[ 292 ] 

 XL 1 1. Proceedings of Learned Societies. 



GEOLOGICAL SOCIETY. 

 [Continued from p. 230.] 



(Abstract of Mr. Murchisons Paper " On certain Trap Rocks in the 

 Counties of Salop, Montgomery, fyc.,'' read May 21st, — continued.) 



Brecknockshire. — A RIDGE of trap rock of about three miles in 

 **■ length and half a mile in width, extends from 

 the stream of Nanteinon on the north-east to the right bank of the 

 Yrfon near Llanwrtyd on the south-west. The gorge, in which the 

 little river Cerdin flows, separates this ridge into two mountains, 

 Gaer Civm and Carn Dwad. 



The predominant character of the trap is porphyritic, including 

 greenstones, compact felspar rocks, &c. Some of the porphyry 

 is columnar. This nucleus is irregularly coated over with a thin 

 and broken covering of highly altered grauwacke schist, which is 

 frequently in the state of Lydian stone, from beneath the dislocated 

 beds of which bosses of trap protrude. Other varieties of altered 

 rock in contorted positions are seen on the flanks of the ridge, in 

 some of which are crystals of iron pyrites and small portions of car- 

 buret of iron. 



Absurd trials for coal, similar to those previously described, have 

 been made on the sides of these hills. The mineral source of Llan- 

 wrtyd issues from the pyritized schist on the banks of the Yrfon, 

 and it was from a conviction that the phenomena in this case might 

 be due to volcanic action similar to that noticed in the cases near 

 Llandrindod and Builth, that the author was induced to examine 

 this remote district. The result justified his anticipations, and led 

 him to the discovery of this intrusive ridge. 



Caermarthenshire. — The only example of trap rock in the large 

 portion of Caermarthenshire which has been examined, was detected 

 last summer in the small rocky knoll of Blaen-Dyffrin-garn, about 

 three miles south-east of Llangadock. This trap is more or less 

 porphyritic, from which state it passes into a rock having a base of 

 compact felspar in parts containing concretions and disseminated 

 green earth. It throws off strata of sandstone of about the same 

 age as those found on the sides of the Wrekin and Caer Caradoc, 

 and the phenomena resulting from the contact are identical with 

 those of Shropshire, the sandstone being converted into a hard and 

 granular quartz rock, having a conchoidal fracture. 



This line of altered rocks runs precisely parallel to the strike of 

 the strata of the grauwacke series, and is traceable for miles to the 

 south-west, the most prominent elevation of the quartz rocks being 

 Cairn-goch, formerly a Roman camp. 



Small metalliferous veins, chiefly of lead, have been found along 

 this line of eruptive elevation, particularly on the right bank of the 

 river Sowdde. 



The only lead mines now in work in Caermarthenshire are at 

 Nant y Moen, seven miles north of Llandovery, and it is highly 

 worthy of remark, that although these are situated so far to the 



