Murchisoris Silurian System. 541 



through the mottled marl, which becomes occasionally an impure 

 limestone. They are arranged in bands occupying vertically from eight 

 to thirty feet each. 



" The finest example of limestone of the old red system in Carmar- 

 thenshire, occurs in the cliff under the castle at Llanstephan, near the 

 mouth of the Tawey. 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 limestones, mottled 

 green, blue, and red, rising in a dome-shape, and slightly inclined; 

 this calcareous mass is overlaid by red and green marls ; 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 mass- 

 es, which are in parts calcareous, and represent the cornstone forma- 

 tion. 



" To the east of Brecon, the stones rise from beneath the uppermost or 

 quartzose strata in the escarpment of the mountains of the Bock forest, 

 where they are much more strongly developed than in Csermarthen- 

 shire, as attested by different lines of lime-kilns which mark the lower 

 limits of the mountains S. and SE. of the town of Hay. Some of the 

 subordinate beds in the immediate vicinity of Hay, afford a most excel- 

 lent thick-bedded freestone, of a delicate green colour, and of which the 

 town is built. The cornstones, which are here so prevalent, rise to 

 considerable heights on the sides of the escarpments, and dipping gra- 

 dually to the south-east, occasionally reappear in deep demiductions 

 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- 

 west coal field. At the northern escarpment of the Skirrid, the re- 

 markable 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 concretions 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 layers have been already alluded to as appearing in 

 the face of the great escarpment of the Blorenge. 



" I refer the reader to the map, to obtain a notion of the large tracts 

 in Brecon and Monmouth where these limestones prevail. A good de- 

 scending section of the whole system of old red has recently been 



