BEDDED TRAP ROCKS, RADNORSHIRE. 



325 



tirely of light yellow clay, the decomposed felspar so predominant in the central ridges ; 

 and particularly when the surface is choked up (as near Llansaintfread) , with numerous 

 loose quartzose boulders, transported from the west. I will first describe those trap 

 rocks which are distinctly interstratified with the fossiliferous deposits, the general rela- 

 tions of which are fully explained in the coloured section, PL 33. fig. 5. They prevail 

 in the northern portion of the district, and their centre may be placed in the hills 

 of Gelli and Gilwern, whence they are prolonged to the south-west by Upper-house, 

 Beili-brith, Carreg-gwiber and Bryn-hir (near the Three Wells) j and to the north-east 

 in the hills of Llandegley and Cefn-Llys. In the Gelli and Gilwern hills, these bedded 

 trap rocks attain the greatest heights, and consist of the following varieties. 



1. Dark grey compact felspar, slightly porpliyritic, having a few imbedded crystals of common felspar. It contains, also, 

 dark green flattened concretions, which mark the laminae of deposit, their edges passing into a kind of steatite or serpentine. 



2. The same rock, with concretions of quartz and felspar, having an agate-like arrangement, the whole streaked by ir- 

 regular films of serpentine, giving it a laminated, green and white aspect. 



3. Grey granular trap, made up of felspar, quartz, and iron pyrites, with a little of carbonate of lime disseminated; the 

 felspar occasionally collected in very small concretions. 



4. Clay-stone of blue milk, and light grey colours, with a few crystals of common felspar, fracture more or less con- 

 choidal, surface dull, with spots or minute concretions, sometimes hollow, and containing earthy matter with pyrites. 



5. Same rock as 4, but more earthy, and of greenish-grey colour. 



6. Do. do. more slaty, has at first sight the aspect of compact felspar porphyry, but when magnified, proves to be a dull, 

 granular claystone, with some concretions of felspar and many crystals of pyrites. 



7. Felspar porphyry, approaching in parts to the clinkstone of Werner, of a dark greenish-grey colour, weathering out- 

 side to a rusty-brown : the decomposing felspar crystals leave small cavities. 



Carbonate of lime occurs in small veins in these laminated traps, occasionally slightly intermixed with the other minerals. 



In the Carreg-gwiber quarries, one of the light-grey felspar rocks, having its lamination marked 

 by dark, rusty-brown, elongated blotches of decomposed green earth, is much extracted as a 

 building stone. In the Berrw and other places, a cream-coloured and delicate green, fine granular 

 felspar rock is used as an oven or fire-stone. 



The prevailing inclination of these bedded traps is to the north-west, as may be seen by crossing 

 the main ridge from the valley of the Eddw to Llandrindod, and the prevailing angle of dip is 40° 

 to 45° j thick and thin beds alternating with the flags. On descending from the north-western face 

 of Gelli, a brook, the chief feeder of the Howddy, rushes down a narrow chasm, the Cwm-re, in 

 which this alternation is admirably displayed ; and as the flags contain the Asaphus Buchii, it is 

 manifest that submarine volcanic operations were in activity during the deposition of the sand, mud 

 and crustaceans which formed the Llandeilo flags. 



The bed of this brook on the side of the hill presents the following very instructive section. (PL 33. 

 fig. 5., and this wood-cut.) 



57, 



