TRAP AND ALTERED ROCKS — BRECKNOCKSHIRE. 



343 



Porphyry intruded in the schist near Llanwrtyd, from a sketch by Mrs. Traherne. 

 (P. Porphyry.) (A S. Altered schist.) 



Trap and Altered Rocks in Brecknockshire. 



With the exception of the small knolls near Builth already described, no observer 

 had previously noticed any mass of trap rock in Brecknockshire. Having heard of the 

 mineral waters of Llanwrtyd, I conceived that as similar springs in Radnorshire and at 

 Builth uniformly issued from altered strata in contact with trap rocks, the same phe- 

 nomenon might be repeated near this spa ; and on visiting it, the inference proved 

 correct. I found a line of intrusive rock, about three miles in length and half a mile 

 in its greatest width, running, like the trap ridges of Radnorshire, from north-east to 

 south-west. A narrow and deep dell, through which flows the rivulet Cerdin, divides 

 this elliptical-shaped ridge into two mountains, Caer-cwm and Garn-dwad, each about 

 1600 feet in height. At the north-eastern extremity of Caer-cwm, trap is seen, for 

 the last time, on the banks of the little stream Nant-einon, alternating in thin courses 

 with slaty schists ; whilst at the south-western end of Garn-dwad the trap crosses the 

 Ithon between Llanwrtyd and the mineral spring, near a boss of rock called Gwern- 

 goch, upon the right bank of that river, and near the farm-house of Dol-y-dymmor. 

 In this ridge of Garn-dwad and Caer-cwm the predominant character of the trap is 

 porphyritic, and the following varieties occur : 



1. A rock, having a base of greenish grey colovir, composed apparently of an intimate mixture of compact felspar and 

 hornblende, spotted white, probably by a separation of felspar from the compound base. 



2. Coarse rock of granular felspar, with minute crystals of common felspar, containing a number of minute grains of 

 quartz. 



3. A variety of the same, containing many well-defined crystals of felspar in a greyish granular base, and a few small 

 elliptical cells filled with green earth. 



2 u 2 



