ALTERNATIONS OF BEDDED TRAP AND LOWER SILURIAN ROCKS. 327 



2nd. Several alternations of porphyritic, bedded felspar, with shelly, brown, Caradoc Sandstones, 

 dipping conformable with the above, the angle increasing with the rise of the hill. The sandstone 

 is worn into depressions, and the trap forms ridges 1 . The former contains the shells. 



3rd. The whole of the hill called Llandegley rocks consists of bedded trap, the direction of the 

 beds being perfectly conformable to those which alternate with shelly sandstone on the exterior 

 slopes; but the angle of inclination having increased in each successive ridge, amounts to 60° 

 and 70°, dip N.N/W. These are the rocks which present rugged escarpments to the south-east, 

 where they are largely quarried as Llandegley building stones. In the depression north-east of 

 this escarpment, and also in descending to the village of Llandegley, are several bosses of zmstratified 

 trap, which seem to rise irregularly through the surrounding deposits, and throw them off on all 

 sides, producing veins of quartz, coats of anthracite, cavities filled with green earth, &c, in the sur- 

 rounding strata. These are felspar rocks, sometimes porphyritic, passing into greenstone and 

 quarried for the roads. 



The Llandegley rocks offer a fine bold front to the north-east and constitute the termination of 

 this trap district. They send out a small dyke or spur of porphyritic greenstone, which traverses 

 the black shale with nodules in the bed of the brook close to the sulphureous wells. The inferences 

 deducible from this fact will be rendered more apparent when we have examined the relations of the 

 rocks in the vicinity of the other sources of mineral water which surround this tract, p. 334. At 

 Blaen Eddw, near the source of the sulphureous spring, is the same juxta-position of unconformable 

 trap and shale. The same at Llandrindod and at the Park Wells near Builth. 



In the vicinity of Blaen Eddw are several small protuberances of highly crystalline, coarse- 

 grained greenstone, with a little lime disseminated, and also white and grey, granular felspar rocks, 

 which tilt the strata to the north-west and south-east, the latter being usually much hardened at the 

 contact with the trap. These bosses are not continuous for any great length ; but they are suc- 

 ceeded on the west by another parallel ridge of similar composition, which terminates in the fine 

 bold rock of Craig-fawr, the direction of which may be followed in the stony hill of Coed-mawr. 

 The prevailing rock in these ridges is greenstone and porphyritic greenstone, highly crystalline, and 

 generally with a little lime both disseminated and in veins 2 . Other bosses, all protruding in pa- 

 rallel lines which range from north-east to south-west, are found near Bettws Disserth and Tin-y- 

 coed, extending from the south end of the Llandegley rhos to the rocks south-west of Llansant- 

 fread. Another and a shorter parallel is seen in the neighbourhood of Llwyn madoc. In all these 

 cases the central bosses of trap have been thrown up in irregular masses without any appearance of 

 bedding, but towards their flanks is frequently a passage into distinct, slaty and bedded rocks, still 

 preserving a trappean character. This is well seen near Coed-mawr, where there is a transition 

 from a coarse greenstone in the centre of a small hill to a slaty exterior of altered flags (in part a 

 complete flinty slate if not a Lydian stone) with many crystals of iron pyrites. Receding from 

 the trap the alteration disappears, and in the valley of Bettws, near the little chapel, is the ordinary 

 thinly foliated black shale of the country. The black colour in this instance has again misled the 



1 A beautiful variety of grey volcanic grit has been recently quarried at the Graig for the construction of 

 the bridge at Pen-a-bont, the strata dipping west 35 p . 



3 Several large blocks of these trap rocks, having a rude columnar form, are arranged in a circle on the dreary 

 common of Rhos maen, about one mile east of Craig-fawr. They resemble the Druidical circles of the Isle of 

 Arran and others which I have met with in geological rambles. I am not aware that this Druidical circle has 

 been described by any antiquary. Its place is marked in the Map t 



2 s 2 



