RAMSAY — PHYSICAL STRUCTXJilE OF MERIONETHSHIRE. 171 



Lingula Davisii. jS'ear Dolgelli they also contain a crustacean, 

 Symenocaris vermicauda. From ]?trynydd Gader they stretch across 

 the Barmouth estuary, being in many places pierced by felspathic 

 traps and lines of greenstone ; finally they rise on the flanks of 

 the great boss of Cambrian grits that form the centre of the 

 ^'Merionethshire anticlinal " of Professor Sedgwick, which extend from 

 Barmouth to Harlech and the neighbourhood of Ffestiniog. 



Down the Bala valley, and along the upper course of the river TVnion 

 towards Dolgelli, there runs a great fault. It is a downthrow to the 

 north-west, and, on its western side, all the rocks of Cader Idris and 

 the Arans are repeated. Thus, from Penmaen to the Arenigs we have a 

 repetition of the interbedded felspathic traps and ashes of the Arans, 

 and underneath them the Lingula-flags again crop out between the 

 west sides of Arenig and the Cambrian strata of Dol-melynllyn and 

 Trawsfynydd. These Lingula-flags circle entirely round the south-east, 

 east, and north sides of the Cambrian boss, and pass into overlying 

 strata, which are imperfect representatives of the Llandeilo-flags of 

 South "Wales. These, in turn, are themselves encircled by a great 

 crescent-shaped mountain-ring of interbedded volcanic felspathic traps 

 and ashes, already described, of which Cader Idris, the Arans, Arenigs, 

 and Moelwyns, form a chief part. Beyond Arenig, these igneous 

 rocks stretch north to Llyn Conwy, from thence striking westward to 

 the Manods and the Moelwyns, beyond which they suddenly die out 

 before reaching Traeth-bach. 



It has been already stated that beneath the Lingula-flags the Cambrian 

 grits constitute the centre of the Merionethsliire anticlinal. They form 

 one of the most rugged tracts of Wales. The centre of the anticlinal 

 is in Gors-goch, north of Llawllech, and east and west, north and south 

 from this centre, the rocks dip steadily. Walking up the road to 

 Trawsfynydd, the instructed eye can readily see, on the cliffs of 

 Rhinog-fawr and Craig-ddrwg, the great terraced lines of hard grit 

 dipping westward ; and, on the opposite hand, the same Cambrian 

 strata dipping east in the broken slopes of Craig-y-Penmaen. From 

 bottom to top, the masses of strata succeed each other, like, as it were, 

 the concentric coats of an onion. The central boss of Cambrian grit 

 throws off on its flanks the superincumbent Lingula-flags, which are, 

 in their turn, overlaid by another casing of slates, associated with 

 the interbedded igneous series. 



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