478 W. J. SOLLAS ON THE SILURIAN DISTRICT 
appeared, the first beds met with on crossing the Rhymney bridge 
having a northerly dip and corresponding with those on the northern 
side of the Pen-y-lan axis. The dip to the north continues through- 
out the Silurians on this side of the river, the Old Red Sandstone 
coming on conformably and dipping at first in the same direction. 
Its dip, however, is soon reversed, indicating a synclinal fold, on the 
axis of which Pen-yr-heol is situated. This brings in the Silurians 
again in a small anticlinal on which Pandy is situated, and which 
corresponds to the fold on the other side of the river near Cefn- 
coed-isaf. Past Pandy the Old Red Sandstone again succeeds the 
Silurian, and is well exposed in a quarry in Coed-y-cwarel, where 
it exhibits the same northward dip as the Silurians on the north side 
of the Pandy anticlinal. 
The following section (fig. 3) illustrates the structure of the 
southern end of the Rhymney range of hills :— 
Fig. 3.— Diagrammatic Section from Rhymney Bridge to Coed-y- 
cwarel. (Scale, 2 inches to a mile.) 
8. N. 
Rhymney Pen-yr- Coed-y- 
Bridge. - Cae Castell. Rhymney. heol. Pandy. cwarel. 
a. Gravel. c. Old Red Sandstone. d. Silurian. 
From the foregoing description, which is founded on a large 
number of careful measurements of ‘‘dips and strikes,” one sees that 
the rocks of our area have been folded from N.N.E. to 8.8.W. into 
a series of folds trending from W.N.W. to E.S.E. 
One must not omit to point out, however, that on traversing the 
district from KE. to W. one discovers the existence of a second set of 
foldings at right angles to the preceding. Thus, on the east of the 
Rhymney the principal Silurian strata dip to the N.N.E. at first, 
but more to the north the dip changes and becomes N.N.W., while 
on the Pen-y-lan side the dip of the corresponding beds is to the 
N.E. or even due E.; so that the valley of the Rhymney, between 
Rhymney and Pen-y-lan, lies in the axis of a synclinal fold. Further 
to the W., again, the dip changes, at first becoming nearly due N. and 
then N.W., giving to the hill which slopes from Pen-y-lan to Llwyn- 
y-grant-uchaf an anticlinal structure. Still continuing to the west, 
we once more find a change in dip, the beds now striking either due 
north or a little east of it. Thus the Pen-y-lan range of hills, 
which attains a greater height than the preceding, les in a wide 
synclinal fold. 
The folds trending more or less north and south are much less 
marked than those trending east and west, The existence of two 
