§ 3. THE COAL MEASURES. 663 



or Old Red sandstone, which was formerly included in the 

 Carboniferous system, but is now regarded as a distinct 

 formation, on account of a peculiar type of organic remains 

 prevailing throughout. 



3. The Coal measures. — The bituminous substance 

 termed coal is simply vegetable matter altered by chemical 

 changes, which will hereafter be considered. It occurs in 

 beds that vary from a few inches to a fathom in thick- 

 ness, and are interposed between strata of shale, clay, 

 micaceous sandstone, limestone, and ironstone ; alternations 

 of this kind occupying circumscribed areas, are termed 

 coal-basins. Mr. Bakewell observes that the strata thus 

 disposed may be explained by a series of mussel- shells, 

 placed one within the other, and having layers of clay in- 

 terposed. If one side of the shell be raised to indicate the 

 general rise of the strata in that direction, and the whole 

 series be dislocated by partial cracks or fissures, the general 

 arrangement- of the beds and the displacements which they 

 have undergone, will be represented ; each shell being the 

 type of a bed of coal, and the partitions of clay, of the earthy 

 strata which separate the carboniferous layers. 



It is the association of iron ore with the limestone that 

 serves as a flux, and the fuel required for the reduction of 

 the ore into a metallic state, that has given rise to the 

 numerous iron-foundries which occupy the sites of our 

 principal fields. The usual characters of a Coal-field, as 

 a series of strata of this kind is termed, are shown in the 

 section of that of South Gloucestershire, {Lign. 118, ante, 

 p. 522.) Here we perceive that the Devonian, or Old 

 Red-sandstone, has been elevated into a position almost 

 vertical, and that the Mountain limestone, which lies 

 immediately upon it, partakes of the same inclination. 

 This is succeeded by conformable beds of Millstone- 

 grit, which are followed by alternations of coal and 

 grit ; the Trias, Lias, and Inferior Oolite, (3, 4, 5,) 



