On the supposed Pre-Cambrian Rocks of St. DavioVs. 365 



unconformable to both ; and he asserts that the basement conglo- 

 merate of the Cambrian system lies quite un conformably on all 

 these rocks, and is in great part made up out of their waste. 



Taking up each of these groups in the order of sequence assigned 

 to them, the author maintained that the "Dimetian group" is an 

 eruptive granite, which has disrupted and altered the Cambrian 

 strata, even above the horizon of the supposed basal conglomerate. 

 He described a series of natural sections where this relation is 

 exposed, particularly one on the coast at Ogof-Llesugn, where the 

 conglomerate has been torn off and involved in the granite, and has 

 been intensely indurated, so as to become a kind of pebbly quartzite. 

 No other rock occurs within the granite mass except dykes of 

 diabase, which rise through all the rocks of the district, but are 

 especially abundant in the granite. The veins of finer granite, so 

 general in granite areas, are conspicuous here. In short, whether 

 studied in hand-specimens or on the ground, the rock is so unmis- 

 takably an eruptive mass that the author could not understand how 

 this view, which was that expressed on the Geological Survey maps, 

 should ever have been called in question. The manner in which it 

 has risen across the bedding of successive horizons in the Cambrian 

 series proves that, instead of being a Pre-Cambrian gneiss, it must 

 be much younger than all the Cambrian rocks of the district. 



The "Arvonian group" consists of quartziferous porphyries, or 

 elvans, associated with the granite, and of the metamorphosed strata 

 in tneir vicinity. Reference was made to natural sections where 

 the actual intrusion of the elvans across the bedding of the rocks 

 could be seen. 



The "Pebidian group" comprises a series of volcanic tuffs and 

 breccias, with inter stratified and intrusive lavas. The author main- 

 tained that this group forms an integral part of the Cambrian system 

 as developed at St. David's. It ha s been broken through by the granite 

 and porphyries, and is therefore of older date. Instead of being 

 covered unconformably by the Cambrian conglomerate, as asserted by 

 Dr. Hicks, the volcanic group is covered quite conformably by that 

 rock ; and seams of tuff are interstratified with the conglomerate and 

 occur on various horizons above it. The conglomerate, instead of 

 being mainly composed of fragments of the rocks beneath it, consists 

 almost entirely of quartz and quartzite, only four per cent, of frag- 

 ments having been found to have been derived from some of the 

 projecting lava-islands underneath it. 



From the evidence now brought forward, the author contended 

 that as the names " Dimetian," " Arvonian," and " Pebidian " had 

 been founded on error of observation, they ought to be dropped out 

 of geological literature. 



April 11.— J. W. Hulke, Esq., F.R.S., President, in the Chair. 

 The following communication was read : — 



" On the supposed Pre-Cambrian Rocks of St. David's." By 

 Archibald Geikie, Esq., LI.D., F.R.S., E.G.S. 



Part II. 

 In this second part of his paper the author gave the results of the 

 Phil. Mag. S. 5. Vol. 15. No. 95. May 1883. 2 D 



