South-western Coal District of England. 219 



the oolite escarpment and the outer ridge of the coal-field. After closely 

 skirting- this ridge, and occasionally overlying it, they finally towards the south 

 surmount the eastern frontier, and pass within the basin. In this south-eastern 

 part of the coal-field, the shafts, by which the coal is won, are frequently opened 

 in the red marl, in the lias, or even in the lower beds of the oolite. 



It is only on the eastern and south-eastern frontiers of the two remaining coal- 

 basins that the horizontal deposits of red marl and lias are found, when they face 

 the similar deposits that skirt the eastern shores of the Severn and the Bristol 

 Channel. From the nature of the islets in the Channel end of the shoals laid 

 bare at the lowest ebbs, it appears that these deposits compose the whole bed of 

 the estuary, extending from shore to shore, except when removed by denuda- 

 tion or the more recent action of the tides. Wherever this removal has been 

 effected, the older ridges are discovered extending across the Channel. 



The coal-basin of the Forest of Dean being altogether mountainous, while 

 the horizontal deposits prevail at the low level of the valley of the Severn, those 

 deposits are confined to the foot of the outer ridges of the basin, which border 

 on the western shores of the Severn and the Bristol Channel. They are not 

 found protruded within the area of coal-field itself, nor do they surmount that 

 mountain-barrier to reappear on its western frontier. 



We may trace their course along the right bank of the Severn from the south- 

 ern extremity of the Malvern hills to the northern prolongations of the Forest 

 of Dean, and thence beneath the eastern escarpment of the ridges of the Fo- 

 rest to Caldecot level at the base of Penca-mawr, and to the flats extending to 

 the mouths of the Usk and Taaf. 



But of the horizontal deposits which occur on this side of the estuary, the 

 most extensive are those in the southern part of the vale of Glamorganshire. 

 Here they occupy the whole tract between the Bristol Channel and the collate- 

 ral limestone range of St. Lythian's Down*, lining the coast, from the mouths 

 of the Ely and Taaf to that of the Ogmore, with a range of cliffs, which face 

 towards the same formations on the opposite coast near Watchet. Lias 

 occupies nearly the whole of this tract, except in the parish of Sully, where the 

 red marl and dolomitic sandstone occur extensively. 



The vale of the Ely is also covered in many places by these deposits, which 

 here rest on old red sandstone. 



They also fillf an extensive cavity, formed by the vale of the Ewenny and 

 the lower part of that of the Ogmore, near Bridgend, whence they pass through 



* See the Sections, PI. XXXIV. figures 2 and 3. 

 t Consult Mr. Greenough's Map of England. 



