COAL MEASURES. 191 



II. FOREST OF DEAN COAL-FIELD. 



This Coal-field extends from near Mitcheldean by Colcford to 

 near Lydney, in Gloucestershire. 



The Coal-measures attain a thickness of 2765 feet, and contain 

 fifteen seams, of which eight are of a thickness of two feet and 

 upwards. The thickest seam is about five feet. 



As remarked by Prof. Hull, this Coal field forms a more perfect 

 "basin" than any other in England. The uppermost strata com- 

 prise about 830 feet of sandstones and shales with thin coals, 

 beneath which are the principal seams worked, and these belong 

 to the Lower Coal-measures. 



There is a remarkable instance of what is called a ' horse ' in 

 this Coal-field, which appears to have been a channel cut amongst 

 a mass of vegetable matter, and afterwards filled with mud, during 

 the Coal-measure period. The Dean Forest stone is used for 

 building-purposes and grind-stones. 



Along the south-west side of this Coal-field, near Lydney, the 

 Millstone Grit and Carboniferous Limestone are overlapped un- 

 conformably by the Coal-measures.^ 



12. BRISTOL AND SOMERSET COAL-FIELDS. 



The principal portion of these Coal-fields extends from Cromhall 

 Heath near Tortworth in Gloucestershire, by Iron Acton, Brislington, 

 and Mangotsfield, between Bristol and Bath, to Radstock, and other 

 places north of the Mendip Hills. The Coal-measures are divided 

 as follows :^ — 



( a. Radstock, etc. 1 c j .. i 1 ^ i 



TT.j.^ C-- l/TT-tr^ j/^i( Sandstones, shales and coaL 



Upper iicries. — \b. 1* arringtonGournay and Coal- } r 



( pit Heath. ) ^°°° '^*^^- 



Middle Series. — Chiefly sandstone. 2000 feet (Pennant Grit, 970 feet). 



■ a. Kingswood, etc. \ Sandstones, si 



b. Bristol, Vobster, etc. j 2500 feet. 



Lower Series [ "" ^"^ing^wood, etc. \ Sandstones, shales, and coal. 



Three Coal-basins are exposed at the surface, that of Bristol and 

 Radstock (which is, however, much concealed by Secondary strata), 

 the little faulted tract of Clapton-in-Gordano, and that of Nailsea. 

 One other has been proved by Mr. C. Richardson, in the bed of 

 the Severn, and this may possibly be connected with the Clapton 

 Coal-field. The Severn Tunnel was excavated partly in Millstone 

 Grit, Coal-measures, and Triassic Rocks.^ While to the south of 



1 See Mushet, T.G.S. (2), i. 288; H. Maclauchlan, Ibid. v. 195 ; J. Buddie, 

 Ibid. vi. 215 ; De la Beche, Mem. Geol. Survey, vol. i. p. 203. 



^ Geology of East Somerset and the Bristol Coal-fields (Geol. Survey), 1876 

 (this contains a list of works by J. M'Murtrie and others) ; J. Anstie, The Coal- 

 fields of Gloucestershire and Somersetshire, 1873 ; E. Wethered, Midland 

 Naturalist, iv. 25, 59 ; H. E. Hippisley, G. Mag. 1S78, p. 345 ; H. Cossham, 

 E. Wethered and W. Saise, The Northern End of the Bristol Coalfield, Colliery 

 Gttardian, Sept. 17, 1875. See also Buckland and Conybeare, T.G.S. (2), i. 2IO. 



^ Evan D. Jones, P. Geol. Assoc, vii. 339. 



