.Vol. 56.] SILURIAN SEQUENCE OF RHAYADER. 137 



the officers of the Geological Survey. Unconformable overlap of 

 the Tarannon and Upper Llandovery Series on older beds had been 

 described, but never satisfactorily proved. The Geological Survey 

 had classed the Lower Llandovery with the Ordovician Series, and 

 the speaker would like to ask the Author whether in the Rhayader 

 District there was any evidence of a passage between the two series. 



Prof. C. Lapworth said that he had been interested in the Mid- 

 Wales country since the late Walter Keeping had discovered Birk- 

 hill graptolites within it, and he had himself pointed out some of its 

 resemblances to the Upland region of Southern Scotland, but he had 

 never found an opportunity of studying an} r of its geology in detail. It- 

 was pleasant to find, from the Author's results, how remarkably the 

 graptolite-fauna of the Iihayader Series corresponded in detail with 

 that of other regions in Britain and North-western Europe. The 

 peculiar phenomena of unconformity and overlap described by the 

 Author were very striking ; but they seemed to be in harmony with 

 well-known appearances in other districts, among the strata laid 

 down during that Valentian period of great crust-movement which 

 intervened between the Upper Bala and the Lower Wenlock. He had 

 twice visited the Rhayader District and neighbourhood, and had been 

 struck by the fine development of the conglomerates and grits at 

 Caban Coch and elsewhere, as contrasted with the masses of shales 

 and slates among which they occurred. The whole country seemed 

 to be an ideal one as a source of water-supply — the broader shaly 

 valleys affording admirable sites for impounding-reservoirs, and the 

 narrow rock-bound gorges for retaining-dams. Mr. Mansergh had 

 already brilliantly utilized part of the region for the Birmingham 

 water-supply, and the speaker looked forward to the time when the 

 rest of it would be similarly utilized by Sir Alexander Binnie for 

 the water-supply of London itself. 



The President and Sir Alexander Binnie also spoke. 



The Author thanked the Fellows for the very kind manner in 

 which they had received his paper. In reply to Mr. Marr, he stated 

 that a drawing of the graptolite Diplocjraptus modestus, Lapw. had 

 been figured as such, but not described. In answer to a question 

 from Prof. Blake, with regard to the relations of the Gwastaden 

 and Caban Groups, he said that the break between the two was 

 more of a stratigraphical than a palaeontological nature, as many of 

 the graptolites occurring in the topmost beds of the Gwastaden 

 Group were found in zones well up in the Caban Group. The 

 brachiopoda obtained from the Iihayader district bore satisfactory 

 comparison with those quoted from Llandovery. As to the strati- 

 graphical relations of the Gwastaden and local Bala rocks, the two 

 sets of deposits appeared to be conformable within the limits of the 

 Iihayader District. 



