322 



CAUSE OF THE LUDLOW AND BRECON ANTICLINALS. 



and the contorted strata which occur on the sides of this altered mass, sufficiently well explain what 

 we have described in the south-western prolongation of the same axis at Old Radnor. 



There is not, perhaps, in Great Britain a finer mass of altered and crystalline limestone than that 

 exhibited at Nash Scar, the principal cliff of which rises to the height of 200 or 300 feet above 

 the adjoining valley of Knill and Presteign. At Woodside and Haxwell, to the south-west of the 

 Scar, the limestone is stratified, as represented in the wood-cut, p. 313, and at the latter place ap- 

 parently folds around a nucleus ; but in receding from this point to the north, the stratified charac- 

 ters disappear and it rises in large and contorted folds towards the chief mass, where all traces of 

 bedding vanish. 



Again, at the north-eastern extremity of this ridge, near the spot marked Folly on the Ordnance 

 map, are thin bands of limestone, subordinate to shale, which dip at a high angle, overlying the 

 grits as at Old Radnor. As these beds recede from the scene of disturbance they become wwaltered, 

 the shale being abundantly charged with Asaphus caudatus, Calymene variolaris, and C. macro- 

 phthalma, the Isotelus (or Bar Trilobite), &c. They mark, indeed, by their position and by their 

 wrapping round the north-eastern end of the ridge, the extreme point of prolongation of the axis 

 of the Old Radnor elevation. 



Although no intrusive trap has yet been discovered beneath or on the sides of the Nash lime- 

 stone, the Carton conglomerate, which has been largely cut into near the north-eastern end of the 

 ridge, and protrudes from beneath the limestone, strongly resembles a volcanic grit. It is excavated 

 to the depth of forty feet, and appeared to me to be zmstratified towards the bottom. The stone 

 has been largely quarried for rough masonry, and the new jail at Presteign is built of it. The 

 matrix is a greyish blue felspar, enveloping fragments of greywacke and quartz. (See p. 320.) 



As the line of broken and altered limestone of Nash and Old Radnor marks the last tract towards 

 the south-west, where any calcareous mass worthy of being worked, is found in the Silurian rocks 

 of South Wales, the demand for the limestone is so great (being transported fifty and sixty miles 

 into the slaty and quartzose districts,) that so much of this great boss of Nash Scar may be consumed 

 as will enable future geologists to discover the intrusive rocks, which I doubt not exist beneath this 

 converted limestone. This inference is deduced from the evidences of eruption and the changes 

 produced in the prolongation of this same line of fissure at Old Radnor, distant only three miles. 



The strata of the Silurian System which are thrown off to the south-east and north- 

 west of the trappean axis of Old Radnor, are the Ludlow and Wenlock formations and 

 a small portion of the Caradoc sandstone. We further know that this eruption must 

 have taken place subsequent to the deposit of the Old Red Sandstone, as stripes and 

 patches of the lower part of that system are observed upon either side of the axis of 

 elevation in highly inclined and dislocated strata, both in the Radnor woods west of the 

 Nash Scar, and again at Llantowel between Old Radnor and Gladestry. (See p. 192.) 



By reference to the map, it will be seen that the volcanic outburst we have just de- 

 scribed, is a key which satisfactorily explains the elevation of other tracts where no 

 trap appears at the surface ; for the line of fissure if prolonged, is precisely coincident 

 with the axis of elevation of the Ludlow promontory and with the great Brecon anti- 

 clinal, to be described in the twenty- seventh chapter. 



We may conclude our notice of this tract by stating that, though upon a less scale 



