Societies and Academies

Abstract

LONDON Geological Society, March 8. D. A. BRYN DAVIES: The Ordovician rocks of the Trefriw district, North Wales. The area described extends from Trefriw and Dolgarrog in the Conway valley south-westwards to Capel Curig. The Glanrafon beds are correlated with the beds immediately underlying the Snowdon volcanic suite in Snowdonia, and with the beds of the same name underlying the volcanic suite at Dolwyd-delan. The Crafnant volcanic series is considered to represent an incomplete development of the Snow-donian suite, the bedded pyroclastic and probably also the upper rhyolitic series being absent. In the neighbourhood of Dolgarrog, the Crafnant series is faulted against a thick development of pumice-tuff and spilitic agglomerate overlain by rhyolite-tuff and rhyolite. These are regarded as contemporaneous with the Crafnant series, but representing, in part at least, the products of a different centre of activity. The fauna of the Llanrhychwyn Slates, comparing closely with that of the Lower Cadnant Shales of Conway, places the underlying volcanic series in the Llandeilian, and indicates its correlation with the upper part of the Conway Volcanic series. V. WILSON: The Corallian rocks of Yorkshire. (1) The Howardian Hills. Three divisions of the Corallian are recognised and their stratigraphical relations are dealt with in detail. The important role played by the spicular remains of the lithistid sponge Rhaxella perforata Hinde in the building up of the Lower Calcareous Grit is recognised for the first time. Calcareous, and many varieties of siliceous, spicules are described, and their distribution is discussed. Though perfect lithological continuity exists between the Lower Calcareous Grit and the overlying Osmington Oolite series, the absence of any representatives of the Hambleton Oolite series and the Middle Calcareous Grit containing fossils characteristic of these two divisions is considered to indicate the existence of a non-sequence in this area. In the later stages of the Osmington Oolite period reefs became established, and numerous facies deposits accumulated from the erosion of the reefs. As the Lower Calcareous Grit passes gradually into the Oxford Clay below, so the Upper Calcareous Grit gradually gives place to the overlying Kimeridge Clay in the North Grimston district, there being no unconformity such as was formerly supposed.