102 



COAL STRATA. 



-markable effect : after this period, metallic veins have been rarely 

 formed, for they seldom rise into the coal strata. The vegetable re- 

 mains that are in the coal strata, appear principally to belong to plants 

 that abound chiefly in tropical climates, as will be subsequently no- 

 ticed. In no country have coal-measures been more extensively 

 worked than in England, or the relations of the strata to the rocks 

 above or below them been more fully examined. 



Every coal district has its peculiar series of strata, unconnected 

 with any other : there is a general resemblance in the nature of the 

 different beds in each. A district, with its peculiar series of strata, 

 is called a coal-field. The foundation rock on which the coal-fields 

 of Derbyshire, Northumberland, Durham, Shropshire, and North 

 and South Wales immediately rest, is the mountain and transition 

 limestone, described in Chapter VII. In Nottinghamshire, York- 

 shire, and Lancashire, the foundation rock has not been sunk to, nor 

 does it rise to the surface ; but we have every reason to believe, that 

 it is formed by a continuation of the same limestone, though this is 

 by no means essential to a coal-field. In some parts of France, I 

 have observed the coal strata resting upon granite ; being separated 

 from it only by a thick bed of conglomerate. A general view of the 

 arrangement of the Derbyshire coal-field may be taken as affording 

 a type of the whole English coal-fields, with certain exceptions, 

 which will be noticed. 



The thick beds of mountain limestone (see Chap. VII.) which 

 form entire mountains, decline in height towards the eastern side of 

 the county, and are covered by the coal-measures. The lowest bed 

 of these measures, or, to speak more correctly, the bed which sepa- 

 rates the coal-measures from the Hmestone, partakes of a mixed 

 character, varying from soft argillaceous shale to hard sandstone ; 

 the prevailing color is a dark reddish or blackish brown. This bed 

 has been called limestone-shah : its total thickness varies from five 

 to six hundred feet, but in some situations is much less. 



The harder strata of which this great bed is composed, are sepa- 

 rated by soft beds that easily disintegrate and fall down ; they form 

 the exposed face of Mam Tor, or the shivering mountain, near Cas- 

 tleton. The peculiar circumstance which renders this bed remark- 

 able is, that though it contains chiefly vegetable remains, it contains 

 also occasional patches or limited strata of dark bituminous lime- 

 stone, with beds and nodules of ironstone, and thin seams of coal, 

 which, however interesting they may be to the geological enquirer, 

 are too inconsiderable to be worked. The next large bed, which is 

 in some situations from three to four hundred feet in thickness, is 

 composed chiefly of strata of hard siliceous sandstone, which is in 

 some places coarse, containing angular fragments of quartz; in other 

 parts it is a fine grained and very durable stone. Some of the strata 

 of this bed were formerly worked for millstones ; from which cir- 

 cumstance it received the name of Millstone Grit» It contains, as 



