THE PEE-CAMBRIAN AGE OF KOCKS IN N.W. PEMBROKESHIRE. 351 



29. Further Proofs of the Pre-Cambrian Age of certain Granitoid, 

 Felsitic, and other Eocks in N.W. Pembrokeshire. By 

 Henry Hicks, M.D., F.E.S., F.G.S. (Eead May 26, 1886.) 



Introduction. 



During a recent visit to Pembrokeshire I further examined some of 

 the sections referred to in my paper communicated to the Geological 

 Society in 1884 * as well as others not there mentioned. The 

 additional facts thus obtained are so important and so thoroughly 

 confirm the views expressed in that paper, that I have felt it my 

 duty to lay them before the Society. I referred so fully in my 

 former paper to the rocks in the immediate neighbourhood of St. 

 David's, and gave such ample evidence in support of my views, that 

 it will be unnecessary to add much to it. Still as it is most 

 important that some of the questions discussed in that paper should 

 be settled beyond the possibility of doubt, I purpose giving such 

 facts as are necessary to complete the evidence. Prof. Bonney has 

 kindly undertaken to furnish notes on some of the rock specimens 

 collected, and I am indebted to him for much valuable information 

 concerning the conclusions at which he has arrived. 



St. David's. 



Undoubtedly the point of greatest importance with which we have 

 to deal, especially in its bearing on the controversy which has taken 

 place in reference to the age of these rocks, is that relating to the 

 Granitoid (Dimetian) rocks. The Director-General of the Geological 

 Survey (Dr. Geikie) has definitely laid this down as the main 

 question in the discussion. We maintain that the Granitoid rocks 

 of this area, whatever their origin may have been, are of Pre- 

 Cambrian age. Dr. Geikie, on the other hand, contends that they 

 are intrusive in the Cambrian rocks. He says that after " the 

 Harlech and succeeding groups of the Cambrian system were depo- 

 sited .... which had been laid down continuously without dis- 

 cordance .... they were invaded by the rise of a mass of granite 

 with the usual peripheral quartz-porphyries "f, the mass of granite 

 referred to being the Dimetian ridge of granitoid rocks extending 

 from the city of St. David's to the coast near Nun's Chapel and 

 to Porthclais and Porthlisky. I proved most conclusively, I think, 

 in my former paper that the evidence relied upon by Dr. Geikie 

 to show that the Granitoid rocks were intrusive in the Cambrian 

 was based on erroneous observations. Not only is there no evi- 

 dence of intrusion (and on this point, it must be remembered, 

 my view is upheld most unhesitatingly by Professors Hughes, 

 Bonney, and Blake, with several other observers), but I was even able 

 to demonstrate that the Granitoid rocks occurred in that area before 



* Quart. Journ. Geol. Soc. vol. xl. p. 507. 

 t Ibid. vol. xxxix. p. 324. 



