310 Gerald M. Part— 



Dr. Andrews concludes that this reptile swam in deep water instead 

 of merely paddling along on the surface like the plesiosaurs. In 

 conclusion, I will only add that we have tried to follow out the idea 

 of " stream-lines ", so important to those who design ships, sub- 

 marines, and air-ships, etc. The teeth are rather too conspicuous, 

 but this was done with a purpose, because otherwise they v/ould 

 hardly be visible, for in life they would be largely hidden by the 

 flesh surrounding the bone of the jaws. 



Similar casts have been presented to the British Museum 

 (Natural History), to the New Museum, Oxford, and to the 

 Geological Society. 



Notes on the Ordovician Lavas of the Mynydd 

 Prescelly, N. Pembrokeshire. 



By Gerald M. Paet, M.A., F.G.S. 



PEMBROKESHIRE is, on the whole— for Wales— a com- 

 -*- paratively low-lying county, a somewhat dissected peneplain 

 not usually rising much above four or five hundred feet above sea- 

 level. Apart from the ragged stacks which rise up above the level 

 of this peneplain in the northern part of the county— Carn Llidi 

 and Pen-berry near St. Davids and the Strumble Head masses near 

 Fishguard, for example — there is only one really promiuent feature 

 in the whole Pembrokeshire landscape, the Mynydd Prescelly, which 

 rise near the western end of their ridge above Rosebush to a height 

 of 1,760 feet. 



It is therefore somewhat surprising to find a great scarcity of 

 geological literature dealing with this part of the county, and the 

 only work of note which I have been able to discover is a paper by 

 J. Parkinson.^ The bulk of this is concerned with the spherulitic 

 rhyolites which occur on Carn Alw and Foel Trigarn at the north- 

 east end of the hills, though Parkinson briefly mentions " certain 

 more basic rocks " which he describes as being " of somewhat 

 uniform and monotonous appearance ", and also the gabbroid 

 dolerites which form many of the prominent " Cams " along the 

 summit of the ridge. 



These have recently added interest to the problems of Stonehenge, 

 Dr. H. H. Thomas having identified the " blue stones " of this 

 monument with the peculiar spotted dolerite which forms Carn 

 Meini and other " Cams " at the eastern end of the Prescellys.^ 



It is to be regretted that even the glamour of Stonehenge 

 can hardly make up for all the epidote, chlorite, and other decom- 

 position products with which these rocks are afiected. Hitherto 

 there has been some doubt as to their exact age, though it is probable 

 that, like the other dolerites of North Pembrokeshire, these 

 intrusions are older than the main folding which is of Valentian or 



1 Q.J.G.S., vol. liii, 1897, p. 465. 



^ Geol. Survey — Summary of Progress, 1920, p. 56. 



