176 CENTRAL OR CORNSTONE FORMATION, CAERMARTHENSHIRE, ETC. 



of the high inclination of the strata, and the clearness of the section, a perfect know- 

 ledge of all the beds comprising the cornstone division, which, owing to their slight in- 

 clination and gentle undulations, are expanded over the low and fertile tracts of 

 Brecknockshire and Herefordshire, and are there rarely well exhibited as a whole. The 

 strata consist of deep red shale, argillaceous sandstone, and hard, quartzose, dingy 

 purple, or brown sandstone, slightly micaceous ; with intercalated calcareous beds, of a 

 concretionary and pseudo-concretionary structure. These calcareous concretions, varying 

 in colour from red to green, and in diameter from half an inch to three or four inches, 

 are disseminated through the mottled marl, which becomes occasionally an impure lime- 

 stone. They are arranged in bands occupying vertically from 8 to 30 feet each 1 . 



The finest example of limestone of the Old Red System in Caermarthenshire, occurs in the cliff 

 under the Castle at Llanstephan near the mouth of the Towey. (PI. 34. fig. 10.) The rock is there 

 from twenty- five to thirty feet thick, the upper part consisting of a number of small concretions, 

 which are underlaid by three massive beds of impure limestone, mottled green, blue, and red. Rising 

 in a dome-shape and slightly inclined, this calcareous mass is overlaid by red and green marls j 

 and further to the south or towards the marine headland, are flagstones, sandstones, and other well- 

 characterized beds of the system. In a subsequent account of Pembrokeshire, I shall have occasion 

 to show that although the calcareous matter becomes much scarcer in the Old Red Sandstone of 

 that county, we still meet with mottled, imperfect, concretionary masses, which are in parts calca- 

 reous, and represent the cornstone formation. 



To the east of Brecon, the cornstones rise from beneath the uppermost or quartzose strata into 

 the escarpment of the mountains of the Black Forest, where they are much more strongly developed 

 than in Caermarthenshire, as attested by different lines of lime-kilns which mark the lower limits 

 of the mountains S. and S.E. of the town of Hay. (PI. 31. fig. 1.) Some of the subordinate beds in 

 the immediate vicinity of Hay, afford a most excellent, thick-bedded freestone of a delicate green 

 colour, and of which the town is built. The cornstones, which are here so prevalent, rise to consi- 

 derable heights on the sides of the escarpments, and dipping gradually to the south-east, occasion- 

 ally reappear in deep denudations in the valley of the Usk, near Abergavenny; and finally disappear 

 under the great mass of overlying sandstone and quartzose conglomerate, which has been described 

 as forming the extreme margin of the South Welsh coal-field. At the northern escarpment of the 

 Skirrid, the remarkable ridge to the north of Abergavenny before alluded to, thick beds of cornstone 

 are exposed, dipping under red, brown, chocolate, and green sandstones, with blotches and con- 

 cretions of red marl. Other courses of cornstone extend along the lower sides of the Skirrid, and 

 are exposed' in the transverse valley between that mountain and the Sugar Loaf ; and a few thin 



1 The inhabitants of the banks of the Sowdde are not aware of the calcareous nature of these beds, there 

 being no occasion to seek for lime in a situation where, from the high inclination of all the strata, the middle 

 and lower members of the Old Red Sandstone are thrown into the immediate neighbourhood of the escarpment 

 of the carboniferous limestone (see section PI. 34. fig. 5.), the great works in which, at Clogan-mawr and Clogan- 

 bach, on the road leading from Pont-ar-lleche to Swansea, supply all the adjoining western districts of Caer- 

 marthenshire with lime very superior in quality to any which can be procured from the cornstones of the Old 

 Red Sandstone. The same statement may be applied to the district immediately to the north of Abergavenny, 

 where the cornstones are exposed, but will not, from their impure quality, stand a competition with the adja- 

 cent carboniferous limestone, and consequently are not worked. (See Map.) 



