I. — On the Geology of Southern Pembrokeshire. 



By H. T. De la BECHE, Esq. F.R., L. and G. S.S. etc.* 



[Read May 16th 1823,] 



xHE district which forms the subject of the present memoir, comprehends 

 the whole of Southern Pembrokeshire. It deserves the attention of the ge- 

 ologist, not only from its including the western extremity of the great coal- 

 basin of South Wales, and exhibiting along the boundaries of that basin, 

 within a narrow compass, the mutual and diversified relations of the carbo- 

 niferous and transition systems of rocks ; but also from its presenting in both 

 those systems striking instances of disturbance and contortion, and displaying 

 perhaps in a more remarkable manner than any other district in England the 

 real character of those varieties of trap which so commonly occur associated 

 with transition formations f. The long series of natural sections presented 



* The author begs to state from what sources he has derived any important information on the 

 subject of the present memoir. — A valuable paper by Dr. Kidd, giving the results of his observa- 

 tions on the trap. rocks near St. David's, made during a journey in the year 1813, was published in 

 the second volume of the first series of the Geological Transactions, in the year 1814. In that 

 paper will be found the most accurate account then made public of the greenstone-rocks asso- 

 ciated with transition formations. The Rev. W. D. Conybeare, who had been the companion of 

 Dr. Kidd on the journey above referred to, again visited the district in the year 1816, and on this 

 occasion succeeded in ascertaining the general structure of the district ; but the tempestuous 

 state of the weather having prevented him from examining the coast, the most decisive evidence 

 was yet wanting to prove the accuracy of his conclusions. The want experienced by Mr. Cony, 

 beare of any thing deserving the name of a map has since been completely remedied by the 

 publication of the Ordnance map of Pembrokeshire : it is scarcely possible to appretiate too 

 highly the assistance which this and the other parts of that splendid survey of England are cal- 

 culated to afford to the geologist. Availing himself of all this information, and particularly of 

 the notes and advice of his friend Mr. Conybeare, in the summer of 1822 the author visited 

 every part of the district, and more particularly directed his attention to the sections presented 

 by the coast. He has drawn up the present memoir from the notes made by him on that excursion. 



t The district derives additional interest from the close resemblance it bears, in geological" 

 structure, to the Isle of Anglesea, at the opposite extremity of the coast of Wales. — See Mr. Hens- 

 low's paper in the Transactions of the Cambridge Philosophical Society, vol. i. page 359. 



VOL. II. — SECOND SERIES. B 



