332 Reports and Proceedings — Geological Society of London. 



section, and indicates the predominance of a Mendip faoips. Much 

 dolomite occurs in tlie lower part of S, indicating sliallow-water 

 conditions during part of Seminnla time. There is a distinct 

 development of the semireticnlatus subzone (SJ, which exhibits 

 a famial overlap between and S ; and the base of S2 is remarkable 

 for its rich and varied fauna. 



The Woodspring Ridge shows a sequence from the Upper 

 Zaplirentis zone (^2) to the top of Cj, the whole sequence being 

 exactly similar to that of Clevedon. In this ridge the contemporaneous 

 igneous rocks occur in horizon 7. In the Weston Worle Ridge 

 they are put above the ' Cnninia Oolite,' that is, about 450 feet 

 below the top of C. It is, therefoi'e, evident that there were two 

 periods of volcanic activity, one of which occurred at the close 

 of Znphrentis time and tlie other early in Syringothyris time. Notes 

 are given on the faunal sequence. 



IL— May 24th, 1905.— J. E. Marr, Sc.D., F.R.S., President, in 

 the Chair. The following communications were read : — 



1. " On the Igneous Rocks occurring between St. David's Head 

 and Strumble Head (Pembrokeshire)," By James Viuceut Elsden, 

 B.Sc, F.G.S. 



The author finds that the contemporaneous lavas of the Llanrian 

 area agree generally in character with the eruptive rocks of 

 apparently Ordovician age in the Strumble Head and Prescelly 

 districts. These are all of an essentially acid type. The intrusive 

 rocks of the area are of later date, and belong to three distinct 

 types : — (1) The gabbros and diabases of the Strumble Head area; 

 (2) the norites and associated rocks of St. David's Head and the 

 surrounding district; and (3) the liine-bostonites and porphyrites of 

 the Abercastle Mathry district. Detailed petrographical descriptions 

 of the different types are given, accompanied in many cases by 

 analyses and comparisons with corresponding or related rocks of 

 other areas. The lime-bostonites, which have affinities with those 

 of Ma3na described by Brogger, but are much more basic tlian the 

 Wicklow keratophyres, are apparently the oldest of the intrusive 

 rocks, and seem to belong to the petrographical province of Soutli- 

 Eastern Ireland. The gabbros and norites were intruded approxi- 

 mately during the same interval ; at a later period the norites, 

 enstatite-diorites, and the rest of the rocks associated with thera 

 spread north-eastward from St. David's Head, and penetrated the 

 area of the Strumble Head intrusions. Similarly, the gabbros and 

 diabases spread to a more limited extent south-westward into 

 the norite area. In the overlapping area the gabbro and norite 

 ))rovinGes ai'e separated by an ill -defined zone, in which some 

 mixture of the two magmas took place. The latest phase of 

 igneous activity was the formation of the Pen-Caer basalt-laccolite, 

 with apophyses penetrating the Garn-Fawr to Y-Garn intrusions. 

 It is not necessary to assume that each of the several intrusions 

 was confined to any single stage of vulcanicity. The laccolites 

 and bosses were probably the result of injections extending over 

 a prolonged interval from co-existing magma basins, or from a single 



