Mass of Anhydrite in Magnesian Limestone, 755 



The distribution of organisms in the Magnesian Limestone was 

 largely influenced by the quantity of sulphates present in the 

 surrounding water. The Shell Limestone is shown to be a chain 

 of reef-knolls, in the building up of which a limited number of 

 forms take part, probably induced by current-action in the Permian 

 Sea and lying more or less parallel with the old Permian shore-line. 

 The increasingly unfavourable conditions prevailing towards the 

 top of the Shell Limestone bring about a dwarfing and gradual 

 extinction of the typical Shell-Limestone fauna. 



The curious distribution and present position of the Upper 

 Magnesian Limestones in Durham is noticed, and an explanation 

 offered. 



The Permian succession is shown to be more complete in tire 

 southern than in the northern area of the county. 



Various sections in the Upper and Upper Middle Limestones in 

 the Hartlepool area are described, among them the recent sinking 

 for BlackhaU Colliery, where all the series were pierced, including 

 the full thickness of the Shell Limestone. 



February 26th, 1913.— Dr. Aubrey Strahan, F.R.S., President, 



in the Chair. 



The following communications were read : — 



1. ' The Geology of Bardsey Island (Carnarvonshire).' By Charles 

 Alfred Matley, D.Sc., F.G.S. ; with an Appendix on the Petrography 

 by John Smith Flett, M.A., D.Sc, F.G.S. 



Bardsey, an island a mile and three-quarters long, lies off the 

 promontory of the Lleyn (Western Carnarvonshire), and forms the 

 isolated extremity of the strip of pre-Cambrian rocks that borders 

 the western coast of the Lleyn from Nevin south-westwards. 



The rocks are principally gritty schistose slates, with many thin 

 and some thick bands of grit, quartzite, and limestone; and they 

 contain an horizon of variolitic lava and tufaceous shale, which 

 indicates that a volcanic episode took place during their formation. 

 Sills of albite- diabase also occur, as well as one or more sills of a 

 crushed granite. 



The rocks have been subjected to intense earth-pressure acting 

 mainly from the north-west, and are mostly in a cataclastic con- 

 dition, the harder rocks being almost always torn up into lenticles. 

 The beds are shown to be arranged on the whole in a number 

 of isoclinal folds, complicated by overth rusting, shearing, and 

 brecciation. Stages in the formation of crush-conglomerates are 

 described. From the nature of the brecciation and the compara- 

 tively small amount of mineral alteration that the beds have 

 undergone, it is inferred that the load of superincumbent rock at 

 the time of the principal earth-movements was not great. 



The rocks are correlated with the lower portion of the Llanbadrig 

 Beds and with the Llanfair y'nghornwy Beds of Anglesey, and they 

 agree also with their Anglesey representatives in the manner in 

 which they have been affected by earth-movement. 



