On the Pre-Cambrian Rocks of Pembrokeshire. 377 



replied also to the criticisms contained in the paper by the Director- 

 General of the Survey, read last year before the Society, and indi- 

 cated 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 gra- 

 nitoid rocks and of the felsites from the basal Cambrian conglo- 

 merates, 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 cha- 

 racter induced in the sedimentary rocks in contact with the granitoid 

 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 

 rocks, the author stated, was impossible if petrological evidence was 

 of any value. The author also produced many facts to show that 

 the conglomerates at the base of the Cambrian constantly over- 

 lapped the different members of the series which he claimed to be 

 of Pre-Cambrian age, and that the unconformity was very marked 

 and to be clearly seen in many coast-sections. The conglomerates 

 were shown also to contain well-rolled pebbles of all the series in- 

 cluded under the names Dimetian, Arvonian, and Pebidian, as proved 

 by careful microscopical examination of the fragments by Mr. T, 

 Davies and himself. An Appendix, by Mr. Davies, describing the 

 microscopic character of the rocks accompanied the paper. 



2. " Note on a specimen of Iron Amianthus." By the Rev. J. 

 Magens Hello, H.A., P.G.S. 



The accompanying specimen was found at the bottom of one of 

 the Wingeworth iron-furnaces, near Chesterfield, and was given to 

 me by Hr. Arthur Carrington, one of the owners. 



The furnaces have been lately blown out for repairs, and in the 

 mass of slaggy refuse at the bottom a thin layer of the curious pro- 

 duct known as Iron Amianthus was interposed between the sand 

 and the iron refuse. 



The red sand at the bottom of the furnace was converted in its 

 upper part into a compact hard white sandstone, an inch or two in 



Phil. Mag. S. 5. Vol. 18. No. 113. Oct. 1884. 2C 





