South-western Coal District of England. 215 



more or less completely filling these depressions. The three insulated mem- 

 bers of the western calcareous frontier are, 1. Broadfield Down, of which 

 Worleberry and the calcareous islet of Flatholm may be considered the extreme 

 western prolongations ; 2. Leigh Down, comprehending the defile of the 

 Avon, and the hills stretching thence to Westbury ; 3. the range commencing 

 at Knoll park and the Ridgeway near Almonsbury, and thence running by 

 Thornbury to Tortworth. 



The old red sandstone is seen in many places accompanying this western 

 interrupted calcareous chain, either forming an arch in its centre, or rising 

 from the foot of its escarpment in the plains to the west of it. The latter is the 

 case towards the northern part of this western frontier, where the anticlinal 

 line, separating the coal-basin of South Gloucestershire from that of the Forest 

 of Dean, seems to pass along the middle of the valley of the Severn. 



Transition limestone emerges from beneath old red sandstone at Tortworth, 

 and, passing the Severn at Pyrton-passage, is united to the exterior chain of 

 the Forest of Dean. 



Near Tortworth, at the northern apex of the triangle,, the range extending 

 from Almonsbury is deflected abruptly to the south, and in that direction, con- 

 stituting the eastern frontier of the basin, it may be traced through Wickwar 

 to Sodbury. From Sodbury to the eastern extremity of the Mendips near 

 Mells, the southern half of the eastern frontier is concealed by overlying depo- 

 sits, except at three points of very limited extent at the northern foot of Lans- 

 down near Wick rocks, where valleys of denudation, cut down through hori- 

 zontal lias, have exposed the limestone, dipping there, as in every visible part 

 of the boundary, towards the centre of the coal-basin. From the foot of 

 Lansdown to the Mendip hills the continuity of the coal-basin is well ascer- 

 tained, either by valleys of denudation which cut through the horizontal depo- 

 sits down to the coal-measures (and in such valleys are situated the principal 

 collieries of Somersetshire), or by frequent shafts opened in the same depo- 

 sits, beneath which the coal is worked. From the dip of the coal-measures, 

 wherever proved in the interval referred to, it may be concluded, that the sub- 

 terraneous prolongation of the eastern frontier of the basin coincides very 

 nearly with a line drawn from Wick rocks to Mells. 



3. Boundary of the Coal-basin of the Forest of Dean. 



This basin occupies an irregular elliptical area, circumscribed by the tri- 

 angle formed by the Wye, the Severn, and the road from Gloucester to Ross, 

 the longer axis from north to south being about ten miles in length, and the 

 transverse from east to west about six miles. Together w'th its exterior 



VOL. VI. 2 F 



