﻿142 MK. W. G. EEARNSIDES ON THE [May I9IO, 



7v The Tremadoc Slates and Associated Rocks of South-East 

 Carnarvonshire. By William George Fearnsides, l&.hjf 

 F.G.S., Fellow of Sidney Sussex College, Cambridge. (Read 

 December 1st, 1909.) 



[Plates XY-XYII— Sections & Map.] 



Contents. 



Page 



I. Introduction 142 



II. Previous Literature 143 



III. The Maentwrog or Eusty Mag Series 148 



IV. The Ffestiniog or Grey Flag and Grauwacke Series 150 



V. The Dolgelly or Black Band Series 152 



VI. The Tremadoc Series 155 



VII. The Arenig and Llandeilo Series 164 



VIII. The Tremadoc Country 166 



IX. The Folding 176 



X. The Older Dolerites 177 



XI. The Cleavage 177 



XII. The Faulting 178 



XIII. The Later Dolerites 179 



XIV. The Jointing 179 



XV. The Age of the Gabbroid Dolerites 182 



XVI. The Pleistocene Accumulations 182 



XVII. General Summary and Conclusions 184 



I. Introduction. 



'The ancient parish of Ynyscynhaiarn lies along the northern 

 shore of Cardigan Bay, at the month of the Afon Glaslyn, and forms 

 the south-eastern corner of the county of Carnarvon. The area to 

 be described in the present paper is the country represented upon 

 the 6-inch Ordnance Survey maps of Carnarvonshire, Sheet XXXIV 

 S.E. (Criccieth) and Sheet XXXIV S.W. (Portmadoc), with portions 

 of Sheets XLII N.E. & X.W. and XXXIV N.E. & N.W. to the 

 north and south of these. It includes portions of all four quarter- 

 sheets of Sheet 75 of the 1-inch Geological Survey map. 



This area is a rectangular block of some 20 square miles, and 

 includes the townships of Portmadoc and Tremadoc (now united as 

 the Urban District of Ynyscynhaiarn) and the nourishing health- 

 resorts of Borth-y-gest and Criccieth. It is traversed by the Cambrian 

 Railway, between Barmouth and Pwllheli, and is approached either 

 by that railway or by the London and North- Western Railway 

 (through Bangor and Afonwen), or again by the narrow-gauge 

 railway from Ffestiniog. 



For its scenery the district has long been famous, and there are 

 few British districts which can show so great a variety of tide-flat, 

 salt-marsh, sea-cliff, sand-dune, park, coppice, meadow, cornland, 

 bog, moor, lake, and mountain, even over an area of many times 

 its size. The bigger hills of the district, rising abruptly almost to 



