INTRUSIVE TRAP AND ALTERED ROCKS NEAR BUILTH. 333 



repeated in jagged forms, and followed on its flank by (6) a porcelain rock of light colour with many vertical joints which 

 resemble true bedding, but the lamina? of deposit are rarely observable in tbis altered rock, and when detected they dip 

 north-west at a high angle. This porcelanite is divided into prisms by two sets of highly inclined parallel joints. In one 

 part the beds are twisted round at right angles to the principal strike. 7. Irregular masses, composed of compact felspar, 

 sometimes concretionary, again appear, throwing off indurated and dislocated strata. 8. Light grey calcareous flagstone, 

 slightly indurated in the central beds or those furthest from the trap. 9. Coarse greenstone, resembling syenite, here pro- 

 trudes in different parts of the bed of the river, and the flag in contact is a lightish blue hornstone irregularly folded over 

 or caught up in the trap. 10. Black flag, slightly calcareous and micaceous, strike south-west and north-east and quite un- 

 altered in the middle. 11. As the strata approach the next boss of trap they become harder, change to a dead dull black, 

 the mica disappears, and large crystals of iron pyrites are abundant; the rock at length acquires a great compactness and 

 specific gravity, yet effervesces briskly with acids, and finally the alteration is still more complete, for the strata are caught 

 up, rolled over, broken, and squeezed in discordant directions, the strike of the indurated and heavy strata being north and 

 south. 12. The greenstone which has caused all this change is projected across the stream in a strong, gnarled, irregular 

 ledge, through which the water has excavated two channels, leaving an island in the centre. The irregular insertion of 

 the trap into the flags can best be comprehended by this ground plan of the two rocks in a part of the bed of the stream. 



IT 



60. 



Some of this greenstone contains carbonate of lime disseminated, and in parts entangles and carries up upon its surface 

 isolated fragments of the calcareous flag, highly indurated, and containing much crystallized iron pyrites with little flakes 

 of anthracite and black calcareous spar. 



13. Again the phenomena of alteration disappear in descending the stream, and thirty or forty paces from the intrusive 

 rock the shale is fissile, slightly micaceous, and incoherent, but on coming near another zone of trap which rises into the 

 plantation hillocks on the right bank, the beds first become as thick as tiles (14), and finally, in contact with the greenstone, 

 are united in a heavy, thick-bedded, dull mass, which splits with a conchoidal fracture. ]5. The trap rock at this spot is 

 a greyish concretionary greenstone with much iron pyrites and carbonate of lime. 



There are two or three other wedges of trap between this projecting rock and the hoiise of Pen ddol, on the sides of 

 which the same appearances prevail, and subsequently the black shale of the district disappears, under the alluvia of the 

 valley of the Wye. 



The relations of these alternating bosses of trap and bands of schist demonstrate that 

 the trap rock when in a state of fusion was forced into the beds of schist, altering and 

 dislocating them in the manner above described 1 . 



1 Dr. Gilby gave a short account of the "trap and clay slate formation" of this district (1820) in the Edin- 

 burgh Philosophical Journal. It contains many good observations, and particularly in reference to the gorge 

 of the Wye above described. " It is curious," he says, " that the slate, or rather the flinty rock at its actual 

 junction with the trap is not stratified, the stratification being only manifest when a little removed from the 

 greenstone. The dip of the slate is everything that is perplexing ; sometimes it appears to dip under the 



ledge In other places the slate seems shattered and turned about in every direction, and several times 



I observed large patches of the indurated schist completely contained in the greenstone." Edin. Phil. Journ. 

 vol. ii. p. 256 and 257. Is it not remarkable that, with such evidence as this, the author could not quite abandon 

 his Wernerian principles ? The section which I have given may seem to differ in a few details from that of 

 Dr. Gilby, but I attribute the discrepancy to my having united observations, made on both banks of the river. 



2 T 



