Reports and Proceedings — Geological Society of London. 325 



Geolc3ical Soceety of London'. 



l.—May 12, 1909.— Professor W. J. Sollas, LL.D., Sc.D., F.R.S., 

 President, and afterwards Dr. J. J. H. Teall, M.A., F.R.S., Vice- 

 President, in the Chair. 



The following communications were read : — 



1. "The Hartfell - Valentian Succession around Plynlimon and 

 Pont Erwyd (^orth Cardiganshire)." By Owen Thomas Jones, M.A., 

 B.Sc, F.G.S. 



In this paper the author deals with the stratigraphical succession 

 and the geological structure of an area of about 40 square miles, lying 

 in the hilly district about 12 to 16 miles east of Aberystwyth. In an 

 historical introduction the work of previous observers, including 

 Sedgwick, Ramsay, and Walter Keeping, is dealt with. 



The rocks within the district are divided into three stages, which 

 are further subdivided into groups and zones, as ibllows : — 



I 



Rhudduant Grits. 

 Rhuddnaut Shales. 

 Blaeu Myherin Mudstones. 

 Dolwen Mudstones. 



Mudstones with thin grit-bands. 



Flags and shales. (Zone of Mono- 



graptus Sedgiviclci.) 

 Shales and mudstones. (Zone of Cephalo- 



graptns cometa.) 

 Mudstones and shales. (Zone of Mono- 



graptus convolutus.) 



8. Leptotheca-banA. 



y- 



Black shales and 



mudstones. 

 (Zone of Moiw- 



graptits com- 



mutiis.) 



Flags and black 



Magnus-\)a.TiA. 

 Triangulatua- 

 band. 

 o. Triangulatns-var . 

 band, 

 shales. (Zone of 



Monograptus cyphics, s.s.) 

 Flags with thin shales. (Zone of 



Monograptus, sp. nov.) 

 Flags and shales. (Zone of Mono- 

 graptus, sp. nov.) 

 Flags, shales, and grits. (Zone of 



Ccphalograptiis ? acuininatus.) 

 Flags with thin shales. (Zone of 

 Glyptograptus perscidptus.) 

 Mudstones. 



Grits, conglomerates, and mudstones. 

 Flags with thin shales. (Zone of 

 Dicellograptus ancips.) 



The Plynlimon Stage is developed in the northern part of the district, 

 between Plynlimon and Pont Erwyd ; the Pont Erwyd Stage along 

 the two valleys of the Rheidol and the Castell, which converge near 

 Pont Erwyd village; while the Ystwyth Stage is developed on the 



