80 THE MEDALS OF CREATION. Cuap. V. 
terrestrial, lacustrine, and marine animals, may accordingly 
be found associated with it.* But though many coal-fields 
(or basins, as they are termed, because they occupy depres- 
sions) have evidently been produced by different, and local 
agencies, the sedimentary deposits and coal-beds comprised 
in the carboniferous formations, setting aside unimportant 
variations, present a remarkable uniformity of character in 
their nature and arrangement, not only throughout Great 
Britain and Europe, but in every other part of the known 
world. 
STRATIFICATION OF A COAL-FIELD.—The group of strata 
constituting a coal-field consists of an alternation of layers 
of coal and of clay, of variable thickness, resting, very gene- 
rally, on grit, or marine limestone abounding in shells, 
corals, and crinoidea. 
My late excellent friend, Mr. Bakewell, used to exemplify 
the manner in which the beds of coal are interstratified with 
layers of clay and shale, by the following apt illustration ; 
let. a series of mussel-shells be placed one within the other, 
and a layer of clay be interposed between each; the shells 
will represent the beds of coal, and the partitions of clay the 
earthy strata intercalated between the carboniferous layers ; 
now, if one side of the series of shells be raised to indicate 
the general rise of the strata.in that direction, and the whole 
be dislocated by partial cracks and fissures, the general 
arrangement and subsequent displacement of the beds will 
be represented. 
The principal feature which arrests attention on the 
the west end of the Crystal Palace, a section of the lowest bed of 
coal from Tividale Colliery in South Staffordshire, the total thickness 
of which was 29 feet, with no intermixture whatever of sediment, 
except some thin shaly partings: the entire mass was composed of 
carbonized vegetables, 
* Sir R. I. Murchison has treated this subject with great ability: 
see Sil, Syst. chap. xi., and the illustrative maps opposite, p. 152. 
