68 REPORT — 1864. 



Hicks) the Cambrian rocks of St. David's for the Glenoid Trilohites mentioned in 

 the last paper, the author paid some attention to the i-elations of the central trap- 

 rock of the dislrict, which runs in a broad mass, a mile or two wide, from Llanrei- 

 than to the headland of St. David's, and is continued out to sea in Ramsey Island. 

 As the purple rocks, sandstones, and slates of the whole Lower Cambrian division 

 are thrown up at high angles, all but vertical, on either flank of this mass, which 

 forms the axis of the whole country, there is no diihculty in studying its behaviour 

 in contact with the Cambrians. If it were an intrusive trap of later date, it would 

 penetrate them here and there, or at least alter them at the point of contact, as the 

 neighbouring granite of Brawcly and Koch actually does. On the contrary, where- 

 ever the boundary can be seen, steatitic and felspathic schist unaltered, and beds of 

 thick conglomerate, mark the line, and are often very conspicuous. These conglo- 

 merates — of quai-tz-rock, jasper, felstone, &c. — may or may not have been derived 

 from the immediate neighbourhood. They are traceable along the south and north 

 sides of the trap-region, and are followed by sandstones of various degrees of coarse- 

 ness, but indicating by the ripple-mark, as well as the coarse material, that they 

 were accumidated in shallow water; and as we know that pebbles, often as large 

 as swan's eggs, are not carried far out to sea, but mark either a submarine shoal or 

 a coast-line, we are compelled to assign them to a source near at hand. The upper 

 beds of the Lower Cambrian formation are finer-grained and lighter-coloured, and 

 pass insensibly into grey and then into thin black beds of the Lingiila-flags, with 

 trappean ashes and lava-flows — the great Upper Cambrian formation. 



Comparing this order of things with what occm's in North Wales, one is struck 

 with the wonderful similarity in the two regions ; coarse conglomerate and purple 

 shale, red sandstones, and then grey rocks, passing into black, deep-water shales. 

 Crossing the channel it is the same ; the Lower Cambrian rocks of Wicklow give 

 evidence of accumulation in shallow water ; and Sir R. I. Murchison has shown us 

 exactly the same thing, even exaggerated, in the conglomerates of North-western 

 Scotland ; but these rest directly on the Old Laurentian rocks, from which they 

 seem to have been derived. The Hebrides and the west coast of Sutlierland were 

 land or shallow water when the Cambrian pebble-beds were formed aromid them. 

 We do not know the land which supplied the pebble-bands and sandstones of 

 North Wales and Shropshire ; but the researches of Dr. H. B. HoU haA'e shown us 

 that the Malvern Hills were a low reef of rocks at this time ; and everything points 

 to a shallow sea, studded with islets and reefs, as the condition of things which 

 existed in our area, probably also in Normandy and the Channel Islands, at this 

 time. Again, the old Lam-entian gneiss is remarkable for its syenitic character. 

 Syenite is common ; true granite is comparatively rare in these old rocks. This is 

 the case in Canada, where they are best seen. Dr. Holl has shown it to be the 

 case at Malvern, and hence we should look for it in Wales. The mass of igneous 

 rock, which forms the backbone of the St. David's peninsula, and which supports, 

 without penetrating them, the shallow-water accumulations of the older Canibrian 

 around it, is syenitic in character. The quartz-veins penetrating it may well have 

 supplied the pebbles ; and the felspathic matter was the origin of the softer schists 

 of the rocks which lie around it. That there was shallow water, with rocky ground 

 close by, is eAident ; and in the absence of any evidence to the contrarj', the author 

 suggests that the syenitic trap of St. David's is a part of the old pre-Cambrian 

 land. As he did not visit Ramsey Island, the evidence is incomplete. It will be 

 necessary to see whether the Cambrians there are affected by the trap, or lie upon 

 it unaltered; as he believes is the case with those of St. David's. 



A Brief Explanation of a Oeological Map of the Bristol Coal-field. 

 By W. Sanders, F.B.S., F.G.S. 

 This map has been constructed by reducing about 220 parish-maps to the scale 

 of 4 inches to the mile, or 20 chains to the inch. The map comprises a large por- 

 tion of the geological series, ranging from the Lower Silurian up to the lower divi- 

 sion of the Cretaceous system. VVith respect to the coal strata, as the deposits of a 

 later age occupy a large portion of the country, only one half of the coal strata of 

 the northern part of the oasin, and only a tenth or twentieth part of the southern 



I 



