TRAP AND ALTERED ROCKS OF OLD RADNOR, ETC. 



317 



Clywedog, the stream flowing by Abbey cwm-hir, they veer round to the S.S.W. in 

 the hills of DeA^anner and Coed trewernan, whence the same strata, sometimes con- 

 taining thick stony bands, but more frequently masses of schistose and friable structure, 

 resume the true strike of the country, traversing the Camlo Hills from north-east to 

 south-west, and ranging into the prominent ridge of Gwastaden south-east of Rhayader. 

 In the latter district the sandstones assume a decidedly slaty character, occupy a vast 

 breadth, and are traversed from north-west to south-east by the river Wye. The lofty 

 hills of Dol-fan, Rhiw-graid, and Gwastaden are distinguished by summits of coarse 

 slaty sandstone and fine conglomerate. These ridges are parallel, ranging from north- 

 east to south-west, and the strata, though sometimes dipping to the south-east, are for 

 the most part thrown over, at greater or less angles, to the north-west. (PI. 33. f. 7.) 

 In the Gwastaden Hills the quartzose slaty sandstones and conglomerates dip from 35° 

 to 50° to the north-west. They pass into coarse roofing slates, which occur in the 

 hills both south-east and north-west of Rhayader, and at the fall of the Wye. Some 

 of the finest examples of these slaty rocks 1 and their associated conglomerates are in 

 the deep gorges of the river Elain, a tributary of the Wye, particularly in the crags 

 called Cefn Craig-y-foel, where very large, apparently concretionary masses of coarse 

 conglomerate, are interstratified with slates, the whole much twisted and contorted, 

 dipping to the north-west. Thin veins of lead occur, and have been worked near the 

 sources and on the banks of the river Elain. 



These Upper Cambrian Rocks graduate into the older slaty rocks of Wales. There 

 is, indeed, no well-defined line of separation between the coarse quartzose slates of 

 Rhayader and the masses of more crystalline slate which are repeated upon parallel lines 

 between that town and Plinlymmon (Plyn-lumon of the Ordnance Map). They all 

 belong to the upper group of the Cambrian System, and are of the same age as many 

 mountains in Montgomeryshire (Moel-ben-tyrch, &c), the whole of which have been 

 proved by Professor Sedgwick to be of younger date than the slates of Merionethshire, 

 &c. 



Trap and altered Rocks in Radnorshire. 



Between the south-western termination of the various ridges of trap of Shropshire 

 and Montgomeryshire, and the rocks of the same character in Radnorshire, is a space 

 of eighteen or twenty square miles entirely void of volcanic matter, being exclusively 

 occupied by undulating strata. Round-backed hills of Upper Silurian Rocks occa- 

 sionally carry over upon their surface the lowest beds of the Old Red Sandstone. 

 The Radnorshire trap rocks diversify these soft hills, by shooting up in distinct stony 

 ridges, running from north-east to south-west. The most eastern is the Old Radnor 



1 By consulting the section, PI. 33. f. 7, it will be seen that these slaty rocks contain within them vast 

 masses of incoherent shale and hard grit. 



2 R 



