INTRUSIVE TRAP AND ALTERED ROCKS NEAR BUILTH. 



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Carneddau. It is rocky and rugged on its flanks, exposing fine precipitous faces under 

 Carneddau Castle and Bwlch on the north, at Graig dhu on the south-east ; and at its 

 southern end it advances in hard knotty promontories, sloping down from the heights 

 of Gaer-fawr and Tan-y-craig to the left bank of the Wye, opposite Builth. The upper 

 surface of this chief mass of trap is chiefly covered with good pasture, through which 

 the bosses of the rock occasionally appear, and numerous cairns of stone diversify the 

 outline. The following are the chief varieties of the Carneddau trap 1 : 



1. Greenstone, fine and coarse-grained, the crystals of hornblende being in some instances distinct. 



2. Porphyritic greenstone. 



3. Felspar rocks, chiefly granular, but passing into 



4. Compact felspar, sometimes very hard. This rock frequently forms the summits. 



5. Amygdaloids of various characters, passing into greenstones on the one hand and into porphyritic felspar rocks on the 

 other. 



6. Porphyry ; some varieties have pink crystals in a dark base of felspar, others contain hornblende. 



These rocks are of grey, blue, and greenish colours, sometimes, when the felspar pre- 

 dominates, weathering to dirty white, and the amygdaloids often on decomposing become 

 vesicular. Where the greenstone, containing numerous crystals of white felspar, abounds, 

 as in the bosses between Gaerfawr and Builth, the rocks are very much rounded off, 

 precisely as in the coarse-grained granites of Devonshire and Cornwall 4 . 



Although I said that the Carneddau Hills are almost entirely composed of trap rocks, 

 yet they throw off upon their sides, and occasionally carry up to considerable heights, 

 dismembered and altered portions of the sandstone, schist, or flag (Caradoc Sandstone, 

 Llandeilo flags). Such examples may be seen near the farm of New House at the 

 height of several hundred feet above the Wye. On the south-western face of the 

 mountain, near the spot called Tan-y-craig, there are extensive quarries, from which 

 Builth has chiefly been constructed. These quarries expose the true nature of this 

 building-stone, which is a variety of bluish grey, volcanic grit. Beneath this lies por- 

 phyritic greenstone, bulging out in large spheroidal protuberances. 



The hard volcanic grit wraps round the swelling concretionary masses of this trap, 

 and near the junction it is almost impracticable to separate the grit from the green- 

 stone. (See parallel examples in the Corndon and Shelve country, p. 269 et seq.) The 

 refuse or top beds of these quarries, however, leave no such doubt, as they are unequi- 

 vocal Caradoc sandstone, containing several casts of fossils, such as Orthis Actonice, 

 0. radians, fyc, though most of them are too imperfect for identification. These beds 

 also fold round protuberances of trap and dip west and south-west. 



1 The sketch facing the first page of this chapter, will convey some notion of the outline of the south- 

 eastern corner of this hilly tract. It is taken from the valley of the Wye, about a mile and a half east of 

 Builth. The distant rocks are those of Gaer-fawr and Craig-dhu. 



2 One of the amygdaloids has long been used as a fire stone by the blacksmiths of Builth, another proof, in 

 addition to that of the rock quarried at the Buries near Llandrindod, that felspar which has lost its alkali will 

 resist the amount of heat employed in ovens or ordinary forges. 



