Reports and Proceedings — Geological Society of London. 281 



April 17, 1912. — Dr. Aubrey Strahan, F.K.S., President, in 

 the Chair. 



The President announced that the Council had awarded the 

 Proceeds of the Daniel-Pidgeon Fund for the present year to Otway 

 H. Little, M.A., Royal College of Science for Ireland, Avho proposes 

 to investigate the chemical and mineral changes which have taken 

 place in the metamorphic limestone of Connemara. 



The following communications were read : — 



1. "The Pre-Cambrian and Cambrian Eocks of Brawdy, Hayscastle, 

 and Brimaston (Pembrokeshire)." By Herbert Henry Thomas, M.A., 

 B.Sc, Sec. G.S., and Professor Owen Thomas Jones, M.A., D.Sc, 

 F.G.S. 



The district dealt with in this paper lies about eight or ten miles to 

 the east of St. Davids, and consists of pre-Cambrian plutonic and 

 volcanic rocks surrounded by, and intimately associated with, 

 sedimentary rocks of the Cambrian System. 



The pre-Cambrian igneous and pyroclastic rocks are brought to 

 the surface along an anticlinal axis which ranges in an east-north- 

 easterly and west-south-westerly direction, that is to say, approxi- 

 mately parallel to the ancient ridge of St. Davids. They are 

 distributed over an area measuring about nine miles in length by 

 two miles in greatest breadth. 



The pre-Cambrian rocks are divisible into two classes, an older 

 volcanic series and a newer plutonic and hypabyssal series, for which 

 Hicks's names of Dimetian and Pebidian are respectively retained. 



The authors have not attempted any detailed subdivision of the 

 Pebidian over the whole area, but it is clear that several stages are 

 represented. The lower exposed portion is generally andesitic in 

 character, the upper being rhyolitic and keratophyric. 



The Dimetian comprises granite, quartz-porphyry, and diorite, 

 which are intruded into the Pebidian, and present a common feature 

 in the abundance of soda-felspar. Petrographical descriptions of the 

 various pre-Cambrian rock-types are given. 



The Cambrian has been divided into two main groups — the Welsh 

 Hook Group below and the Ford Beds above. The Welsh Hook 

 Group compares bed for bed with the Caerfai and Lower Solva Series 

 of Hicks, and, like similar beds at St. Davids, consists of basal con- 

 glomerate, green sandstones, red shales, and purple sandstones. 



The position of the Ford Beds, which are mostly shales, is not so 

 certain ; but the evidence is in favour of their belonging to the 

 Upper Solva stage, and their having transgressed lower members of 

 the Solva Series. 



The basal bed of the Cambrian apparently rests upon rocks of 

 different ages in different parts of the district ; and this fact, taken 

 into consideration with other evidence, indicates that the Cambrian 

 reposes un conformably on a complex series of tuffs and lavas and 

 of plutonic rocks intruded into these volcanic rocks. 



The structure of the district is that of a horst, faulted on all sides 

 and surrounded bv much younger beds. The main fractures follow 



