CHIEF SILURIAN DRIFT IN HIGH AS WELL AS LOW POSITIONS. 515 



above varieties of rock, and also with a number of fragments of coal measure grits, 

 mountain limestone, and basalt. As all these materials occur in situ in the Clee Hills, 

 distant only a few miles north-west from the spot, we have in this case additional evidence, 

 that all the ancient submarine currents of this district followed the same direction. 



Let us now follow the Wye from its source into the plain of Hereford, and we shall 

 there find precisely the same phenomena as in the valley of the Teme, — the transported 

 matter changing its contents with every successive zone of rock. In the upper or 

 north-western region of the river, about Rhayder, the fragments have been entirely de- 

 rived from the slates and quartzose conglomerates of the Cambrian System. Similar 

 materials with broken schists of the district are spread over the arid tract between 

 Dolfan and the Builth Hills, but having passed the volcanic chain of Builth, we meet 

 with vast numbers of large porphyritic and greenstone bowlders, altered quartzose 

 strata, &c. piled up in the hollows or upon the slopes of the outer zone of Silurian rocks. 

 These coarse fragments, are one and all referrible to the group of rocks lying to the 

 north-west, and which mantle round the volcanized region extending from Builth to 

 Llandrindod and Llandegley ; and they are often lodged at heights of many hundred 

 feet above the bed of the Wye. They are not arranged in terraces, as if produced by 

 any possible fluviatile action, but are piled up in confused and irregular heaps in the 

 mountain combs. 



The reader should be here again reminded, that this coarse drift is by no means con- 

 fined to the sides of the transverse chasms through which the Teme, the Wye, and other 

 streams escape into the low country, although such fissures appear to have afforded the 

 detritus a ready egress, it being usually propelled further to the south-east in the pro- 

 longation of these openings than elsewhere. On the contrary, this coarse drift is 

 strewed at intervals over the sloping dry combs and high grounds • particularly where 

 they lie to the south-east of any tract which has been the theatre of volcanic eruption 

 or violent elevation of the strata. For example; large bowlders of trap and hard 

 quartzose rocks occur on various hills east of Llandrindod and Builth, (PI. 33. f. 7.) 

 and the same may be seen on the south-eastern slopes of the Hergest ridge, near Kington, 

 or south-eastern talus descending from the volcanic group of Old Radnor. (PI. 33. f. 4.) 

 The vast accumulation of coarse detritus, already alluded to, as impoverishing a wide 

 tract between Kington and the Hay, was also drifted from the same centre of action. 



In descending the gorge of the Wye to the south-east, we find the materials of all 

 the rocks derived- from the volcanic district of Builth, rapidly diminishing in size, and 

 lodged in great mounds against the escarpments of the Old Red Sandstone. As we 

 advance to the south-east, these materials become smaller, and at length in the environs 

 of Hereford they appear only as fine gravel and silt. Again from Hereford to the mouth 

 of the Wye, the bowlders change with the lithological characters of the districts tra- 

 versed, and are referrible to the grits and conglomerates of the coal measures and Old 

 Red Sandstone. 



