PEBIDIAN ROCKS OF PEMBROKESHIRE. 169 



In reply to the President's question, he said he believed all the 

 great faults pointed out were of post-Carboniferous age, as they 

 affected not only the Cambriau rocks, but also the Silurian, and in 

 some cases brought Carboniferous beds to rest directly on Cambrian 

 beds. Some of the other faults he believed were of much earlier 

 date, possibly pre-Cambrian. 



To Prof. Hughes, Mr. Hicks said he was sorry that they would 

 have to disagree on one or two points. He sympathized with him 

 in trying to claim all he could for his eminent predecessor ; but he 

 was certainly wrong in stating that Prof. Sedgwick had ever fore- 

 told that these ribs of porphyries were of pre-Cambrian age ; he had, 

 on the contrary, stated distinctly in some of his latest papers, in 

 describing the Cambrian rocks between Bangor and Caernarvon, that 

 they (the Cambrians) were " cut through by a great intrusive rib of 

 syenitic porphyry of a different epoch," and in nearly the same 

 words he described also the greater mass near Llanberis, &c. 

 Again, he could not possibly believe with Prof. Hughes that faults 

 or any other accidental cause could have produced the unconformity 

 between the Dimetian and Pebidian. The two series are in every 

 case, wherever exposed, unconformable to one another; and the 

 presence of large angular masses of the Dimetian rocks continually 

 in the agglomerates at the base of the Pebidian, together with the 

 certainty that these agglomerates were for the most part cast out of 

 a subaerial volcano, are, he thought, more than sufficient proof of 

 physical changes at the time, which would be followed by a marked 

 break in geological succession. 



Prof. Hughes, in reply to Mr. Hicks, said he thought he might 

 be pardoned for exhibiting some satisfaction at finding additional 

 evidence of the accuracy of Prof. Sedgwick's original work, which 

 had been so long under a cloud in that Society. As to the argu- 

 ment inffavour of an unconformity between Pebidian and Dimetian 

 from the occurrence of fragments of the older in the newer rocks, 

 he said that, if he was right in considering the whole as one 

 volcanic series, it was quite in accordance with what is observed in 

 connexion with volcanic activity in modern times that fragments of 

 any part of older volcanic accumulations, or of hardened necks and 

 flows of lava, should be thrown out in subsequent eruptions, — and 

 that the agglomerates of the Bangor and Pebidian rocks themselves 

 agreed in containing numerous fragments which appeared to belong 

 to the very series in which the agglomerates occurred. With regard 

 to the possibility of its turning out that the pre-Cambrian volcanic 

 series were only metamorphosed Cambrian, he pointed out that the 

 lower part of the Cambrian series was well known : and if there 

 were no other stratigraphical and petrological difficulties there 

 would be this objection, that there was in the Cambrian of that 

 district no sequence of beds at all corresponding to what would be 

 required to furnish any thing like the Bangor group. In answer to 

 a question by Mr. Maw, he said he thought that the only uncon- 

 formity for which there was any evidence was at the base of the 

 Cambrian Conglomerate and Grit. 



Q. J. G. S. No. 134. n 



