244 Rev. James Yates on the Structure of the 



The Breidden group, not far from Welchpool^ consists chiefly of various 

 kinds of trap rocks^ some of which contain globules of dark brown calcareous 

 spar, crystallized occasionally in diverging radii, and probably derived from 

 the fusion of small fragments of limestone. Sometimes instead of containing 

 globules of spar, the rock contains small angular fragments of limestone. 



Trap, similar to that of Standard Hill, occurs again in a hill on the left of 

 ihe road from Welchpool to Montgomery, immediately to the east of Offa's 

 Dyke. The trap rises in an abrupt and almost square mass above the verge 

 of the hill; it has the remains of a small fortification on its summit, and is 

 called Nant Cribba Moat, being encompassed by a trench. It contains con- 

 siderable quarries ; it is divided like the Standard Rock near Welchpool, by 

 a vertical structure of separation. Large veins of breccia pass through the 

 rock in directions parallel to its great planes of separation, and resemble that 

 of the Standard Rock, except that here the base is trap, and not limestone. 



Another large and conspicuous mass of trap is the site of Montgomery 

 Castle ; it consists almost entirely of a base of felspar. The trap of Mont- 

 gomery, like that of Nant Cribba and Welchpool, includes veins of breccia, 

 and also of calcareous spar with coal. 



A variety of shale or slate-clay is seen in immediate contact with the trap at 

 Montgomery, and occurs at intervals through the whole extent of country 

 from that place to Bishop's Castle. This slate-clay easily crumbles down on 

 exposure to the weather ; it has a conchoidal fracture, and divides into flat- 

 tened nodules like those of clay-ironstone, and it is remarkably like the slate- 

 clay usually found in connection with clay-ironstone and coal. It passes into 

 the rhomboidal grauwacke slate occurring in the whole surrounding region. 



Comdon HiU rises, at the distance of some miles to the north-east, to the 

 height of 1700 feet above the sea, and contains the Jlag-stone, which is only 

 a variety of this rhomboidal grauwacke slate, and which abounds in the Forest 

 of Brecon and other parts of South Wales. Corndon also rises exactly like 

 the hiUs in that district, to a sharp edge, with an escarpment towards the lofty 

 mountainous region. 



At the village of Norbury, about four miles north of Bishop's Castle, and at 

 the southern extremity of the Stiperstones, we find a siliceous in connection 

 with a calcareous deposition. The siliceous portion is a gray quartz with a 

 .splintery fracture, and somewhat crystafline, dividing into irregular rhomboids 

 and tetrahedrons. In a quarry on the western side of the viflage the stratifi- 

 cation of this rock is distinctly seen, and a large proportion of its beds is fuD 

 of cavities derived from the testaceous parts of different animals. The casts 

 are chiefly those of a pentamerus, of the turbinated madreporite of Parkinson, 



