1 04 Old Red Sandstone. 



Channel, the south and south-eastern borders of the 

 Silurian rocks of Caermarthenshire, Breconshire, Rad- 

 norshire, and Shropshire, and the long line of Carboni- 

 ferous, Silurian, and New Red Marl strata that runs from 

 Colebrook Dale to the Severn, east of Dean Forest. 

 Fancy in your ' mind's eye ' the Carboniferous rocks 

 of the great South Wales Coalfield, and of Dean Forest, 

 to be stripped away, and the whole of the region men- 

 tioned, of 120 by 90 miles in length and breadth, would 

 consist entirely of Old Red Sandstone. The lower part 

 is chiefly composed of beds of red marl and sandstone, 

 with cornstones ; and the upper part contains strata of 

 sandstone and conglomerate, forming the Beacons of 

 Brecon, 2,860 feet high, these being the loftiest moun- 

 tains in South Wales. 



Cornstones are impure concretionary limestones, 

 often imbedded 'in marl. In these, at the base of the 

 series, near Ludlow, are species of Pterygotus and 

 Pterichthys, and higher up, of Onchus and Cephalaspis, 

 thus correlating them by fossils to the Old Red Sand- 

 stone of Scotland (fig. 26). Along the border of this 

 formation, where the uppermost Silurian strata join the 

 Old Red Sandstone, there is a gradual passage both 

 palseontologically and in the colour and texture of the 

 strata. The Eurypteri and Pterygoti chiefly belong- 

 to these passage-beds, and in the same strata at the 

 very base of the Old Red strata, in which there are no 

 mollusca, are species of fish of the genera Auchenaspis, 

 Onchus (?), Pteraspis. Cephalaspis, and Plectrodus. 

 The Silurian marine mollusca, in fact, quickly'disappear 

 where the beds begin to get red in colour, notwith- 

 standing the perfect conformity of the two sets of strata 

 in England and the borders of Wales, as, for example, 

 in the neighbourhood of Ludlow. At Kington and 



