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ON THE SILURIAN GRITS OF CORWEN, NORTH "WALES. 20'
12. On the Silurian Grits 0/ Corwen, Xorth AY ales. By Prof. T.
McKenni Hughes, M.A., F.G.S. (Read December 20, 1876.)
I beg to offer a few notes, made chiefly in the summer of 1876,
while endeavouring to correlate the base of the Silurian rocks of
North "Wales with the corresponding beds in the Lake-district and
South "Wales.
I found that some important changes in the mapping and conse-
quent classification of the rocks in the neighbourhood of Corwen
were necessary. These corrections being made, I obtained a key by
which I was able to detect, over the whole of the district I then
examined, a very variable but yet a more satisfactory and easily
recognized base for the Silurian rocks.
It will be seen by reference to the Geological-Survey map of the
Corwen district, that the Denbigh Grits are represented as thrown
down by a fault (running W.S.W. and E.X.E.), while no Grits are
are shown on the map between this and Pennyglog Quarry.
I found, however, that the Grits at Corwen were not the Denbigh
Grits, as seen e. g. on top of the Flags at Penyglog Quarry E.S.E.
of Corwen, but that the Corwen Section is as follows (rig. 1) : —
Fig. 1. Diagram Section from Corwen to Penyglog.
.WNW
Nant-
Corwen. caweddu. Nant Llechog. Penyglog.
j i ,
■(I ^~-
The section runs from W.N.W. to E.S.E. from Corwen to Nantcaweddu,
then nearly W. to E. to Nant Llechog, thence E.N.E. to Penyglog,
/ /'. Denbigh-Flag series. g. Grit in Denbigh-Flag series.
h. Band of nodules near base of,/'.
h. Pale slates (including part of what was previously called Bala).
I. Corwen Grit. m. Bala beds.
The nearly vertical Grits of New Corwen seem to be brought
in by faults, two of which are pretty clear. South and east of these
faults there are two large quarries near the church — one in Bala
beds, which have yielded Stygina latifrons, Illcenus Davisii, GJauco-
7iome disticha, Atrypa marginalis, various species of Orthis, and
corals.
The other quarry is a little further west. In it the Grits, to
which I propose to give the name of the " Corwen Grits," are
worked, chiefly for road-metal. These are obviously thrown down
by a small fault from the grits seen in the cliff immediately to the