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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