COAL 829 



rather by its position than by any very vide difference in its general character or 

 organic remains. According to the measurements of Mr. Mnshet, the total thickness 

 of the mountain limestone in the Gloucestershire coal-field is about 120 fathoms. 

 The zone of limestone belonging to this coal-basin is from a furlong to a mile in 

 breadth on the surface of the ground, according as the dip of the strata is more or 

 less rapid. The angle of dip on the northen and western border is often no more 

 than 10, but on the eastern it frequently amounts to 80. 



The aggregate thickness of the coal-measures amounts 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 Bristol coal-basin, of a red siliceous grit, alternating with 

 conglomerate, used for millstones ; and with clay, occasionally used 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 extensively for flag-stone, 

 ashlers, and fire-stone. 3. A bed of grit, 25 fathoms thick, quarried for hearth-stone, 

 separates the preceding series from the following, or the 4th, 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. It contains several thin seams of coal, from 

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

 consisting of 3 seams of coal alternating with shale. 7. This is covered with alter- 

 nate beds of grit and shale, whose aggregate thickness is about 100 fathoms, occupying 

 a tract in the centre of the basin about 4 miles long and 2 miles broad. The 

 sandstone No. 5 is probably the equivalent of the Pennant in the preceding figure. 

 The floor, or pavement, immediately under the coal-beds is, almost without exception, 

 a greyish-slate clay, which, when made into bricks, strongly resists the fire. This fire- 

 clay varies in thickness from a fraction of an inch to several fathoms. Clay-ironstone 

 is often disseminated through the shale. 



The above description holds generally correct for the great coal-fields of south- 

 western England, where we have coal-measures, millstone grit, and mountain limestone 

 in regular order, the latter being at the base of the system. As we proceed northward 

 to Yorkshire and Northumberland, however, the limestone begins to alternate with the 

 true coal-measures, the two deposits forming together a series of strata about 1,000 

 feet in thickness. To this mixed formation succeeds the great mass of genuine 

 mountain limestone. In Fifeshire, in Scotland, we observe a still great departure 

 from the type of the south of England, or a more complete intercalation of dense 

 masses of marine limestone, with sandstone and shales containing coal. 



At Brora, in Sutherlandshire, we have a coal-formation belonging to the lower 

 divisions of the Oolite period ; and in the nort-east of Yorkshire, we have a similar 

 formation. 



The Brora coal-field, to which of late much attention has been directed, is a remark- 

 able example of a coal-basin among the deeper secondary strata, but above the New 

 Eed Sandstone formation. The Kev. Dr. Buckland and Sir C. Lyell, after visiting it 

 in 1824, had expressed an opinion that the strata there were wholly iinconnected 

 with the proper coal-formation below the New Red Sandstone, and were in fact the 

 equivalent of the oolitic series ; an opinion fully confirmed by the subsequent re- 

 searches of Sir E. Murchison. (' Geol. Trans.' for 1827, p. 293.) The Brora coal- 

 field forms a part of those secondary deposits which range along the south-east coast 

 of Sutherlandshire, occupying a narrow tract of about 20 miles in length and 3 in its 

 greatest breadth. 



One stratum of the Brora coal-pit is a coal-shale, composed of a reed-like striated 

 plant of the natural order Equisetacea, which seems to have contributed largely 

 towards the formation of that variety of coal. From this coal-shale the next transi- 

 tion upwards is into a purer bituminous substance approaching to Jet, which 

 constitutes the great bed of coal. This is from 3 feet 3 inches to 3 feet 8 inches thick, 

 and is divided nearly in the middle by a thin layer of impure indurated shale charged 

 with pyrites, which, if not carefully excluded from the mass, sometimes occasions 

 spontaneous combustion upon exposure to the atmosphere; and so much indeed is 

 that mineral disseminated throughout the district, that the shales might be generally 

 termed ' pyritiferous.' Inattention on the part of the workmen, in 1817, in leaving 

 a large quantity of this pyritous matter to accumulate in the pit, occasioned a sponta- 

 neous combustion, which was extinguished only by excluding the air ; indeed, the 

 coal-pit was closed in and remained unworked for four years. The fires broke out 

 again in the pit in 1827. 



The purer part of the Brora coal resembles common pitcoal ; but its powder has 

 the red ferruginous tinge of pulverised lignites. It may bo considered one of the last 

 links between lignite and true coal, approaching very nearly in character to jet, 



