300 Proceedings of Philosophical Societies. [April, 



lias and red ground between this place and Tortwoith, it now 

 becomes totally so from hence to the east extremity of Mendip, 

 with the exception of the three small distant spots at Lodming- 

 ton Comt, Wick Rocks, and Tracey Park, where a denudation 

 of the lias and red ground exposes to view the subjacent continua- 

 tions of the calcareous border of the coal basin in the direct line 

 between Sodbury and the east extremity of Mendip. The 

 remainder of the calcareous border is completely buried under 

 strata of oolite and lias, and the young red sandstone in the 

 interval between Tracey Park and Mells ; and immediately 

 within it, shafts are frequently suiik through horizontal beds of 

 lias and red ground to obtain coal, which is usually dipping at a 

 high angle, unconformably to the more recent beds that cover it. 



The importance of this district in demonstrating the relations 

 of the coal measures to the young red sandstone formation, 

 which have hitherto been so little understood, must be at once 

 obvious. 



It is probable that the whole area of the South Gloucester and 

 Somerset coal field was at one time entirely buried by beds of 

 lias and young red sandstone, and that where the coal measures 

 are now visible at the surface is in consequence of the removal 

 of the strata by denudation. 



A large proportion of the Somerset collieries are won by 

 shafts beginning in lias or red ground ; so also is that of Puckle- 

 church on the border of Kingswood coal field. 



The points within the limestone border, at which the coal 

 measures occupy the surface, and seem at first sight to consti- 

 tute distinct cosil fields, are five in number: as they are considered 

 to be laid open in consequence of the removal by water of their 

 horizontal coverings, they are described as so many denudations 

 made up of the central or Pensfold denudation; northern, or 

 Kingswood ditto ; southern, or Nettlebridge ditto ; and eastern, 

 or AVick and Newton St. Loo denudation. Of these, the north- 

 ern is the largest, its greatest breadth being four miles ; these 

 five masses of denuded strata occupy usually the lowest grounds ; 

 and the intervening hills are composed of horizontal strata of 

 young red sandstone, lias, and occai^ionally of oolite. The atten- 

 dant coal fields of Nailsea appear also to be exposed in conse- 

 quence of a similar denudation. 



The strata, whose aggregate composes the entire coal field of 

 South Gloucester and Somerset, are reducible to three leading 

 subdivisions. 



1. Lowest, or Brandon Hill Grit, being the same with the 

 Millstone Grit of Derbyshire; it is best displayed at Brandoa 

 Hill, near Bristol, and at the village of Clifton. 



2. Lower coal seems alternating with beds of grit and shale, 

 iu which the shale largely predominates. 



3. Upper coal seems alternating with a few beds of shale, and 

 thick beds of gritty freestone, or Pennant stone. All these coal 



