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 consequent 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.N.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 (fig. 1):—
Fig. 1. Diagram Section from Corwen to Penyglog.
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,
f. f'. Denbigh-Flag series. g. Grit in Denbigh-Flag series.
h. Band of nodules near base of f'.
k. Pale slates (including part of what was previously called Bala).
l. 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, Illænus Davisii, Glauconome 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 south.
