[ 615 ] 



LXIX. Procedings of Learned Societies. 



GEOLOGICAL SOCIETY. 



[Continued from p. 192.] 



May 24th, 1905.— J. E. Jtfarr, Sc.D., F.R.S., President, 

 in the Chair. 



rPHE following communications were read : — 



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

 and Strumble Head (Pembrokeshire).' By James Vincent Elsden, 

 B.Sc, E.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 lime-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 Msena described by Brogger, but are much more basic than the 

 Wicklow keratophyres, are apparently the oldest of the intrusive 

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

 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 them 

 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- 

 provinces are 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-Eawr 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 coexisting magma-basins, or from a single 

 differentiated magma. There are clear evidences of some further 

 differentiation in situ, but the full extent to which this took place 

 offers a large field for future investigation. 



