550 LECTURES ON GEOLOGY. 



IRowley to Sedgeley, and from Walsall to Wednesfield. Besides 

 these extensive interruptions to its continuity, there are many 

 faults or breaks in the strata, some of which are traceable to centres 

 of volcanic action at Rowley and Pouch Hill, near Walsall. This 

 coal formation consists of eleven beds of coal, separated by others 

 of clay and shale, containing ironstone. There are five above and 

 as many below the main, or ten-yard coal, which is formed by 

 thirteen different layers, some lying close together, others sepa- 

 rated by thin " partings" of slate clay. None of the beds above the 

 main coal are worth working, their united thickness amounting 

 only to nine feet 3 but those below it are of great value, being col- 

 lectively fourteen yards in thickness. The general dip of the strata 

 is south-easterly, and as the extent of the field, to the east, is not 

 known, it is possible that it may join the Warwickshire field, (the 

 dip of which is to the south-west,) and thus form a basin, whose 

 centre would be about Castle Bromwich, or Coleshill. 



The main coal crops out at Bilston, and is not found far to the 

 north of that town, from whence it is continued southerly to Dud- 

 ley, where it is elevated by the limestone on the north of the town, 

 and abuts against, if it does not under-lie, the basalt on the east. 

 South of Dudley it rests upon the limestone on one side, and upon 

 the basalt on the other, and goes on to Brierley Hill and Cradley, 

 -where it again crops out. It is continued easterly from Bilston to 

 Wednesbury and West Bromwich, where it sinks rapidly beneath 

 the new red sandstone. 



Of this bed of coal, the thickest in England, if not in the world, 

 so much is consumed by the iron furnaces, and neighbouring 

 manufactories, that its exhausted state, in some parts of the field, 

 has induced the miners to be less wasteful in their mode of work- 

 ing J and the slack, or small coal, is now pretty generally raised to 

 the surface, to supply the engine fires. This, with the pillars 

 supporting the roof of the mine, constituting together a third of 

 the whole, was formerly left in the pits, where, becoming ignited 

 by the decomposition of the iron pyrites, it has set fire to the 

 remainder of the bed, in several instances, thus forming what is 

 termed a pseudo-volcano. The action of the fire upon the clay and 

 shale converts them into porcelain, jasper, and vitreous scoriae, 

 which form good materials for roads. Much sulphur and sal 

 ammoniac is sublimed round the vents by which these fires come 

 to the surface. 



There are many different kinds of coal, distinguished by their 

 colour, lustre, fracture, &c. but viewed in reference to their chem- 

 ical composition, they may be divided into three species, according 

 to the proportions of bitumen they contain. The first yields about 

 forty per cent, of bitumen when distilled, this is the Newcastle coal. 

 The second, the slate coal of this district, contains only about 

 twenty per cent j and the Welch, or stone coal, is almost free from 

 bitumen. Dr. Maculloch considers coal, however, as forming a 

 link in the series of bitumens, which vary by the proportion their 

 hydrogen bears to their carbon, rather than as a mixture of bitumen 



