South-western Coal District of England. 285 



in many parts of the neighbouring counties, where it often affords the only 

 supply of that necessary material. 



The upper portion of the old red sandstone, is characterized by the pre- 

 sence of siliceous conglomerate containing siliceous pebbles. This position 

 of the conglomerate in the old red sandstone, is verified by numerous sec- 

 tions along the banks of the Wye, from Ross to Monmouth, and also by the 

 fine section in the hiil immediately to the west of Mitchel Dean. The sum- 

 mit of this hill presents the millstone-grit or lowest grit of the coal-measures; 

 the middle region shows the outcrop of the mountain limestone, from beneath 

 which, towards the base, the conglomerate beds of the old red sandstone 

 emerge in a manner equally distinct. The summits of the loftiest hills near 

 Monmouth, abound with enormous blocks and small insulated tors composed of 

 this conglomerate, of which the rocky projections on the summit of the Kyming- 

 hill, and the insulated tor, at no great distance from it, in the direction of the 

 Forest, called the Brick-stone, and usually mistaken for a druidical monu- 

 ment, are examples. This conglomerate, also forms the upper stratum of 

 many of the highest ridges between the Wye and Usk. It is applied ex- 

 tensively to the fabrication of millstones near Monmouth, and on the banks of 

 the Wye. 



Beds of red and variegated soft, slaty, marl, and marly sandstone, occur 

 indifferently from the top to the bottom in every part of this formation. 

 These marly beds are often precisely similar, both in colour and substance, 

 to those of the newer red sandstone ; but in the older formation, they are 

 usually more distinctly slaty, and alternate more rapidly with thin layers of 

 sandstone-slate. We have before alluded to the remarkable fertility of these 

 marly beds in the old red sandstone, in Monmouthshire and Herefordshire. 



The old red sandstone encircles the Forest, with a ring of very elevated 

 ground, whose long and lofty ridges on the eastern frontier overhang the 

 valley of the Severn. The trough of the river, is filled for the most part with 

 the horizontal formations, but the sandstone is exposed in one short interval, 

 between the villages of Lydney and Blakeney, there forming a range of cliffs 

 alons: the edij-e of the water to the distance of 2 miles, which face the similar 

 rocks occurring on the opposite bank, to the south-west of Pyrton Passage- 

 house. On the western side of the Forest, the old red sandstone is traversed 

 from Ross to Tinterne, by the defiles of the Wye, which are so steep and 

 narrow as scarcely to be perceived at a small distance from the river. To the 

 west of the river, the mountain chain of sandstone is nearly continuous, and 

 increases in height as it advances, until it forms the ridge of Pencamawr be- 

 tween Monmouth and Newport. Not far from this ridge, but not immedi- 



