n. mcxs ox the pre- Cambrian rocks of st. dayid's. 237 



Besides flanking the Dimetian axis as already described, the 

 Pebidian rocks are again exposed supporting the Cambrians at 

 several places in the neighbourhood. The part coloured as intrusive 

 greenstone in the Survey map, running nearly parallel with Ramsey 

 Sound, is made up of these rocks with a few intrusive dykes ; and 

 the bedding isf or the most part quite distinct. The width, how- 

 ever, is greater at the upper part than that which is coloured as 

 greenstone, since it includes here in addition the part coloured as 

 altered Cambrian, which extends on the west side from Rhosson to 

 the fault at the Burrows. It seems tolerably evident also that the 

 space intervening between this patch and the St.-David's ridge will 

 be found to be occupied entirely by these rocks, as wherever an ex- 

 posure is seen the rocks have more the character of the Pebidian 

 than of the Cambrian beds. 



The appearance of the rocks in this exposure is much like that 

 at St. David's ; but there is a larger proportion of greenish and 

 purplish schists alternating with the compact porcellanitic shales. 

 As the beds lie at "a high angle, seldom under 70°, there is probably 

 exposed here at the widest part a thickness of no less than 3000 feet. 

 The Cambrian conglomerates, entirely unaltered, rest unconformably 

 on them to the west of Rhosson rock, and can be traced almost 

 continuously along the western side. On the eastern side, in 

 Porthlisky harbour and at Trevithan, some of the conglomerates at 

 the base may be seen. They seem identical in character with the 

 conglomerates in the St.-David's valley ; so there is every probability 

 that we have a repetition of the whole series in the intervening 

 space, and that the altered beds here do not belong to the Cambrian 

 series, as marked on the maps and sections. Another patch may be 

 seen at the south-western side of Ramsey Island, a triangular 

 mass rising up into a bold hill about 400 feet in height. This is 

 coloured as greenstone intruded into the Arenig rocks in the Survey 

 maps. The beds composing this hill are compact porcellanites, like 

 those near St. David's ; and the bedding is also easily traced. The 

 Cambrian conglomerates are to be seen resting unconformably upon 

 the beds along the north-east edge. 



The fault in Ramsey Sound, along with the numerous ones to be 

 traced in the island, have so altered the position of this portion that 

 the line of strike of the beds has here been quite changed as com- 

 pared with that in the other, greater masses more inland. There 

 are but few beds of the Lower Cambrian series exposed in the 

 island ; but fortunately the conglomerates at the base are well seen 

 at the spot already mentioned, with a strike from AV.N.W. to 

 E.S.E., and, as in the other places where the3 r rest on the Pebidian 

 rocks, in an unaltered state. On the east side of this mass is a 

 great fault, which has brought the upper beds of the Arenig series 

 down upon the Pebidian rocks. 



The faults in Ramsey Island are highly instructive ; for the 

 results usually are of enormous importance, and such as would be 

 productive of almost insurmountable difficulties in an inland or 

 covered district. Here, however, they can be traced in precipitous 



