[56] The immense thicknesss of some coal-beds, without any intercalations of earthy materials, seems to be inexplicable on any other supposition but that of accumulations of drift-wood and plants. In the Great Exhibition of 1851, there was exhibited, on the outside of 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. [57] Sir R. I. Murchison has treated this subject with great ability: see _Sil. Syst._ chap, xi., and the illustrative maps opposite, p. 152. [Sidenote: STRATIFICATION OF A COAL-FIELD.] 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 generally, 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 examination of the section of a coal-pit, is the uniform presence of a thick bed of clay beneath every layer of coal; but a still more extraordinary fact remains to be mentioned, namely, that a common plant of the coal strata, called Stigmaria, (hereafter described, see _Lign. 36, 38_,) invariably occurs, more or less abundantly, in this bed of under-day, although very rarely to be met with in the coal or shale above. This phenomenon, long since noticed by Martin, Macculloch, and other authors, but whose value was not duly estimated till the recent observations of Mr. Logan, (_Geol. Proc._ vol. iii. p. 275,) is also found to prevail throughout the Welsh coal formation, which is upwards of twelve thousand feet in thickness, and contains more than sixty beds of coal, and as many of clay with stigmariæ; the Appalachian coal-measures of the United States present the same characters.[58] To place this fact before the student in a clear point of view, I will describe one of the triple series of beds which compose a coal-field. [58] See Prof. Rogers, in the Proceedings of the American Geologists, p. 453; and Sir C. Lyell's Travels in America.