GENERAL RELATIONS OF THE GROUP OF USK. 



441 



Horderly Section (p. 220), and in a similar position near the south-western termination of the 

 Malvern Hills, (p. 416). If they were found to contain the Asaphus Buchii, these beds would at 

 once be considered Llandeilo flags 1 . The sandy flagstone is also visible in the high grounds of Cefn 

 Isla, and is, apparently, thrown unconformably against the superior formations, in the ridge opposite 

 the town of Usk ; but there the quantity of wood and the absence of clear sections, coupled with 

 reversed dips and dislocations, prevent accurate observation. In like manner a patch of impure 

 limestone occurs in the hill above Rudor Farm, and seems to be associated rather with the Caradoc 

 than the Wenlock formation, for it is thrown off upon the side of a low hill, in which are certain thin 

 bedded sandstones. (See Map.) 



On the whole this tract is of high geological interest. It is, indeed, the most 

 southern point of Great Britain, where a passage from the Silurian Rocks into the Old 

 Red Sandstone can be studied. Geologists, therefore, who cannot travel to those parts 

 where the best types exist, may, it is hoped, be induced to visit a district which is also 

 attractive, on account of its ruined castles and picturesque features 2 . 



Rising into concentric ridges, which are separated by deep intervening hollows and 

 circle round a common centre, the strata are instructively laid open by many transverse 

 breaks, and dip to nearly every point of the compass, at angles varying from 5° to 70°. 

 There are also some good examples of the folding over of strata, particularly in the 

 Ludlow Rocks above Llangibby, where small patches of Old Red Sandstone are pro- 

 duced in upland troughs, distant from the principal mass of the formation. 



A great body of coarse gravel, which will be adverted to in subsequent chapters, is 

 spread over the surrounding country, while no debris occurs within the central area ; 

 and hence it is inferred that this valley has been denuded by the same process as that 

 of Woolhope (p. 436) 3 . 



In concluding the sketch of this group, it may be further observed, 1st. That the 

 river Wye flows through a great fissure, transverse to the axis of the group. 2ndly. 

 That this tract having been thrown up in close contiguity to the coal-basin of South 

 Wales, its axis (from N.N.E. to S.S.W.), though parallel to the adjacent portion of the 

 margin of the coal-field, is nearly at right angles to the major axis of the South Welsh 

 coal basin, which trends from west and by north to east and by south. 



1 Though I examined these black shale beds in an excellent section (Bryn Craig) without detecting organic 

 remains, the weather on that occasion was so unfavourable, that I should not be surprised if other observers 

 were more fortunate. 



2 Ragland and Usk. The latter place is a favourite resort of the angler. (Read Sir Humphrey Davy's Sal- 

 monia, and do not pass by the sign of the Three Salmons, an excellent inn.) 



3 This observation applies only to the central part of the elevated tract, which dips from the common centre 

 of Prescoed Common on the right bank of the Usk. The Hills of Trostrey and Clytha, and the higher plateaux 

 of Ragland, Penrose, &c„ are nearly as much obscured by gravel and even by heavy boulders, as the low 

 grounds adjacent to the river. (See Map and concluding chapters.) 



