﻿152 MR. W. G. FEARNSIDES ON THE [May I9IO, 



The Lingulella Band. 



The topmost layers of the Ffestiniog Beds are characterized by 

 the presence of very numerous specimens of Lingulella davisii in a 

 bright-rusting flaggy grey slate. The fossiliferous flags are about 

 50 feet thick, and can be mapped continuously. They come in 

 from the sea at the cliffs south of Rhiw-for-fawr, near Ogof-ddu 

 (Criccieth), and pass northwards by Tstumllyn House to the quarry 

 by the turnpike at Pencraig. Thence by Gareg-felen to Glanbyl 

 they are drift-covered, but at the farmyard of Gwern-ddwyryd they 

 reappear, striking east and west. Prom the little reservoir on the 

 stream east of Bryn Cilynen, south-eastwards along the hillside 

 behind Wern, they form a dip-slope and make a great exposure. 

 South of the railway they occupy the slopes of Coed-y-chwarel, and 

 through the Coed-y-capel plantation can be followed to the meadows 

 south of Bron-y-foel. Thence over the col by Beudy Pencerbi they 

 pass to the farm of Llanerch, where a fault intervenes displacing 

 them some half a mile to the south. 



On the southern shores of Llyn Gareg-wen they reappear, and so 

 by the cowhouses on the hill can be traced through the quarry 

 near the schoolhouse and on through the village to form the southern 

 shore and promontory of the harbour at Borth. 



The topography of the Pfestiniog Series is in this district very 

 characteristic. The rock weathers very slowly, and bracken, gorse,. 

 bell-heather, bramble, and a thin wire-grass form the usual vege- 

 tation. Even these flourish only upon the flaggy outcrops and along 

 the shatterings due to joints, and the grauwacke grits stand up as 

 lichen-covered cliffs and rock-knobs, which even close to sea-level 

 give the impression of a rough mountain-country and are exceedingly 

 picturesque. 



Though thus giving rise to a diversified moorland topography, 

 the Ffestiniog Series does not, as a whole, behave as a resistant 

 formation ; and it is noticeable that in this district it forms the low 

 ground of Llyn Ysturallyn at the centre of the anticline, and does 

 not at any point attain an elevation greater than 400 feet. 



V. The Dolgelly or Black Band Series. 



The Dolgelly Series follows directly upon the Lingulella Band. 

 Whether we consider the series in its original signification as 

 including only the dark slates of the Black Band, or whether we 

 allow it to include all the beds up to the horizon of the Dictyonema 

 (as suggested in my Arenig paper), 1 it is in this district perfectly 

 well defined, and lends itself beyond all other series to the plotting 

 upon a large-scale map. 



The lithologically defined Black Band is especially convenient 

 from this point of view, and, following round the Ynyscynhaiarn 

 anticline outside the line of the Lingulella Band, above described, 



1 Quart. Journ. Geol. Soc. vol. lxi (1905) pp. 614-15. 



