402 



TRAP ROCKS OF SOUTH PEMBROKE. 



by Dr. Kidd (Geol. Trans., Old Series, vol. ii. p. 79.)> and they have also been alluded to by Mr. De 

 la Beche. Among them, however, is a variety which appears to have escaped the notice of these 

 authors. It is a highly crystalline, large-grained greenstone, made up of albite, mixed with minute 

 dark crystals of chromate of iron. The rock also contains some quartz and the oxides of iron and 

 of chrome, and an earthy mineral substance which may be decomposed augite 1 . The compact 

 felspar rocks are frequently coloured green by a diffusion of epidote or chlorite. 



Some remarkable examples of the most completely altered rocks, such as flinty schist passing 

 into flint, are in the stony ridges to the west of Goodie Sands. Most of the irregular masses of 

 trap along the north side of St. Bride's Bay, between St. David's and the coal-field, have been 

 mapped and described by Mr. De la Beche, who has also marked trap rocks as occupying the Island 

 of Ramsey with most of the adjacent islets (Bishop and his Clerks, &c.) 2 . 



One of the most striking of the trap ridges of Pembrokeshire is that of Trafgarn, which trends 

 from Roche Castle near the coast, in a zig-zag form and a north-easterly direction to Trafgarn, 

 where it bifurcates and is continued to the north-east in two great bands, which terminate in Wallis 

 Common and the rugged tors of Ambleston. At Roche Castle and at Trafgarn the rock is a mass 

 of compact felspar, quite undistinguishable from that of the Wrekin and many rocks previously de- 

 scribed. At Ambleston it becomes a porphyry and porphyritic greenstone, &c. The intrusion of 

 this rock has produced a powerful effect upon the adjacent strata, particularly on those masses 

 inclosed between the forks of trap in the gorge of the river at Trafgarn, where the red and green 

 sandstones are converted into a brittle, siliceous substance, resembling the ribbon-jasper of mi- 

 neralogists. It is largely used as a wall and building stone. Though offering no traces of true 

 bedding, the compact felspar of Trafgarn is divided into rude prisms by two sets of planes or 

 vertical and horizontal joints, giving rise to square-topped masses like ruins, which stand out in 

 bold contrast to the prevailing outline of the county, somewhat like the serrated quartz rocks of the 

 Stiper Stones in Salop. This range of trap, as well as a thinner band of porphyry, lying to the 

 north of it and crossing the river at Scillyham, appear to be intrusive ; and rocks of the same 

 character re-occur at various points along the higher slaty chain, even to the eastern extremity of 

 Precelly. Without dwelling, however, upon this north-eastern district, to which I gave compara- 

 tively little attention, let us now consider the trap rocks to the south of Haverfordwest, where 

 they are intimately associated with the Silurian and younger stratified deposits, — the special objects 

 of my research 3 . 



1 It is upon the authority of Professor Miller of Cambridge, who examined the specimens, that this minera- 

 logical statement is made. 



2 Since that author wrote, small veins of copper have been observed in the altered rocks near Solfach, adja- 

 cent to certain trap rocks. There are also two copper veins running across the promontory of Pen-maen near 

 St. David's, from S.S.E. to N.N.W., and these are flanked on the east by a most remarkable quartz vein, twelve 

 yards thick. 



3 In reference to the points and lines of trap between Precelly and Newport, it is by no means contended 

 that North Pembroke is so accurately worked out, that future observers may not add much to our knowledge. 

 Through great portions of this tract the alternations of trap are so very frequent, that it is almost imprac- 

 ticable to lay them down upon a map of the scale which I have adopted. 



