236
H. HICKS ON THE PRE-CAMBRIAN ROCKS OF ST. DAVID S.
sections. Most of the harder compact conglomerates have been
dropped in the hill north of the Cathedral by a fault running in
a N.E. direction almost at the junction there of these beds with
the Dimetian axis. It is more than probable, however, that the
conglomerates on the west side of the St. David's valley are these
conglomerates repeated, as they have none of the usual appearances
of the Cambrian conglomerates. A tolerably thick series may also
be seen in section in the valley between Llanhowcll and Carvoriog ;
and the hard conglomerate beds creep up above the general surface
near the road-side between Llanhowell and Treglemaish, and near
Bryn close by the fault. To examine the series generally, however,
the first two sections mentioned, viz. Nun's Well and Caerbuddy
Valley, offer the best opportunities. The lower conglomerates are
well seen on the St. -David's side of the valley just above Clegyr
Bridge, and the fine compact porcellanite beds in some quarries
in the valley about midway between the bridge and the mill.
Immediately above the mill the conglomerates of the Cambrian rest
unconformably upon the porcellanites ; and masses of the latter may
here be seen in the conglomerates. At Nun's Well Cliff the porcel-
lanite beds are seen in section perfectly vertical, with the conglome-
rates again resting upon them unconformably. A thick dyke of
compact quartz porphyry cuts across the beds at this point, but does
not penetrate the Cambrian above *. This is shown in the following-
section : —
Section at Nun's- Well Bay, extending from above the Well to the
Coast.
N.N.W.
S.S.E.
X.i'metlu.-n. (
TfO/J
v
Pebidinn.
^) Cumbxlan

• Prof. Judd reports as follows of these porphyries, the best specimens of
  

which are found in the Clegyr valley near Trepuet : — " The base shows little
besides zeolitic minerals and patches of ' viridite,' the latter probably derived
from some hornblendic constituent. But the quartz veins are numerous, and
of large size, ar.d exhibit the stone and (liquid ?) enclosures so common in
quartz-porphyries. Some of the large quartz grains are beautiful double
pyramids, with the intervening prism scarcely, if at all, developed. There are
large felspar crystals, some of which are very little altered ; and the latter are
certainly plagioclase. There cannot be the smallest doubt, therefore, that this
rock was a quartz- forphy rite.
" The rock presents the most remarkable identity of character with the gold-
bearing quai-tz-porphyrite of Cstatye, near Vorospatak, in Transylvania, which
is an eruptive rock of Neogene age. It is probably the oldest quartz-