ALTERED ROCKS— LLANWRTYD WELLS, ETC. 



345 



"Whilst the porphyritic trap occasionally peeps out in rugged bosses along the sum- 

 mits and sides of the hills of Caer-cwm and Garn-dwad, the little transverse dell of the 

 Cerdin lays bare the true nature of this nucleus in a rock called Craig castell, which 

 towers above the left bank of the stream. This precipitous cliff is a porphyry, the 

 exterior of which is black, but the interior is grey compact felspar, with minute white 

 crystals of common felspar. It is arranged in slender four-sided prisms, from 25 to 30 

 feet in length by 5 or 6 inches in diameter, and crossed by transverse joints, the planes 

 of which dip to the north-west. The large and broken masses below are partly of the 

 same rock, partly of other varieties. This is one of the most impenetrable rocks met 

 with in the whole of the country I examined, being very analogous both in compo- 

 sition and relations to some of the porphyries of Snowdon, Cader-Idris, &c, and like 

 them its forms arise from joints, separating the mass into four-sided prisms. As 

 the trap of the mountain of Caer-cwm tapers away to the north-east and finally ceases 

 near Blaen-einon, so that of Garn-dwad thins off to the small protuberances and 

 bands which traverse the Ithon at Llanwrtyd. The bed of this river offers many 

 beautiful examples of highly silicified and indurated strata in contact with trap, strictly 

 according with those described on the banks of the Wye near Builth. The analogy 

 is also rendered quite striking by sulphureous mineral waters issuing from the ad- 

 joining shale; and, judging from the appearance of the veined and altered strata 

 which are exposed on the sides of these trap hills, we can hardly doubt that the mine- 

 ralization of this spring is due to the decomposition of sulphuret of iron, which has 

 been largely accumulated at some of those points where the trap has been intruded into 

 pyritous shale, in the manner similar to that pointed out at the Park Wells near Builth, 

 and at Llandrindod. These waters of Llanwrtyd have not hitherto been completely 

 analysed, but it is sufficient for my present purpose to state that they are sulphureous 

 and slightly saline. 



Lead ores occur in the chain of slate hills on the west, and veins of copper have 

 latterly been discovered. These veins lie far within the Cambrian frontier. With 

 respect to the precise age of the altered strata of the Llanwrtyd hills it is difficult to 

 speak with confidence, since very slight traces of organic remains have been hitherto 

 observed ; yet, judging from analogy and the range of the associated rocks, I believe 

 they may be placed in the upper division of the Cambrian System. (See next chapter 

 for an account of these rocks in Caermarthenshire.) 



I cannot more effectively convey to my readers a clear notion of the arrangement 

 and succession of the strata in Brecknockshire and the adjacent counties of Radnor 

 and Caermarthen, than by annexing a semi-panoramic view, taken by Mrs. Traherne, 

 from the mountain of Esgair davydd, south-west of Llanwrtyd. The spectator is placed 

 on the old slaty rocks of the Cambrian System, and looking E.N. E. and S.E., his range 

 of vision embraces the outlines of two great systems which it is my object to illus- 

 trate. In the distance is the Old Red Sandstone, occupying the highest mountains of 



