216 Messrs. Buckland's and Conybeare's Observations on the 



chains it constitutes a mountain group, whose average height above the sea 

 is about 900 feet. 



With one partial interruption, on the south-east frontier near Lidney, a band 

 of mountain limestone succeeded by old red sandstone may be traced entirely 

 round the basin. To the west and south-west exterior chains of mountain 

 limestone and old red sandstone cross the Wye, and are prolonged to a con- 

 siderable distance between that river and the mouth of the Usk, forming the 

 mountain chains of Penca-mawr and the district of Gwent. Transition lime- 

 stone is found on the western side of the basin near Usk. The narrow and 

 rocky defile of the Wye, from a little below Ross to Chepstow, traverses 

 strata of mountain limestone and old red sandstone, the coal-measures being 

 entirely confined to the district north of that river. On the north-east also, 

 near Mitchel Dean and Longhope, exterior chains from this basin are pro- 

 longed to some distance beyond the coal-field. They exhibit in succession 

 mountain limestone, old red sandstone, and the inferior beds of transition lime- 

 stone, resting finally against the elevated strata of greywacke, which compose 

 May-hill. 



The transition limestone of Tortworth, after crossing the Severn, together 

 with the old red sandstone at Pyrton-passage, may be traced due north through 

 Flaxley to the exterior chain in question, which, forming a narrow ridge, ex- 

 tends from Longhope to the Marcle hills and Stoke Edith, four miles east of 

 Hereford. 



3. Eastern Boundary of the great Coal-basin of South- Wales. 



The general features of this basin are well known from the description of it 

 by Mr. Martin, published in the Philosophical Transactions for 1806. The 

 following general statement may be added concerning the exterior chains of 

 its eastern portion, as they serve to connect it with the coal-basins of Bristol 

 and the Forest of Dean. 



A band, formed by the outcrops of the beds of mountain limestone, sur- 

 rounds the eastern part of this coal-basin. This is succeeded by the old red 

 sandstone, the fundamental rock on which the basin here reposes. The 

 beacons of Brecon, the loftiest mountains in South Wales, being 2862 feet 

 above the sea, are entirely formed of this rock, as is the whole chain connected 

 with them which skirts the basin parallel to the valley of the Usk. Grey- 

 wacke appears at the Eppyent hills in Brecknockshire, to the north of this chain 

 of sandstone. On the east, the old red sandstone occupies the base of the 

 limestone escarpment, and extends over nearly the whole of the district be- 

 tween this escarpment and the similar one of the Forest of Dean. On the 



