RAMSAY — PHYSICAL STRUCTURE OF MERIONETHSHIRE. 171 
Lingula Bavisii. Near Dolgelli they also contain a crustacean, 
Hijmenocaris vermicauda. Prom Mynydd Garler 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 Wnion 
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 Merionethshire anticlinal. They form 
one of the most rugged tracts of Wales. The centre of the anticlinal 
is in Gors-goeh, north of LlawUeeh, 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. 
Q 2 
