328 PEor. 0. T. JONES ox the [Sept. 19 12, 



18. Tlie Geological SiErciUEE of Centeal Wales and tJie Adjoining 

 Regions. B}- Prof. Owen Thomas Jones, M.A., D.Sc, F.G.S. 

 (Read April ITth, 1912.) 



[Plate XXXIY— Geological Map.] 



Contents. 



Page 



I. Introduction 328 



II. Historical Review 329 



III. The Distribution of the Eock-groups 332 



(a) The Ordovician Rocks. 

 (6) The Sihiriau Rocks. 



IV. The Structure of the Area 339 



V. Relation of Structure to Topographic Features 342 



YI. Summary 343 



I. Inteodttction. 



The region herein discussed is that portion of AVales which lies, 

 roughly, between the River Dovey aud Pembrokeshire ; it is 

 bounded on the west by the coast of Cardigan Pay, and on the 

 east by an irregular line coinciding in part with the base of the 

 Wenlock rocks and in part with the margin of the Upper Palaeozoic 

 rocks. 



It comprises, therefore, the whole of Cardiganshire and a great 

 part of Carmarthenshire and Pembrokeshire, together with portions 

 of Montgomeryshire, Breconshire, and Radnorshire — an area ex- 

 ceeding 1800 square miles in extent. This tract is represented 

 upon geological maps by ' Lower Silurian ' colour, only diversified 

 by small areas indicating igneous rocks, or by tracts of yellow dots 

 indicating grits and sandstones. 



Ever since the days when Sedgwick and Murchison laboured to 

 introduce order among the old slaty ^nd volcanic rocks of Wales 

 the region defined above has always remained something of a 

 geological puzzle : the apparent uniformity and unfossiliferous 

 character of the strata over wide areas, and the compression which 

 they have suffered by folding, faulting, and cleavage, combined to 

 defy for a long time all attempts at unravelling its structure. 



In more recent years the discovery of numerous fossiliferous 

 beds among these strata has made possible the application of the 

 zonal method, based in this case upon the range in time of various 

 genera and species of graptolites. By this method, combined with 

 close attention to the lithological characters of the rocks, attacks 

 have been made upon this ' geological wilderness,' which have met 

 with a certain measure of success. 



At the present time the general rock-succession of this territory 

 is firmly established, and the characteristic organisms of the 

 different groups are well known ; it is believed also that enough 



