PRE-CAMBRIAST ROCKS OF ST. DAVID'S. 281 



In the definition of the lithological characters here ascribed to the 

 rocks in question I am disposed generally to agree. These mineral 

 masses are partly eruptive quartz-porphyries, and partly highly sili- 

 ceous strata of sedimentary origin to which the names halleflinta, 

 hornfels, porcellanite, chert, Kieselschiefer, adinole, &c. might in dif- 

 ferent places be applied. But here my agreement ends. Instead of 

 finding evidence that these rocks lie with a discordant strike uncon- 

 formably against the so-called " Dimetian " below, and are covered 

 unconformably by Cambrian or " Pebidian " beds above, Mr. Peach 

 and I discovered that Dr. Hicks had really created a separate stra- 

 tigraphical " group " out of the zone of quartz-porphyry bosses 

 and dykes with the accompanying indurated sedimentary rocks that 

 surround the central core of granite. 



I shall discuss the phenomena of intrusion and metamorphism in 

 the second part of this paper. There are only two questions that 

 need be considered here. In the first place, Dr, Hicks asserts that 

 his "Arvonian" rocks usually present a north and south strike, and 

 are unconformable to his " Dimetian" group. This assertion has been 

 virtually disproved by the evidence which I have now advanced as 

 to the true nature of what he calls " Dimetian." But, for the sake 

 of precision, I may here state that my colleague and I made careful 

 observations of the strike of the rocks all round the granite, and 

 found the dominant trend to be parallel with the granite ridge — that 

 is, generally in a north-easterly and south-westerly direction. "Where 

 the average strike changes, it is rather towards east and west than 

 towards north and south, as is more especially noticeable on the 

 coast between Ogof-llesugn and Caerbwdy. Nowhere could we 

 detect a prevalent north-and-south strike, nor any general tendency 

 in the rocks to strike at the granite. 



The quartz porphyries, which appear to constitute a great part 

 of the so-called " Arvonian group," show no strike. They are really 

 as devoid of any semblance of bedding as an eruptive rock can 

 well be. Dr. Hicks remarks that the junction of the "Arvonian" 

 and " Dimetian " rocks is to be seen at St. David's ; but he immediately 

 adds that there is a line of fault at the locality between the two 

 groups. He describes the "Arvonian" rocks as "striking up 

 towards the ridge " and " the lowest beds " as being visible near the 

 Deanery. The rocks exposed over that area are of a type not un- 

 frequently observable round the edge of the granite and doubtless 

 connected with it, to which I shall refer more in detail in Part II. 

 They are entirely amorphous, eruptive masses, without the least 

 trace of any kind of bedding. On the road- side between the Deanery 

 and Bock House, among the numerous joints there is one set that 

 runs in a north-and-south direction, the joint-planes being inclined 

 at high angles towards the west. This was the only structure that 

 I could discover which might have suggested the idea of bedding. 



In the second place, Dr. Hicks alludes to the view expressed on 

 the Survey Maps that the igneous rocks in question are intrusive, 

 but only to dismiss it without further notice and to substitute for it 

 the statement that " they consist in reality of flows of rhyolitic 



