TRAP AND ALTERED ROCKS OF OLD RADNOR, ETC.

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Clywedog, the stream flowing by Abbey cwm-hir, they veer round to the S.S.W. inthe hills of Devanner 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 tosouth-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 vastbreadth, and are traversed from north-west to south-east by the river Wye. The loftyhills of Dol-fan, Rhiw-graid, and Gwastaden are distinguished by summits of coarseslaty 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 forthe 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 thehills both south-east and north-west of Rhayader, and at the fall of the AYye. Someof the finest examples of these slaty rocks 1 and their associated conglomerates are inthe deep gorges of the river Elain, a tributary of the Wye, particularly in the cragscalled Cefn Craig-y-foel, where very large, apparently concretionary masses of coarseconglomerate, 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 thesources and on the banks of the river Elain.

These Upper Cambrian Rocks graduate into the older slaty rocks of Wales . Thereis, indeed, no well-defined line of separation between the coarse quartzose slates ofRhayader and the masses of more crystalline slate which are repeated upon parallel linesbetween that town and Plinlymmon (Plyn-lumon of the Ordnance Map). They allbelong to the upper group of the Cambrian System, and are of the same age as manymountains in Montgomeryshire (Moel-ben-tyrch, &c.), the whole of which have beenproved 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 spaceof eighteen or twenty square miles entirely void of volcanic matter, being exclusivelyoccupied 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 stonyridges, 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 vastmasses of incoherent shale and hard grit.

2 It