PRE-CAMBEIAN ROCKS OF ST. DAVID's. 279 



features not unfrequently observable along the edge of the granite, 

 being fine-grained and of a diffused greenish tint. On the beach it 

 becomes in places very quartzose and much impregnated with calcite, 

 some portions weathering with a nodular surface not unlike that of 

 a conglomerate. Possibly there may be some dislocation along the 

 line of junction, and the calcareous portions may be due to infiltra- 

 tion along the lines of fracture *. Though I searched the locality 

 very carefully with Mr. Peach, and subsequently with Mr. Topley, 

 I could not trace bedding in the granite such as has been described 

 by Dr. Hicks. As already mentioned, the granite is much jointed 

 here, and the joints are in some places close and rudely parallel ; 

 but they are mere joints, readily distinguishable from any original 

 structure of the rock. 



It is worthy of remark that the schists which abut on the granite, 

 and extend across the Bay of Porth-lisky, dip at high angles towards 

 N.N.W. They strike at the granite, so that, apparently, lower beds 

 come out as they are followed seawards. I shall afterwards show that 

 all these strata are inverted, and that, consequently, the most easterly 

 beds at Porth-lisky are stratigraphically higher than those immedi- 

 ately to the west of them. Mr. Peach and I observed these peculiar 

 schists at Ramsey Sound lying below the conglomerate ; indeed, if 

 the rocks of Porth-lisky could be traced for a quarter of a mile 

 further out to sea, the conglomerate would doubtless make its 

 appearance f. 



If now we turn to the Map (PI. VIII., p. 268), it will be seen that the 

 granite, in its course from St. David's to the sea, cuts across succes- 

 sive horizons of Cambrian beds, penetrating deepest into them on the 

 north and east, and reaching its highest platform on the south. 

 The way in which it has broken through and pushed aside the con- 

 glomerate is peculiarly striking. That band of rock has been assumed 

 by Dr. Hicks to be the base of his Cambrian system ; we find, how- 

 ever, that the granite not only invades it, but ascends across the over- 

 lying shales and sandstones. 



One further statement calls for notice here. Dr. Hicks, having 

 satisfied himself that the granite of St. David's is a bedded metamor- 

 phic rock, has ventured upon estimates of its thickness ; in his 

 paper of May 1878, he remarks that the thickness previously claimed 

 by him for his " Dimetian" group, viz. 15,000 feet, is not an over- 

 estimate %. But, as will be afterwards pointed out, the only bedded 

 rocks that occur between St. David's and Porth-lisky lie on the 

 flanks of the granite, and strike with the ridge instead of across it, 



* Mr. Tawney, who had the advantage of being guided oyer the locality by 

 Dr. Hicks himself, regarded the calcareous bands as "due to the decomposition 

 caused by water filtering down joints, removing alkaline silicates, and depositing 

 carbonates of lime and magnesia " (Proc. Bristol Nat. Soc. N. S. vol. ii. pt. 2, 

 p. 116). I cannot doubt that this is the true explanation of the limestone and 

 dolomite bands described by Dr. Hicks from this locality. 



t In connexion with this southward prolongation of the conglomerate, we 

 must look on the mass at Ogof-llesugn as having been torn off from the main 

 body, which must lie somewhere beneath the granite. 



{ Quart. Journ. Greol. Soc. vol. xxxiv. p. 154 ; see also vol. xxxiii. p. 230 (1877). 



y2 



