316 



CAMBRIAN ROCKS WEST OF THE ITHON. 



Rock, and contain the Pleurotomaria Corallii, the TerebratulaJVavicula, Tentamerus sulcatvs, &c, 

 they probably represent the Aymestry limestone. The heights occupied by these rocks in Radnor- 

 shire are marked upon the map, Radnor Forest, the highest, being 2200 feet above the sea. Whilst 

 the strata of these mudstones extending over the north -western highlands of Radnorshire are soft, 

 incoherent, and perfectly useless, with the exception of the very uppermost beds and the lower 

 flagstone, a large portion of the Upper Silurian Rocks on the banks of the Wye are much indurated 

 and very hard, as in the contorted strata in the gorge of Craig-pwll-ddu and in many parts along 

 the banks of the Wye, and also between Erwood and Henault. They are underlaid by very thinly 

 fissile beds, quarried as tiles. It would be useless to specify all the undulations and dislocations 

 of these rocks, in the hills forming the great featureless, argillaceous tracts between Radnor Forest 

 and the Ithon. The new road leading from Builth and Llandrindod to Newtown, ascending by the 

 left bank of the Ithon, is cut through promontories of these rocks, and affords many illustrations 

 of such phenomena, the strata dipping to the E.N.E., E.S.E., and even to the north-west. (Castle 

 Twmpath, Llanbadarn Fynidd, &c.) 



In this district, there being no trace of limestone, it is impracticable to subdivide the 

 mass into formations, or even to endeavour to separate with accuracy the Ludlow from 

 the Wenlock formation, the whole representing, as before stated, the Upper Silurian 

 Rocks. They occupy in fact a broad and deep trough, lying between the ridges 

 of the older slaty rocks of the Longmynd on the north-east, and those of similar age 

 on the west, which we are about to describe. By this arrangement the Lower Silurian 

 Rocks never rise to the surface, the upper or mudstone strata being unconformably 

 recumbent along their western frontier upon strata of the Cambrian System. We have 

 before adverted to this phenomenon, which extends along the line of junction of the Si- 

 lurian and Cambrian Rocks from Montgomeryshire to the borders of Caermarthenshire, 

 (See PI. 33. f. 1 . p. 310, and outline of Map.) 



" Cambrian Rocks." 



The right bank of the Ithon exhibits a decided change of mineral character in the 

 mountainous masses called the Foel, the Raltt, Bedugre, and other hills which range 

 from the Valley of the Severn towards Abbey Cwm-hir. (PI. 33. f. 1.) These moun- 

 tains are composed of quartzose grits, subordinate to slaty sandstone, probably of the 

 same age as that of the Longmynd or mineral axis of Shropshire, or of Moel-ben-tyrch 

 and other rocks in Montgomeryshire. It is remarkable, that the highest of these 

 mountains, Bryn-dir, the Maen Rocks, and others in the neighbourhood of David's 

 Well (a very slightly sulphureous spring), have all a very determined strike from north 

 to south, by which arrangement they are flanked unconformably by the Upper Silurian 

 Rocks, there being, as has just been stated, no traces in this district of any forma- 

 tions to fill up the interval and to represent those of Caradoc and Llandeilo or the 

 Lower Silurian Rocks. As the ridges of Cambrian rocks approach the valley of the 



