South-western Coal District of England. 283 



shale and limestone embrace the roots of the mountain^ and are continuous 

 from Huntley to Flaxley. 



Above the calcareous strata the shale becomes gradually coarser, and 

 harder; it changes in colour from the prevailing greenish-brown to red, and 

 passes insensibly into the lowest strata of the old red sandstone formation. 

 This upper portion of the shale, which immediately covers the transition 

 limestone, is known in the Shropshire coal-field by the name of die-earth* ; 

 and with this die- earth, the upper beds of the range between Ledbury and 

 Malvern agree both in character and position. 



A similar series of beds to those which we have now described, as be- 

 longing to the transition limestone on the north-eastern border of the Forest 

 of Dean, is found also on its south-western frontier, and there emerging from 

 beneath old red sandstone, exhibits the lowest formation that occurs between 

 this basin and that of South Wales. These opposite points, at which the 

 lowest strata are protruded, determine the natural limits of the basin that we 

 are now describing. 



It is to the west and north-west of Usk, over an area extending in length 

 from N. to S. about 7 miles, and in breadth from E. to W. about 3 miles, 

 that the transition limestone appei^^rs. Its northern extremity lies between Rag- 

 land and Abergavenny, at Clytha Castle, where a section of the hill by the 

 road-side, shows its dip to be to the north-east. It is visible also on both sides 

 of the road from Clytha Castle to Usk, maintaining the same dip as far as 

 that town, which stands upon it. In the hills to the west of Usk, which are 

 situated near the axis of this group, the dip is both rapid and variable ; on 

 the southern and western limits of the group, it dips outwards on either side, 

 and sinks under the conformably inclined strata of old red sandstone, which 

 mantle round it. 



It consists principally of thin laminated strata of tender argillaceous slate, 

 of a greenish-grey colour, containing subordinate beds of similarly coloured 

 sandy slate, and of argillaceous limestone, abounding for the most part, with 

 the characteristic organic remains of this formation, and particularly with 

 orthoceratites. The thickness of the calcareous beds is very variable, that of 

 the separate strata seldom exceeding 2 feet. The calcareous matter is often 



* The upper portion of the transition limestone shale, has acquired this appellation, because 

 in Shropshire, the coal-measures die or terminate on coming into contact with it. This contact 

 arises in the coal-field of Coalbrook Dale, from the total absence of the two formations of 

 mountain limestone and old red sandstone ; occasioning the lowest of the coal-measures, to re- 

 pose immediately on the uppermost beds of the formation of transition limestone. In the coal. field 

 of Dudley, there is ?. similar contact. 



