H. HICKS ON THE PRE-CAMBRIAN ROCKS OF ST. DAVIl/s. 229 



15. On the Pre-Cambrian (Dimetian and Pebidian) Eocks of St. 

 David's. By Henry Hicks, Esq., F.G.S. (Read November 22, 

 1876.) 



[Plate X.] 



Introduction. 



In the Geological-Survey Map of North Pembrokeshire, published in 

 1857, there is in the promontory of St. David's a wideband, coloured 

 as a mass of intrusive syenite and felstone, with altered Cambrians on 

 the north of it, and on the south Cambrians unaltered. In the year 

 1864, when making a section from this so-called syenite to the south 

 through the Cambrians, I noticed that a considerable portion of it 

 was made up of altered rock, in which the bedding was quite dis- 

 tinct. This portion I then believed to be altered Cambrian, like 

 that marked on the north, and to be quite distinct from the central 

 mass, to the influence of which I thought possibly this change might 

 Lave been due. On further examination, however, it seemed clear 

 that the syenite did not penetrate any of these beds, and hence that 

 the alteration was in no way due to it. Moreover the masses of 

 rock forming the Cambrian conglomerate appeared to have been 

 derived from rocks apparently identical with those forming this 

 ridge. This discovery led Mr. Salter (whom I guided over the sec- 

 tion in that year) and myself to suspect that the ridge of so-called 

 syenite therefore was a portion of what had previously been a " Pre- 

 Cambrian island ; " and these views were communicated for the first 

 time by Mr. Salter to the British Association in 1864. At that 

 time no lines of bedding had been discovered in the central or more 

 crj'stalline part; and we were satisfied to look upon this portion as 

 a great mass of eruptive rock, but Pre-Cambrian in age. In our 

 report to the British Association in 1866 I ventured, however, to 

 suggest that this also for the most part was not of volcanic origin, 

 and that it was chiefly made up of altered sedimentary rock. 



In a paper communicated by Prof. Harkness and myself to the 

 Geological Society in 1871, other facts were mentioned tending to 

 prove not only the Pre-Cambrian, but also the sedimentary origin of 

 these rocks, viz : — that the bedding of the central portion had an in- 

 variable striko from N.W. to S.E., and hence discordant with that of 

 the overlying rocks ; that the chemical composition of the rock 

 proved it not to be true syenite ; and that it had associated with it 

 bands of hard greenish-coloured ashy-looking shales, considerably 

 altered in character, but in many instances possessing distinct traces 

 of foliation. In subsequent papers I added a few additional facts; 

 but hitherto the subject has been dealt with imperfectly, as I then 

 devoted most of my time to the immediately overlying series, to 

 which those papers chiefly referred. During the last two years, 



