PEBIDIAN ROCKS OF PEMBROKESHIRE. 167 



to which the papers just read related. He said that, in considering 

 the divergence of the opinions advocated by recent authors from 

 those which had been long entertained, it must always be remem- 

 bered that the later investigators possessed a means of research which 

 was not at the command of their predecessors, the great progress of 

 the study of petrology during the last few years furnishing them 

 with a means of distinguishing and correlating these rocks com- 

 parable to that offered to the student of later sedimentary deposits 

 by the fossils contained in them. He asked Mr. Hicks whether he 

 could say any thing as to the age of the great crumplings and 

 foldings described. 



Mr. Tawney stated that there were porphyries near the fault taken 

 as the base of the Dimetian by Mr. Hicks, which, as in the quarry 

 below the Board-school, contain dihexahedral quartz &c, and are 

 spherulitic ; again, near the Church-school, another coarser porphyry 

 having much the aspect of ordinary Dimetian, but distinguished by 

 large dihexahedral quartz, is also somewhat spherulitic, the radiating 

 structure enveloping the quartz crystals. These porphyries are ap- 

 parently eruptive, and seem to have produced contact-alteration ; for 

 the Dimetian beneath them is dense and fine-grained, as if the con- 

 stituents had been partially rearranged ; their nature, however, re- 

 mains the same, not even the chloritoid mineral being absent. We 

 may presume, then, that the Dimetian had assumed its crystalline 

 character (metamorphic, Hicks) before the intrusion of the porphy- 

 ries. Now, light is thrown on the age of the latter by the curious 

 agglomerate above Clegyr bridge, which bed is placed by Mr. Hicks 

 at the base of the Pebidian : fragments of analogous spherulitic rock 

 enclosed therein have the same mineral composition, and are probably, 

 roughly speaking, contemporaneous with the intrusive masses above- 

 mentioned in the Dimetian. Hence, presuming the metamorphic 

 origin of the Dimetian, we arrive at the later age of the Pebidian 

 series, and its total unconformity to the Dimetian. Moreover the 

 Dimetian is crystalline throughout, while the Pebidian contrasts 

 strongly by its bedded shaly character. 



Mr. Hudleston had travelled over the districts in company with 

 Mr. Hicks and Prof. Hughes, and found that he had undertaken to 

 furnish a short Appendix to Mr. Hicks's paper. Upon instituting a 

 comparison between the St.-David's district and North Wales, the 

 principal datum-line seems to be the great conglomerate taken as 

 the base of the Cambrian, which may be deemed fairly synchronous 

 in both areas. The point at issue was whether the beds below this, 

 forming the subject of the present papers, were really pre-Cambrian, 

 or had been metamorphosed and intruded at a subsequent period. 



The contents of the conglomerate were very much in favour of the 

 author's views. 



Beginning now with a description of the lowest beds of the 

 double series, there was considerable general resemblance between 

 the highly crystalline and granitoid rock of Caernarvon and many of 

 the Dimetians of St. David's, held by the authors to occupy an 

 analogous position; but there was much greater complexity of 



