332 



INTRUSIVE TRAP AND ALTERED ROCKS NEAR BUILTH. 



To the west of these quarries of Tan-y-craig several low ridges (about half the elevation of Car- 

 neddau) range in parallel lines from north-east to south-west, some of them terminating in low and 

 narrow bands in the bed of the Wye, others being prolonged about half a mile beyond that river 

 into the knolls of the Park Wells of Builth. These ridges on the north-east contribute to the beauty 

 of the grounds of Pen cerrig and Wellfield. As they are eminently instructive in explaining the 

 intrusive nature of the trap, and present the most unanswerable evidence of that rock having been 

 in fusion when inserted amid the beds of flag and schist, I propose to give a rather full account of 

 this locality, concluding with an account of the probable cause of the mineralization of the springs 

 of the park. 



On the sides of the new approach to the house of Pen cerrig the shale is in its usual rotten, 

 shivery condition, containing fragments of trilobites, but is dislocated on approaching the little knoll 

 of greenstone, and within a few yards of it, losing its laminated structure, is converted into a hard, 

 veined, flinty flagstone of light green and black colours, thus : 



59, 



a. Trilobite Shale. c. Altered Shale. b. Trap. 



In the quarries of coarse flagstone and wallstone, west of the house of Wellfield, are also evidences 

 of induration and dislocation. Here the trilobite shale, containing some of the largest and finest 

 specimens of Asaphus Buchii, is thrown off at an angle of 40° to the west : the stone although slaty 

 on the great scale, breaks into irregular rhombs, and the finely laminated structure is again oblite- 

 rated. In this case no junction with the trap rock is visible, although we know from the proximity 

 of the summit of the little wooded trap hill which rises up behind this quarry that the intrusive rock 

 is very near at hand 1 . 



On following these several bands of trap and dislocated Silurian strata as they descend to the 

 south-west, their connection is still more clearly explained. First in some ravines (particularly at 

 Gwern-y-fad), on the sides of which the altered and hardened flags are thrown up into nearly vertical 

 strata between protuberances of trap, and secondly in the bed and on both banks of the Wye. From 

 the west of the hamlet of Gwern-y-fad to the spot called Pen-ddol, the Wye for upwards of half a 

 mile, lays bare a most illustrative transverse section, which is, however, only well seen when the 

 river is low, and then we meet with the following succession in proceeding from west to east. 



1. For some distance west of the gorge, black schists containing spheroidal concretions, with orthocera, graptolites, &c, 

 are thrown off dipping to the west. These beds reminded me of the lowest Wenlock shale. 2. Grey calcareous grit which 

 passes into a coarse rock inclosing fragments of schist, which is much altered, acquiring a conchoidal fracture near the 

 trap, dip 35° north-west. It contains kernels of calcareous spar, and its characteristic fossil is Pentamerus ollongus. This 

 band must therefore be classed with the Caradoc sandstone. It dips beneath the shale before described. 3. Irregular bosses 

 of coarse and fine-grained greenstone with a little lime disseminated, thus passing into a finer grained crystalline rock with 

 many veins of white calc spar and crystals of iron pyrites. This trap rises on the right bank of the river and extends to 

 the south-west, constituting the hillocks near the Pump House. 4. In contact with this trap is an altered black flagstone, 

 highly charged with crystals of pyrites, sometimes in geodes. 5. Greenstone, with many veins of carbonate of lime, is 



i Some of the finest specimens of Asaphus Buchii occur in these quarries. The Trinucleus fimbriatus and the 

 new species PL 25. were found in the highly inclined flagstones of Gwern-y-fad. Notwithstanding the mention 

 of these fossils, the student must not look for any proofs of the order of succession in this disturbed tract. 



