South-western Coal District of England. 287 



of Caldecot Level, and on the south-east it dips away gradually from the old 

 red sandstone of Penca-mawr, and sinks under the overlying formations, finally 

 disappearing beneath the bed of the Severn, between the New Passage-house 

 and themouth of the Wye. 



5. Coal-measures. 



The aggregate thickness of the coal-measures amounts, according to 

 Mr. Mushet, to about 500 fathoms. 1 . The lowest beds, which repose on the 

 mountain limestone, are about 40 fathoms thick, and consist here, as in the 

 basin of Bristol, of a red siliceous grit, alternating with conglomerate used 

 for mill-stones, and with clay occasionally worked for ochre. 2. These beds 

 are succeeded by a series about 120 fathoms thick, in which a grey gritstone 

 predominates, alternating in the lower part with shale, and containing 

 6 seams of coal. The grits are of a fissile character, and are quarried ex- 

 tensively for flag-stone, ashlers, and firestone. 3. A bed of grit, 25 fathoms 

 thick, quarried for hearth-stone, separates the preceding series from the 

 following, 4. which is about 115 fathoms thick, and consists of from 12 to 

 14 seams of coal alternating with shale. 5. To this succeeds a straw-coloured 

 sandstone, nearly 100 fathoms thick, forming a high ridge in the interior of 

 the basin, from Serridge-hill, behind Churchway Engine on the north, to 

 below Park-end on the south. It contains several thin seams of coal, from 

 6 to 16 inches in thickness. 6. On this reposes a series about 12 fathoms 

 thick, consisting of 3 seams of coal alternating with shale. 7. This is 

 covered by alternating beds of grit and shale, whose aggregate thickness is 

 about 100 fathoms. They occupy a tract in the centre of the basin, about 

 4 miles long and 2 miles broad, between the Ancient Hall called the Speech- 

 house, in which the courts held under the forest laws were formerly con- 

 vened, and Foxes' Bridge. It is difficult to estabhsh the identity of beds in 

 different coal-basins, even though contiguous. Mr. Mushet regards the grey 

 rock No. 2. as holding the place of the Pennant rock of the basin of Bristol : 

 we are inclined, however, to consider the sandstone No. 5. as the equivalent 

 of the Pennant; and to refer the three series, Nos. 2, 3, and 4, to the lower 

 coal-shale, and Nos. 6 and 7 to the upper coal-shale of that district. 



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