1846.] SEDGWICK ON THE FOSSIL SLATES OF N. WALES, ETC. 143 



slates south of Tremadoc ; and alter a careful search among these 

 beds south of Festiniog, I found the Tremadoc LingulcB and Fucoids 

 in great abundance. By help of these beds we have therefore a base- 

 line common to the systems of Carnarvonshire and Merionethshire. 

 The preceding remarks will enable my hearers to comprehend the 

 meaning of two sections which I proceed shortly to notice — one 

 drawn from the Merioneth anticlinal near Trawsfynydd to the hills 

 north-west of Festiniog — the other from the same point, but in an 

 opposite direction, to the hills near Dinas Mowddwy. 



Section IV. 



From the great Anticlinal of Merionethshire to the Mountains 

 N. TV, of Festiniog. 



From Trawsfynydd to Cynicht, 9 miles. 



N.N.W. 

 Moel Wyn. Cynicht. 



S.S E. 

 Trawsfynydd 



2 3 4 5 



The numbers refer to the description below. 



If we follow this anticlinal line to the north side of the great Bar- 

 mouth estuary, we there find the beds on the opposite sides of it 

 dipping at a great angle ; but near Trawsfynydd the anticHnal avrh 

 becomes much flattened, so that the beds on the opposite sides of 

 its vertex dip only at 10° or 12°. Following the line still farther 

 north, to the latitude of Festiniog, we lose all distinct traces of an 

 anticlinal ; but the beds in the neighbourhood (still dipping at a low 

 angle) may be traced on the map round the prolongation of the an- 

 ticlinal line in the form of an arch, dipping N.W., N. and N.E., as 

 before stated. The section about to be described passes, however, 

 considerably to the N.W. of these flexures, and does not appear to 

 be aflected by them. Commencing then a little to the S.E. of 

 Trawsfynydd, we have the following ascending series : — 



1. A series of earthy pyritous slates. The structure is extremely 

 varied, and some quarries are partially worked for economical use. 

 A little south of Trawsfynydd these slates have a red colour and 

 alternate with bands of contemporaneous porphyry. A still lower 

 part of this series is exposed on the north shore of the Barmouth 

 estuary. Thickness unknown. 



2. Very coarse grits, marked by bands of small quartz pebbles, 

 alternating with finer bands of grit, and thin bands of slate resem- 

 bling the Festiniog flagstone. They are divided into rhombohedral 

 masses by good perpendicular dip and strike joints. They also alter- 

 nate (especially in their upper portion) with bands of contempora- 

 neous porphyry. This group is of considerable thickness, and forms 

 great precipices on the flanks, and among the gorges, of the chain 

 which runs from Barmouth towards Maentwrog. Average dip about 

 12° or 14°. 



3. Numerous alternations of ylate, trappean shales, porphyry, &c. 

 This group is also of considerable thickness, and runs into the crests 

 and eastern flanks of a portion of the chain above mentioned. 



