the last twenty years, made frequent excursions to 

 Arenig, alone and also as leader of large classes of Cambridge 

 students. He has in this way collected a vast amount of ex- 

 tremely useful material, and all this, together with the corresponding 

 field-notes and sketch-maps, he most generously placed at my 

 disposal. 



Mr. G. J. Williams was long resident at Ffestiniog, which is 

 only about a dozen miles away, and more than twenty years ago 

 discovered the fascinations of the Arenig district as a collecting- 

 ground. Unfortunately, change of residence and increased pressure 

 of work have latterly prevented him from continuing his researches, 

 which have never been concluded ; and when I announced my 

 intention of mapping the whole country on the 6-inch scale, he 

 kindly gave me a day over the ground, pointing out fossil-localities. 

 Moreover, since that time he has always allowed me full access to 

 his fine and carefully-localized collection of Welsh fossils. 



II. Description of the Rock-Succession. 



The general succession (see also figs. 1 & 2, pp. 610-11) of the 

 sedimentary rocks of the district is as follows : — 



( Dicranographis-Skales. 



Oaradoc. 



Derfel or O^'s-Limestone. 



Khyolitic Ashes = Upper 1 TT A , e 



Massive Ashes = Middle V ^ ei ' Ashes of 



Acid Andesitic Ashes = Lower J reni g- i 



Daerfawr Shales. Zone of Didymograptus Murchisoni. f 



Platy Ashes. 1 Lower (Hypersthene-Andesites) ' 



- Great Agglomerate, j Ashes of Arenig. J 



Olchfa or Bifidus-Shales. Zone of Didymograptus ) 



bifidus. 

 Filltirgerig or Hirundo-Beds. "} Zone of Didymograptus \ 

 Erwent or Ogygia -Limestone. J hirundo. )■ Arenig. 



Henllan or Calymene- Ashes. \ „ <? -n-i *,.„ I 



Llyfnant or Extensus-m^gs. Zo ™ ° f ^P^graptus 



I Basal Grit. J exUnms ' ) 



Unconformity. 



1 ' Manod & the Moelwyns ' Quart. Journ. Geol. Soc. vol. xlvii (1891) 

 up. 373-74. 



2 Geol. Mag. 1904, pp. 199-211. 



Q.J. G.S. No. 243. 2 v 



