TRAP ROCKS, CAERMARTHENSI1IRE. — CASTEL COGAN, ETC. 365 



matter, as the induration and alteration of the beds ; for even this zone of Carmarthenshire trap is 

 not without its metalliferous combinations, although they are of small extent. In the younger 

 strata of the Silurian rocks, which are parallel to the ridge of Blaen-dyffrin-garn, and Cairn-goch, 

 veins of lead have been partially worked, passing through the vertical strata on the banks of the 

 Sowdde, and similar veins have been more extensively cut into in the prolongation of these altered 

 strata near Pen-cae-sarah, and in the adjacent deep dingle, small filaments of copper ore have also 

 been discovered along the flanks of the quartz rock of Cairn-goch, precisely analogous to those on 

 the sides of Caer Caradoc. 



Trap of Castel cogan, fyc. 



A much more extensive outburst of trap than that of Blaen dyffrin-garn, occurs be- 

 tween the rivers Towy and Taaf, in the south-western district of Caermarthenshire. 

 This trap has also been erupted on a north-east and south-westerly fissure, and is 

 traceable at intervals for three to four miles, from a spot called the Glog, by Capel 

 and Llangynog, to Castel cogan and Gallt-y-minde, where it terminates in rugged hills 

 on the left bank of the Taaf. At Gallt-y-minde the rock is a hard, dark-grey, cream- 

 coloured or pink, compact felspar, projecting in irregular and angular masses, with 

 veins containing crystals of quartz, and minute crystals of iron pyrites. Thence it rises 

 into the highest ground in this tract, surmounted by the remains of the ancient camp 

 of Castel cogan 1 ; the rampart of which has been entirely constructed of this rock. 



The line of eruption towards Llangynog is marked by masses of amygdaloid, protruding through 

 contorted schist and grit. From this point the trap subsides for some distance, but it reappears 

 at Capel in the form of a concretionary felspar rock, in parts having a brecciated aspect, and 

 passing into an imperfect greenstone. The line of outburst is also flanked by masses of volcanic 

 grit, passing into a felspar conglomerate. At the Glog, where it has been deeply cut into for a 

 road- stone, the base of the quarries exposes a very hard rock, full of oblique rents and cracks, 

 made up essentially of compact felspar, for the most part concretionary, whilst in the upper 

 part, small pebbles of quartz become apparent, and are frequent near the summit. This felspar 

 conglomerate, with quartz pebbles of the size of eggs, varies from that state to a grit, and when 

 deeply laid open, consists of a concretionary and solid mass of felspar. It occupies several emi- 

 nences of 500 to 600 feet in height, along the northern face of the line of eruption. (Pen moelfre, 

 &c.) Like the volcanic conglomerates described in Radnorshire and elsewhere, the mass becomes 

 bedded where the quartz pebbles predominate, and therefore constitutes one of those rocks formed 

 by the mixed agency of fire and water. 



The discovery of this volcanized tract upon a line of fissure, parallel to the direction of the 

 Silurian rocks, and running so near their junction with the Old Red Sandstone, satisfactorily explains 

 many singular lithological appearances and fractures in the Upper Silurian group, which previously 

 surprised me. It has already been shown, that at several points on the left bank of the Towy, 

 quartzose grits,, containing much felspar, and altogether differing from the regular measures of the 



1 The camp of Castel cogan is probably a Roman work, though the parallellogram is not of the regularity 

 observable in Cairn-goch. 



2 z 



