344 



INTRUSIVE TRAP — ALTERED AND VEINED ROCKS. 



4. Dark grey and green, concretionary, Compact felspar. 



5. Greenstone; highly crystalline, hoth fine and coarse-grained, and sometimes very hornblendic. 



6. Greystone, or grey granular felspar, intimately mixed with hornblende and a few crystals of carbonate of lime) 



7. Amygdaloidal trap, cellular on the weathered surfaces 



The greenstones are best segn near Pen-y-banc, and the more porphyritic ^~ks occupy 

 the centre of the hill. Associated with a porphyritic greenstone, in the bed of the 

 Yrfon at Dol-y-dymmor, is a greenish grey, close-grained amygdaloid, having the cells 

 filled with calcareous spar, generally coated with pellicles of green earth, and varying 

 in size from mustard-seeds to almonds. In some cases the stratified rocks in contact 

 appear to have undergone a kind of "boursoufflure," and are scarcely to be distinguished 

 from the amygdaloid. On the sides of the principal ridge of trap, the changes produced 

 in the vertical and dislocated strata are numerous and clear. On its lower flanks and 

 south-western extremity near Pen-y-banc, the trap is coated with a thin and broken 

 covering of schist, which is silicified or in the state of hornstone, highly translucent at 

 the edges, of a scaly fracture and dark grey colour with cloudy streaks, as if formed 

 by an imperfect separation of hornblende. Other varieties are black Lydian stones, 

 ringing under the hammer, and splitting with a fine conchoidal fracture ; some of them 

 containing a number of bright metallic spots, probably oxide of iron. The upper parts 

 and summits of these hills exhibit numberless rugged and irregular bosses of trap, 

 sometimes carrying up fragments of altered or indurated schist, which occasionally 

 contains crystals of carbonate of lime irregularly disseminated, giving to the rock a 

 pseudo-porphyritic structure. (Sides of Caer-cwm.) The flanks of the hills, where 

 partially eroded, offer beautiful examples of contorted, broken, and altered schists, par- 

 ticularly near the little transverse valley of the Cerdin, and about half a mile north-east 

 of the farm of Pen-y-banc. Mrs. Traherne (being on a visit to the baths in 1833) has 

 kindly enabled me to present the reader with a perfect view of the relations of the trap 

 to the broken strata. (See p. 343.) Irregular knobs of trap penetrate the schist, which 

 is dislocated, shattered, and bent. Much iron pyrites occurs near the point of contact ; 

 the shale is indurated and exhibits polished surfaces with some quartz and traces of 

 plumbago (carburet of iron). A little further down the side of this abrupt ravine, the 

 schists weather to a whitish colour, in the same manner as some of the killas of 

 Cornwall. On the other side of the gulley, and removed from the ridge of trap, the 

 slaty schists are no longer contorted or broken, but rise with perfect regularity, having 

 a slaty cleavage, the planes of which dip north-west 65° to 70°. (See the previous 

 wood-cut.) In the little comb of Nant-yr-odyn, north of the gorge of the Cerdin, 

 the black and highly inclined shale has been penetrated by galleries in search of coal ; 

 the mania for this speculation, to which I have already so much alluded, appearing to 

 strengthen in proportion to the impossibility of success ; these stratified, slaty deposits 

 being unquestionably a part of the Cambrian System, for to the west of this ridge they 

 pass into true roofing slates. 



