Geological Society of London. 331 



Bridge a Cambrian conglomerate, overlain by Olenus-sk&les, is to be 

 seen resting on the eroded edges of the Trefgarn series. The author 

 examined this section lately, and obtained from the Conglomerate 

 some very large pebbles of the characteristic rocks called halleflintas, 

 and of the ash-bands, both of which are found in situ in the quarry. 

 He therefore maintained that there was the most ample evidence to 

 show that there was a great group of pre-Cambrian rocks exposed in 

 N.-W. Pembrokeshire, and hence that he had proved conclusively 

 that Dr. Geikie's views in regard to these rocks, as given in his 

 paper and more recently in his text-book, are entirely erroneous. 



2. " On some Bock-specimens collected by Dr. Hicks in North- 

 western Pembrokeshire." By Prof. T. G. Bonney, D.Sc, LL.D., 

 F.B.S., F.G.S. 



The author stated that he had examined microscopically a series 

 of specimens collected by Dr. Hicks, and compared them with those 

 described by Mr. T. Davies, in vol. xl. of the Quarterly Journal, and 

 with some in his own collection. He agreed with Mr. Davies's 

 conclusions in all important matters. 



The Chanter's-Seat conglomerate contained many grains of quartz 

 and felspar, curiously like those minerals in the so-called Dimetian, 

 together with numerous small rolled fragments, about a quarter of 

 an inch in diameter, exactly resembling the finer-grained varieties 

 of that rock, besides bits of felsite, similar to some which occur in 

 the St. David's district, quartzite, a quartz-schist, and an argillite. 



The rocks in situ in the Trefgarn quarry were indurated trachytic 

 ashes, together with the curious flinty rock which was the most 

 typical of the so-called halleflintas. One of the pebbles from the 

 overlying conglomerate perfectly corresponded with the last-named 

 rock, others appeared to be most probably from an altered trachytic 

 ash, differing only varietally from those in situ. 



After prolonged examination of this " halleflinta " of Trefgarn 

 and the similar rocks from Boch, he was of opinion that while it was 

 possible that some specimens might be altered ashes, most of them 

 were originally rhyolites or obsidians, devitrified, and then silicified 

 by the passage of water which had contained silica in solution. 

 The Trefgarn group obviously could not be intrusive in the Lower 

 Cambrian, and it was extremely improbable that the Boch Castle 

 series was newer than the basement conglomerate of that district. 



The Brawdy granitoid rock might be a granite, but at any rate it 

 presented considerable resemblance to the ''Dimetian." 



It was therefore evident that the Cambrian conglomerate of St. 

 David's was formed from a very varied series of rocks, some of them 

 much older than it, and that the Dimetian could not be intrusive in 

 it. Moreover, even if the Dimetian should be proved ultimately to 

 be a granite, and the core of a volcano which had emitted the rhyo- 

 lites, sufficient time must have elapsed after its consolidation and 

 prior to the making of the conglomerate to remove, by denudation, 

 a great mass of overlying rock. Hence, whatever its nature, it was 

 pre-Cambrian. 



3. " On the Glaciation of South Lancashire, Cheshire, and the 



