156 Geological Society : — 



country, shows the complete succession. In any area some one 

 group is always better developed than others ; and some portion of 

 at least one group is certain to be missing. The lowest, or Gwastaden 

 Group, has a maximum thickness of over 1800 feet. It is underlain, 

 apparently conformably, by highly-cleaved dark blue shales, and 

 overlain unconformably by both the Caban and Rhayader Groups. 

 The base of the Gwastaden Group is formed of a thick mass of 

 grauwackes, which thin to the east and thicken to the west. They 

 contain Climacograptus, and pass up into mixed flags and grits with 

 Climacograptus and Diplograptas. These are succeeded by shales 

 and miidstones in which the first Monograptidse appear. These 

 become dominant in the upper part. 



The succeeding Caban Group has a maximum thickness of 

 1500 feet. Its lower division consists of two massive con- 

 glomerates, separated by shales ; its higher division is made up 

 of fine-grained grits, shales, and flags. Each member of the 

 group is overlapped to the east by the next sub-group above it, 

 until eventually the whole group disappears beneath the Rhayader 

 Pale Shales, which in the eastern areas rest directly on the 

 Gwastaden rocks. 



The Rhayader Group consists of pale green, blue, and grey shales 

 and mudstones, which overstep on to the Gwastaden beds; and may 

 possibly pass completely over them, and rest on the dark shales 

 farther east. 



After the Gwastaden rocks were laid down, the sea-floor appears 

 to have been elevated and denuded, a hollow being scoured out to 

 the eastward. Rapid sinking followed ; and the sea filled the 

 hollow with the Caban sediments, practically levelling it up by the 

 time that the deposition of the Pale Slates began. 



Tables of fossils enable the author to establish a complete com- 

 parison of the whole of the local zones of the Rhayader district with 

 those of Southern Scotland, "Wales, and Sweden. In the Rhayader 

 area we find, for the first time in Britain, the entire Yalentian suc- 

 cession developed in one general sequence ef rocks, with a more or 

 less common lithological character and with a fauna composed 

 throughout of similar palEeontological types. 



December 6th.— W. Whitaker, B.A., F.R.S., President, 

 in the Chair. 

 Dr. Blanfobd said that he had been asked by Prof. Jijjdd, who was 

 unable to attend, to say a few words about certain photographs sent 

 by Mr. E. H. L. Schwarz, and representing the Dwyka boulder-bed 

 and the rounded and grooved underlying surface, in the neighbourhood 

 of the Orange River near Hopetown and Prieska. The importance 

 of these photographs lay in the evidence which they afforded on a 

 disputed point. Dr. Sutherland and Mr. Griesbach had called 

 attention to the evidence of ice-action presented by the Dwyka 

 Conglomerate in Natal, and additional evidence had been brought 

 forward by several observers, especially by Mr. Dunn from the Orange 



