1. _Under-clay_; the lowermost stratum. This is a tough argillaceous earth or clay, which on drying becomes of a grey colour, and very friable; it is occasionally black, from an intermixture of carbonaceous matter. This bed almost invariably contains an abundance of _Stigmariæ_ (see Plates XXII. XXIII.), of considerable length, with their rootlets attached, and which extend in every direction through the clay (as shown in the figures 1, 2, 6, pp. 199, 201). These roots commonly lie parallel with the planes of the stratum, and nearer to the top than to the bottom. 2. _Coal._--A carbonized mass, in which the external forms of the plants and trees composing it are obliterated, but the internal structure, in many instances, remains. Large trunks, and stems, and leaves, are rarely found in it. 3. _The Roof_, or upper bed.--This consists of slaty clay, abounding in leaves, trunks and branches, fruit, &c.; it includes layers and nodules of ironstone, inclosing leaves, insects, Crustacea, &c. In some localities beds of fresh-water shells, in others of marine shells, are intercalated with the shale; finely laminated clay, micaceous sand, grit, and pebbles of limestone, sandstone, &c. are also often interstratified. The principal illustrative specimens of the leaves, fruit, &c. (as those in Plate XXX. to Plate XXXIV.) are found in this bed. Thus an uninterrupted series of strata, in which triple deposits of this kind are repeated, (often thirty or forty times, and through a thickness of several thousand feet,) constitutes the predominant character of the ancient coal formations wherever they have been explored. The difficulties attending a satisfactory solution of this problem, are fully stated in the last edition of my Wonders of Geology (Vol. ii. Lecture vii.), and to that work I must refer the reader for a more extended consideration of this highly interesting subject. [Footnote 104: See Medals of Creation, vol. ii. chap. i.; and Thoughts on a Pebble, pp. 20, 69.] VII. Coal.--The numerous fossil plants from the carboniferous strata that are figured in this work, render it necessary to put the general reader in possession of a concise view of the nature and mode of formation of those ancient accumulations of vegetable matter, which now constitute the beds of mineral fuel termed coal.