South-icestern Coal District of England. 249 



3. On the eastern frontier of the Coal-basin. 



At Tortworth the exterior chain is deflected somewhat suddenly to the 

 S.E., extending- in that direction through Wickwar to Sodbury. It presents 

 a low chain of mountain limestone with old red sandstone along its outer 

 border. The junction of the two rocks may be seen at Wickwar^ the church 

 being built upon the sandstone. Overlying red marl abuts against the foot of 

 the sandstone escarpment^ and just below the church of Wickwar a cap of lias 

 also rests upon the sandstone slope. To the east of Wickwar, on the other 

 side of a narrow valley, lias forms a broad terrace, above which rises the lofty- 

 oolitic ridge of the Cotswold. To the south of Sodbury the exterior chain 

 becomes so low, that the lias terrace extends over it, and passes within the 

 basin. The calcareous limits of the basin, however, are exposed in valleys of 

 denudation between Sodbury and the Avon at three points lying due south of 

 each other, and in the line of prolongation of the exterior chain, viz. 1. At 

 Codrington Court ; 2. At Wick Rocks ; 3. At Granham Rocks Of these 

 Wick Rocks alone require particular notice. Hills of lias here surround the 

 valley of the Buoyd on the east, north, and west. That brook here deserts 

 the broad valley to the east of the rocks, in order to traverse a narrow defile 

 f of a mile in length. At the northern extremity of the defile we meet first 

 with old red sandstone ; then with lofty precipices of mountain limestone, 

 crowned with ancient entrenchments defending the opposite sides of the pass ; 

 and further on, before we issue through the southern gorge, we see the mill- 

 stone grit resting against the limestone. Beyond the grit, horizontal dolomitic 

 conglomerate lines the mouth of the valley. All the inclined strata dip due 

 west at an angle of 70°. 



At Beach Lane and Granham Rocks, a little to the south of Tracy-park, the 

 mountain limestone is just seen in the bottom of the valleys excavated through 

 the lias. The millstone grit is also seen at the latter spot. In the wide inter- 

 val between the Avon and the Mendip Hills the limestone frontier is entirely 

 concealed by overlying deposits. 



On the Coal measures of the Basin. 



The coal-measures appear to be distributed throughout the greater part of 

 the area included within the outer calcareous ridges of the basin, though con- 

 cealed in many places by the overlying formations. It seems probable that 

 these later formations were at one time much more extensive than at present, 

 and that they have been furrowed out and removed in those places where the 

 coal-measures are now exposed. These uncovered areas I shall term the 

 Northern, the Central, the Southern, the Eastern, and theWestern Coal-tracts. 



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