88 



porphyries, greenstone, and much concretionary rock, frequently of 

 very large size. These trap rocks burst through strata of grauwacke, 

 in some of which fossils of the Ludlow and Wenlock formations 

 have been found. The strata in contact are much hardened and 

 fractured, and contain many veins of carbonate of lime, and sul- 

 phate and carbonate of barytes, &c. &c. 



The other chief ridge, or that on which Rodney's Pillar stands, 

 is composed in great part of coarse-grained columnar greenstone, 

 passing at its extremities into fine concretionary trap and clinkstone. 

 The Criggan, a third or minor ridge, included between these, ex- 

 hibits bosses of greenstone, with altered and siHcified schists on 

 their sides. 



A new locality of singularly spotted trap rock piercing through 

 an impure limestone full of shells, is noticed at the Cefn, between 

 the Moel y Golfa and Welch Pool. 



The celebrated quarries of building-stone at Welch Pool expose 

 a broad dyke of columnar greenstone, passing in one part into con- 

 cretionary trap traversing strata of the same age, and producing 

 great changes in them in contact. The direction of this great dyke 

 is in the prolongation of the volcanic axis of the Breiddin Hills. 

 The last traces of these concretionaiy traps are observable in Powis 

 Castle Park. 



Radnorshire. — The trap rocks in Radnorshire run in distinct 

 ridges from north-east to south-west : the most eastern are those near 

 Old Radnor ; the central or chief masses range from Llandegley 

 and Llandrindod to Builth ; a third and unimportant ridge occurs at 

 Baxter's Bank, five miles north-west of Llandrindod. 



Old Radnor Group. — These trap rocks occupy two parallel ridges 

 comprising Stanner Rocks, Worsel Wood, and Hanter Hill on the 

 south-east, and Old Radnor Hill and its dependencies on the north- 

 west. These rocks are distinguishable from all others mentioned in 

 this memoir by the abundance of hypersthene, and exhibit many 

 passages from a coarse crystalline hypersthene rock to a fine-grained 

 greenstone. Unlike the hypersthene rocks in the Isle of Skye, their 

 base is for the most part of compact felspar, which passes into gra- 

 nular felspar, porphyry, &c. 



In Old Radnor Hill, besides hypersthene rock and greenstone, 

 there are concretionary traps, bastard serpentine, &c. Towards 

 Harpton Court these latter rocks throw off a peculiar conglomerate, 

 having a baseof felspar inclosing pebbles of quartz, which the author 

 supposes to have been formed by a mixture of volcanic matter with 

 submarine detritus. 



The eruptive masses of trap tilt the strata of the Ludlow forma- 

 tion on the one side, and those of the Wenlock rocks on the other, 

 the most interesting phaenomena being observable near Old Radnor 

 church and the quarries to the south of it, where the limestone has 

 been cleared away from the abrupt faces and points of the intrusive 

 rock. Bands of imperfect serpentine are frequent between the trap 

 and the limestone. The latter, near the contact, is wholly unstratified, 

 crystalline, and hard, and only resumes its ordinary appearance at 



