Reports & Proceedings — Geological Society of London. 229 



3. March 28, 1917.— Dr. Alfred Harker, P.B.S., President, in the 



Chair. 



The President announced that the Council had awarded the 

 Proceeds of the Daniel Pidgeon Pund for the present year to Arthur 

 Holmes, D.I.C., B.Sc, F.G.S., who proposes to conduct researches 

 in connexion with the Geology of the Dartmoor Border, around 

 Okehampton and Belstone. 



The following communication was read: — 



"The Carboniferous Limestone Series on the South-Eastern Margin 

 of the South Wales Coalfield." By Prank Dixev, M.Sc, F.G.S., and 

 Thomas Pranklin Sibly, D.Sc, P.G.S., University College of South 

 Wales and Monmouthshire, Cardiff. 



The outcrop dealt with extends from the valley of the Ewenny 

 River near Bridgend (Glamorgan) to that of the Ebbw Biver at 

 Bisca (Monmouth), a distance of about 19 miles from west-south- 

 west to east-north-east. It is traversed by the Bivers Ely, Taff, and 

 Bhymney. 



Traced north-eastwards along this outcrop, the Carboniferous 

 Limestone Series suffers much attenuation, and becomes mainly 

 dolomitic, as shown by the officers of H.M. Geological Survey during 

 the recent re-survey of the coalfield. 



The table on p. 230 summarizes the succession in the western part 

 of the district. 



The Lonsdaleia subzone (D2) has not been recognized, and if 

 represented, it is probably incomplete. Contemporaneous dolo- 

 mitization is of comparatively small vertical extent. 



At Bisca the total thickness has diminished from 2,700 feet to 

 800 feet, made up as follows : — 



C 2 + Si) Feet. 



Ci \ . . . 675 Main Limestone. 



Z j 



K . . . . 125 Lower Limestone Shales. 



800 



There the Main Limestone is almost wholly dolomitic, and nearly 

 barren of fossils. Its lower portion consists of crystalline dolomites, 

 which represent contemporaneously altered standard limestones. Its 

 upper portion, composed of dolomite mudstones with bands of crystal- 

 line dolomite and a very small thickness of calcitic oolite and calcite 

 mudstone, is a Modiola phase extending from some level in Ci up into 

 Si. The Lower Limestone Shales maintain a normal character. 



The great reduction of thickness is due to two factors, in about 

 equal degree : — 



1. Unconformable overstep by the Millstone Grit. This cuts out the 

 Dibunophyllum beds and the Main Seminula Zone. (Overstep is very gradual 

 along most of the outcrop, but it becomes rapid at those points where the zones 

 of the Carboniferous Limestone swing more sharply north-eastwards.) 



2. Attenuation of the surviving zones. (This is rapid in the area east of 

 the Taff.) 



