.[ 683 ] 

 LXT. Proceedings of* Learned Societies. 



GEOLOGICAL SOCIETY. 

 [Continued from p. 160.] 



March 10th, 1920.— Mr. K. D. Oldham, F.R.S., President, 

 in the Chair. 

 npHE following communication was read : — 



' The Lower Palaeozoic Rocks of the Arthog-Dolgelley District 

 (Merionethshire).' By Prof. Arthur Hubert Cox, M.Sc, Ph.D., 

 F.G.S., and Alfred Kingsley Wells, B.Sc, F.G.S. 



This paper gives an account of the geology of the country 

 between the Cader Idris range and the Mawddach Estuary. The 

 stratigraphical succession is as follows : — 



? Llandeilo Lower Basic Volcanic Series. 



( Cefn-Hir Ashes. 



| Crogenen Slates. 

 Lower ■{ Bryn Brith Beds. 



Llanvirn | Moelyn Slates. 



1^ ' China-Stone ' Ashes. N , 



, . > Pont Kings Slates. ! Lower Acid or Mynydd Cader 



' " " r t a i, i Volcanic Series. 



. J Lower Ashes. 



Arenig ]_ Basement Series. J 



\ vi. Upper Pencil -Slates. 

 rUnconformity. | v. Upper Dictyonema Band, 

 j iv. Asaphellus Beds. 

 fTremadoc Slates. | iii. Lower Pencil-Slates. 

 Upper ! Dolgelley Beds. I *f Lo ^ er Dictyonema Band. 



n, I „. .. . ^ -. 1 i. Niobe Beds. 



Cambrian «{ Ffestiniog Beds. v - 



j Maentwrog Beds. [Not seen south of the Mawddach 

 ^ Estuary.] 



The Upper Cambrian beds are similar, both lithologically and 

 faunally, to the corresponding beds on the north-east and north- 

 west of the Harlech Dome. Discontinuity between the Cambrian 

 and Ordovician Systems is shown by the marked contrast in litholog}^ 

 between the uppermost beds of the Tremadoc Slates and the suc- 

 ceeding arenaceous Basement Series of the Ordovician System. 

 The Ordovician succession includes in its lower portion two distinct 

 volcanic series ; the lower one immediately succeeds the Basement 

 Series, and consists mainly of rhyolitic ashes, with occasional 

 rhyolite-flows which thin out westwards. Coincidently with the 

 thinning of the volcanic rocks, slate-bands with an Arenig and 

 Lower Llanvirn fauna appear interbedded with the rhyolitic rocks. 

 The higher volcanic series is separated from the lower by the main 

 mass of the Bifidus Beds, and includes a great thickness of 

 'andesitic' ashes and agglomerates, together with abundant spilite- 

 flows often occurring as pillow-lavas. 



Intrusive igneous rocks in the form of transgressive sills and 

 laccolitic bodies occur at all horizons in the succession. They aiv 

 of two main types: (i) diabases of normal Welsh types, and 

 (ii) granophyres (eurite type). Basic intrusions are the more 



