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 

 south. , 



