Geolocjlcal Society of London. 285 



Tbe meaning of the name " Dyas," which has become well estab- 

 lished abroad, was illustrated, since it is often overlooked by English 

 geological writers ; and a Dyassic order was pointed out as existing 

 to some extent in the English series. 



Some general reasons, based on the physical characteristics of the 

 Dyas-group, were given for regarding it as much more closely allied 

 to the preceding Carboniferous than to the succeeding Trias. 



The last portion of the paper was more speculative, and in it an 

 attempt was made to trace, in the facts we know of the geology of 

 Central Europe, and the inferences drawn from them, the causes of 

 the apparent anomaly between the fauna of the Post-Carboniferous 

 strata of more northern Europe and tliat of the Alpine Trias. 



II.— May 14, 1884.— 1. " On the Pre-Cambrian Eocks of Pem- 

 brokeshire, with especial reference to the St. David's District." By 

 Dr. Henry Hicks, F.G.S., with an Appendix by Thomas Davies, 

 Esq., F.G.S. 



The author, in this paper, gave further detailed evidence in 

 addition to that already submitted by him, to show that the Geological 

 Survey Map of the district of St. David's and of other parts of Pem- 

 brokeshire is incorrect in some of its most essential features, and 

 inaccurate in very many of its petrographical and stratigraphical 

 details. Some new areas in South Pembrokeshire were also referred 

 to. He replied also to the criticisms contained in the paper by the 

 Dii-ector-General of the Survey, read last year before the Society, 

 and indicated that Dr. Geikie had completely misunderstood the 

 sections and the order of succession of the rocks at St. David's. He 

 pointed out that the views so elaborately worked out by the Director- 

 General, to show the evidence of metamorphism in the rocks, were 

 based on the entirely false supposition that the granitoid rocks were 

 intrusive in the Cambrian rocks, and that the felsites were merely 

 peripheral masses. He showed, by producing abundant fragments 

 of the Granitoid rocks and of the Felsites from the basal Cambrian 

 conglomerates, that the granitoid rocks were the very oldest rocks in 

 the district, and that they must undoubtedly be of Pre-Cambrian age. 

 He proved, from microscopical evidence, that the rocks supposed to 

 have been altered by the intrusion of the granitoid rocks, were in 

 the condition in which they are now found before the Cambrian 

 rocks were deposited, and moreover, that the supposed concretions 

 in the Porcellanites and Conglomerates, claimed to have been due 

 to metamorphism, had turned out, on microscopical evidence, to be 

 actually fragments of old Pre-Cambrian rhyolites enclosed in the 

 sediments. It was shown also that at the points indicated by the 

 Director-General, where the evidences of intrusion were supposed 

 to be seen, there was not the slightest change of a metamorphic 

 character induced in the sedimentary rocks in contact with the grani- 

 toid rocks. The only difference that could possibly be recognized in 

 them by the aid of the microscope was such as is well known to be 

 the result of crushing when in the neighbourhood of faults. Indeed 

 there was the clearest evidence possible to show that the junctions 

 were merely fault-junctions. The supposed fold in the Pebidian 



