214 Messrs. Buckland's and Conybeare's Observations on the 



The coal-measures, therefore, will occupy the interior of all the basins thus 

 constituted, and will be surrounded by exterior bands of mountain-limestone 

 and old red sandstone, in the order of the outcrop of the subjacent beds. 



The principal basins of the district, which include as many principal coal- 

 fields, are three : — 



1 . That of Somersetshire and South-Gloucestershire ; which for brevity we 

 shall call the Basin of Bristol. 



2. That of the Forest of Dean. 



3. The great coal-basin of South-Wales. 



We shall now trace the boundaries of these three coal-basins, as far as they 

 fall within the geographical limits to which we have confined ourselves. 



1. Boundary of the Coal Basin of Bristol *. 



This basin occupies an irregular triangular space, the base of which on the 

 south is the chain of the Mendip Hills, a high range of mountain-limestone, 

 running from east to west, and resting on an arch of old red sandstone. The 

 sandstone appears, with some interruptions, in the central region of the chain, 

 of which the longer axis coincides with the anticlinal line. The calcareous 

 isle of Steepholm is placed on the prolongation of this southern frontier, and 

 connects it with a collateral branch of the hmestone boundary of the coal-basin 

 of South Wales. 



The northern apex of the triangle is at the village of Tortworth in Glouces- 

 tershire. The western frontier of the basin is continued from the Mendips 

 to this apex ; not by an unbroken chain, but by three lofty insulated groups, 

 placed on the same line of bearing, and parted by narrow intervals, of which 

 the broadest falls short of three miles. These interruptions, or rather depres- 

 sions in the chain, are occasioned by undulations in the bed of limestone, 

 which dip (to emerge from them again) beneath the horizontal deposits ; 



* A valuable Paper by Dr, Bright, describing the natural section of the strata at the defile of 

 the Avon, and supplying the key to the geological structure of the entire district, was read at the 

 Meetings of the Geological Society in the year 1811, and published in their Transactions in the 

 year 1817. The Bristol Coal-basin has been well described by Mr. Townshend in his " Cha. 

 racter of Moses," published in 1813; and subsequently by Dr. Gilby in a Paper inserted in the 

 Philosophical Magazine. — In a Note appended to the Paper of Dr. Bright, Mr. Warburton has 

 made an important observation on the dolomitic conglomerate, regarding it as the equivalent of 

 the magnesian limestone of the North of England ; and in a Paper inserted in the same volume 

 of the Geological Transactions, Dr. Gilby has taken the same view of the subject. Some Notes 

 of the late Professor Tennant on the dolomite of the Mendips will be found annexed to this Me- 

 moir. The foregoing are the principal authorities from which we have derived advantage, in 

 drawing up our account of the " Bristol Coal.basin." 



