Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/265

Rh like those in the valley south of Llansantfrraid, on the road to Llanarmon.

Along the strike of the Llansantffraid mudstones toward the S.E. we find a curious bed, indicated by a blue line in the Survey Map. This consists of alternations of bands of Limestone and fine Sandstone with wavy lines, very like that into which the Corwen Grit passed in several places. In this I saw several traces of fossils. The only ones I was able to make out were Favosites alveolaris and a large form of Orthis calligramma.

Both the Limestone and the tramway mudstone pass under the Pale Slates; and these are overlain by the Denbigh Flags, which are worked in the large quarries near Llansantffraid. A bed of Sandstone seen on the hillside north of the village may represent the Grit of Penyglog, near Corwen.

The points that seem to me clear are:—that the Corwen Grits are distinct from the Penyglog Grits; that there is more evidence of a discordancy at their base than at the base of the Pale Slates or of the Penyglog Grits; that there are generally some beds of conglomerate, sandstone, or limestone with sandstone on the horizon of the Corwen Grits; that the general facies of the few fossils obtained from these beds in the district examined is that of the May-Hill rocks; that the mudstones of the ravine north of Plasuchaf are the same as those of the tramway-cutting south-west of Llansantffraid.

Other questions remain to be worked out. Are these mudstones (which have not yet been found immediately underlying the Corwen Grits) merely a local development of those Grits? or are they a lower part of the same group locally developed to a greater thickness? or are they a higher part of the underlying Bala series here and there overlapped by the Silurian Rocks?

I have thought it better to bring forward what I have done, and invite cooperation along the same line of investigation, rather than to wait till I could offer more definite results.

Prof. said that about Builth and all round the Longmynd area, and, indeed, over a great part of South Wales, we find Cambrian and Lower Silurian rocks overlain unconformably by the Pentamerus-limestone or Upper Llandovery rocks, upon which, north of the Builth country, and also in part of South Wales, come the Tarannon shales and the Denbigh grits with Wenlock fossils, overlain again by the ordinary Wenlock shales. He was particularly pleased, therefore, to hear that Pentamerus oblongus occurred in the beds where Prof. Hughes said it was present, as this was strongly in favour of his own opinion that the Upper Silurian strata were transgressive in the Corwen district.

Mr. considered the Lower (or Corwen) Grits to be the equivalents of the grits at the base of the Lower Llandovery rocks in South Wales. The Upper (or Penyglog) Grits are nearer to the horizon of the May-Hill Sandstones. There was no visible