South-western Coal District of England. 225 



that river aside from the valley leading through Long Ashton and Nailsea to 

 the Bristol Channel^ and conducts it through the limestone chain of Leigh 

 Down into the Severn, is a magnificent example of the latter. 



Exterior Chains of the Coal-basin described. 



We now proceed to trace the old red sandstone and the mountain limestone 

 in their joint progress around the frontier of the Bristol coal-basin. 



L On the southern frontier or Mendip chain. 



This is the loftiest and most extensive of the exterior chains of the basin. 

 Its whole length from Frome on the east to Uphill bay on the west, where it 

 is cut off by the Bristol Channel, is about 26 miles. It consists (as we have 

 before stated in the introductory chapter) of mantle-shaped strata of moun- 

 tain limestone, investing a central axis of old red sandstone. This axis is par- 

 tially covered by the limestone, and partially emerges from beneath it, and 

 then rises into four * conspicuous ridges, of which the highest points ascend 

 to between 100 and 300 feet above the elevated calcareous table-land of the 

 Mendips. The calcareous slopes on either side of the chain are encumbered 

 by the overlying formations, and especially by the dolomitic conglomerate. 

 These deposits are also frequently insinuated into the valleys between the two 

 exterior calcareous bands, and invest extensive portions even of the high central 

 table -land. 



The first point where the inclined strata appear at the eastern extremity of 

 the chain, is on the line of the intended canal close to Whatcomb farm, half a 

 mile west of Prome. They are concealed along great part of the low table- 

 land, that encircles the eastern termination of the chain, by strata of inferior 

 oolite, and are visible only in the valleys of denudation through which the se- 

 veral streams pass that unite between the village of Mells and Frome. Beau- 

 tiful sections may be seen in the precipitous sides of these valleys, exhibiting 

 the oolitic strata in an absolutely horizontal position, reposing on the truncated 

 edges of highly inclined strata of mountain limestone f . The character of 

 these valleys is exceedingly wild and romantic from the craggy steepness of 

 their sides, though their depth is very inconsiderable. Hence few more striking 



* There is a fifth appearance of the sandstone^ but to a very limited extent, in a valley at the 

 head of Emborrow Meer. 



f Vide Plate XXXV. fig. 2. At the village of Whately the two formations adhere so closely, 

 that we have collected firmly cemented masses, of which one half is oolite, and the other mountain 

 limestone, the strata of the two rocks intersecting at an angle of 45". In this neighbourhood a bed 

 of breccia is sometimes interposed, occasioned probably by the entanglement of loose gravel, pre- 

 viously lying on the surface of the limestone, in the first deposits of the oolite. 



2g2 



