COLLECTED BY DK. HICKS IN N.W. PEMBEOKESHIRE. 363 



We have, then, in Pembrokeshire exactly the alternative presented 

 in Caernarvonshire and Anglesey * ; and in either case it results that 

 the granitoid rocks, whether igneous or not, are much older than 

 the conglomerate, so that the latter forms a good base to the Cam- 

 brian series. Erom these conclusions I see no way of escape, except 

 by denying the facts on which they are founded. 



Discussion. 



The President said that these papers dealt with one important 

 branch of the discussion concerning the age of the rocks of St. 

 David's. It is a crucial point to ascertain whether detritus derived 

 from the rocks called Dimetian occurs in the Cambrian ; and that 

 this is the case the Authors consider that they have succeeded in 

 proving. 



Prof. Blake thought the question had been fairly threshed out 

 before ; he only regretted that the examination had not been trans- 

 ferred to a locality where the evidence is clearer than at St. David's. 

 He suggested that the Harlech area would afford important results. 



Mr. MaPvR said that Mr. Roberts and himself, in their paper on the 

 neighbourhood of Haverfordwest, had spoken with caution about the 

 age of the volcanic ashes they had observed, and the subsequent dis- 

 covery of ashes in the Lingula-fiags justified their caution ; but the 

 evidence now brought forward by Dr. Hicks confirmed their sug- 

 gestion. He called attention to the resemblance of the halleflintas 

 to siliceous sinters. 



Mr. Teall said that the specimens of rock from Trefgarn which 

 he had partially analyzed for Mr. Marr yielded SiO.^ 97'5 %, Al^Og, 

 Fe^Og, 1-1 %? loss on ignition 1-8 Yo- 



Dr. Selwyn said it was so long since he had examined the Welsh 

 rocks that he had almost forgotten them, but he had seen rocks of 

 similar aspects in Canada, and was now convinced that the meta- 

 morphic rocks of Anglesey and Caernarvonshire were not altered 

 Cambrians. So far as North Wales was concerned, he was inclined 

 to agree with Dr. Hicks. At the same time it was extremely diffi- 

 cult to classify these ancient formations, and generally it was 

 impossible to determine which was the top and which the bottom 

 of the sequence. In Cape Breton, J^ova Scotia, however, unaltered 

 Lower Cambrian rocks were found resting unconformably on highly 

 altered rocks, gneiss, &c., presumably of Laurentian or Huronian age. 



Dr. Hicks, in reply, thanked the Pellows, and especially Dr. 

 Selwyn, for thefr remarks, and said he had more evidence to bring 

 forward, if necessary. He considered Dr. Selwyn's original descrip- 

 tions of the North- Wales rocks most valuable. 



Prof. BoNNEY thought it doubtful whether any of the rocks at Eoch 

 Castle were originally sinter. The hollow structure is probably 

 vesicular. At the same time he was not surprised to find that the 

 rock consisted almost entirely of silica, for the microscope showed 

 that the felspathic constituent in the rock had been almost wholly 

 replaced by silica. 



* Quart. Journ. Greol. Soc. vol. xl. p. 200. 



