. 1820.] Geological Society. 213 



it into several smaller basins and irregular concavities which may 

 be considered subordinate to the general structure of the 

 district. 



This district occupies a triangular area bounded on the south 

 by the Mendip hills, and on the other two sides by lines drawn 

 from the two extremities of the Mendip chain to the village of 

 Tortworth, in Gloucestershire. 



The exterior ridges which bound the area of this coal forma- 

 tion consist for the most part of an interrupted chain of mountain 

 limestone reposing on old red sandstone ; and these two forma- 

 tions are so closely associated, that in describing their geogra- 

 phical extent, it is necessary to treat of them in combination. 



Of these extensive ridges, the Mendip chain is most important 

 in extent and elevation ; it consists of a central axis of old red 

 sandstone, Hanked by a double line of mountain limestone ; the 

 sandstone strata, however, are not visible in one continuous line ; 

 but present themselves in a series of four ridges, forming the 

 • four most elevated parts of the chain, though not exclusively 

 confined to them. 



Each of these ridges forms a saddle-shaped nucleus of sand- 

 stone, usually inclined at an angle exceeding 45°, around which 

 the incumbent beds of limestone are wrapped as a mantle dipping 

 in every direction, conformably to the subjacent sandstone. 



Between the sandstone and mountain lime is interposed abed 

 of shale, in which several fruitless attempts have been made in 

 search for coal : this bed it is proposed to designate as the 

 lower limestone shale. The Mendip chain is thus divided into 

 four regions, each containing as its nucleus one of the ridges of 

 old red sandstone. 



The most easterly of these is the ridge of Masberry Castle, 

 near Shepton Mallet ; the east central is Pen Hill, near Wells ; 

 the west central, Priddy North Hill and the western Blackdown, 

 near Cross. 



The total length of the Mendip chain is about 26 miles run- 

 ning east and west from near Frome to Uphill, on the Severn ; 

 on the south-east flank of this chain a curious circumstance is 

 exhibited in the contact of overlying horizontal beds of lias and 

 inferior oolite, with the inclined strata of mountain lime and old 

 red sandstone, several small rivers in this part exhibitino- roman- 

 tic difis of mountain lime in highly inclined strata, crowned with 

 horizontal beds of oolite. 



These strata sometimes adhere to each other as firmly as if they 

 had been parts of one and the same contemporaneous formation. 

 The detail of the description of the subdivisional ridges of the 

 Mendip chain is too minute to allow of abridgment. 



The mountain limestone corresponds in all its characters with 

 tijat of Clifton, and has its usual properties of engulphing rivers, 

 and exhibitingextensive caverns and abru[)t romantic precipices, 

 the most maarkable of which are at Chedder clitls. The old 



