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



other on the south of the ridge, having its centre at Radford, and extending to 

 the hmestone of the Mendips. The northern trough is of much less extent 

 than the southern, including only the northern part of the northern denuded 

 coal-tract : the southern trough embraces as well the southern part of the 

 northern coal-tract as the whole of the central and southern. 



The principal and most permanent subdivisions of the coal-measures that we 

 have been able to trace in this basin are the following, beginning with the lowest. 



1. The Millstone-grit ; 2. the Lower coal-shale; 3. the Pennant grit ; 4. the 

 Upper coal-shale. There is also another coal-grit, occurring between Comp- 

 ton Dando and Pensford, whose relations are equivocal, but which occupies a 

 place perhaps above the upper coal-shale. 



I. The millstone -grit* agrees in position and seems to be of cotempoia- 

 neous origin with the millstone-grit of Derbyshire. It differs however from 

 the grit of Derbyshire and the North of England in being highly charged with 

 red oxide of iron, and so compact as often to assume the character of a close- 

 grained cherty quartz-rock. In these respects it agrees with the lowest mem- 

 ber of the Warwickshire coal-field between Atherstone and Nuneaton. 



The character of this grit is well exhibited in Brandon-hill above Bristol, 

 the rock at which place may be taken as the type of its general appearance in 

 the Bristol coal-basin. It has been described by Dr. Bright in the 4th volume of 

 the Geological Transactions, under the name of ferruginous sand and siliceous 

 ironstone ; but it may be doubted whether the author has sufficiently distin- 

 guished the millstone-grit of Brandon-hill from an overlying conglomerate 

 resting against its southern slope, and belonging to the new red sandstone. This 

 conglomerate is much more highly ferruginous than the millstone-grit itself 



Associated with the compact and cherty beds of this grit, there often occurs 

 a siliceous conglomerate containing large quartzose pebbles. This might be 

 applied, as it is in the Forest of Dean, to the fabrication of millstones. The 

 beds of grit are divided by way-bandsf, and often by thick seam.s, of red fer- 

 ruginous clay. The lower beds of grit contain impressions of the shells, the 

 Anomia producta for instance, which usually occur in the mountain limestone, 

 and also impressions of those vegetables which are commonly found in the coal- 

 measures. The alternating beds of grit, shale with coal-seams, and limestone, 

 observed at the junction of the millstone-grit with the mountaiji limestone, have 

 already been described under the title of the Upper limestone- shale. 



* The term ^^ millstone'' is applied by tlie miners in this district to thedolomitic conglomerate. 



t These waybands of clay render the highly inclined masses of grit, which skirt Brandon-hill, 

 a very insecure foundation for buildings, since after frosty seasons the upper strata are disposed, 

 if overloaded, to slide over the inclined planes presented by the loosened argillaceous beds. 



