198 Geological Society: — 



distributed over an area measuring about 9 miles in length by 

 2 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 rbyolitic 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 

 conglomerate, 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 unconformably 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 by much younger beds. The main fractures 

 follow an east-north-easterly to west-south-westerly direction, but 

 frequently branch and run together, thus enclosing lenticular 

 masses. 



Much of the faulting is of pre-Carboniferous age, but that it 

 continued into Carboniferous times is shown by the manner in 

 which the Carboniferous rocks of the district have been affected. 



2. ' The Geological Structure of Central Wales and the Adjoining 

 Region.' By Prof. Owen Thomas Jones, M.A., D.Sc, F.G.S. 



This paper deals with the structure on a large scale of an area 

 of about 1800 square miles, comprising the western portion of 

 Wales between the River Dovey and South Pembrokeshire. 



In a historical introduction the work of earlier observers, notably 

 Sedgwick, Ramsay, and Walter Keeping, is referred to. 



The paper is accompanied by a map whereon is indicated the 

 distribution of certain rock-groups : this map is based partly on 

 personal observations, and partly on information gathered from 

 various publications. 



On the map the structure of the area is easily perceived. There 



