374 ME. H. H. THOMAS AND PROE. O. T. JONES ON THE [Sept. I912, 



21. On the Pre-Cambrian and Cambrian Rocks of Brawdt, 

 Hayscastle, and Brimaston (Pembrokeshire). By Herbert 

 Henry Thomas, M.A., B.Sc, Sec.G.S., and Prof. Owen Thomas 

 Jones, M.A., D.Sc, P.G.S. (Eead April 17th, 1912.) 



[Plate XL — Geological Map.] 



Contents. 



Page 



I. Introduction 374 



{a) Historical review 375 



{h) Bibliography 377 



II. The Pre-Cambrian Rocks 377 



(o) The Pebidiau 377 



(6) The Diinetian 380 



(c) Petrography 384 



III. The Cambrian Rocks 390 



(a) Their Succession and Distribution 390 



{b) Their Relation to the Pebidian and 



Dimetian 394 



(c) Post-Cambi'ian Basi c Dykes 396 



IV. Tectonics 396 



V. Summary and Conclusions 398 



I. Introditction. 



During the recent revision of the geology of the Pembrokeshire 

 coalfield by the Geological Survey, a portion of the area dealt with 

 in this paper had to be surveyed by one of us on the scale of 

 6 incbes to the mile. The work had not progressed far before it was 

 recognized that this area was one of exceptional interest, and one 

 in which the relations of the Lower Cambrian to the pre-Cambrian 

 rocks could be made out with little difficulty. We therefore decided 

 to make an unofficial survey, on the same scale, of that portion of 

 the district, containing Lower Cambrian and older rocks, which 

 lay beyond the limits of the proposed publications of the revised 

 geological survey. 



The area surveyed and represented by the appended map 

 (PI. XL) is that portion of Pembrokeshire which stretches from 

 the Uiver Cleddau at St. Lawrence to St. Bride's Bay at Newgale, 

 and includes the villages of St. Lawrence, Hayscastle, and Brawdy. 

 Its total length from east to west is about 9 miles, and it has a 

 maximum breadth of about 2 miles. 



The country is of low relief, seldom rising to more than 400 feet 

 above sea-level, and forms a somewhat elevated part of the West 

 Pembrokeshire plain. The surface-configuration has but little 

 relation to the nature or texture of the rocks beneath, and natural 

 exposures are of rare occurrence, except on the sides of the deeper 

 valleys and in the sea-clifis. 



