CAKBONIFEROUS DEPOSITS. 181 



state in which vestiges of these animals occur. Sometimes the cast of each chamber is isolated, 

 so as to present a series from the innermost to the outermost cell. Sections of those casts, in 

 which the chambers are filled up with spar, constitute specimens of great beauty and interest. 

 The so-called snake-stones are, of course, mere casts of Ammonites ;' those of Whitby, from the 

 lias limestone, are well known to every collector ; the casts of a very large species are common in 

 the oolite, especially at Swindon, in Wiltshire, and in the neighbourhood of Bath. 



VI. The Carboniferous Deposits, or Coal Measures. — The various deposits of Coal have 

 manifestly been formed under different local circumstances. Some have been peat-bogs, to 

 which repeated additions have been made by successive subsidences of the land ; others have 

 been deposited at the bottom of lakes and rivers, and these ai-e associated with remains of fresh- 

 water shells and Crustacea; others have accumulated in the abyss of the ocean, having been 

 formed by the drifting and engulfing of immense rafts of trees and other vegetable matter, like 

 those of the Mississippi ; others in inland seas, the successive layers of vegetables having been 

 supplied by periodical land-floods. There can be no doubt that coal has been, and may be, 

 produced under all these conditions ; and at different periods, and in various localities, all these 

 causes may have been in operation. But the great series of ancient coal-formations present a 

 remarkable uniformity of character, not only throughout Europe, but also in America and other 

 parts of the world. A coal-field (as a group of strata of this kind is commonly termed), is 

 generally composed of a series of layers of coal, clay, shale, and sand, of variable thicknesses, 

 based on grit or limestone, abounding in marine shells and corals; and the most remarkable 

 phenomenon is the constant presence beneath every bed of coal, of a thick stratum of earthy clay, 

 and of a layer of shale or slaty clay above it. One of the series of triple deposits of which a coal- 

 field consists, presents therefore the following characters : — 



1. Under-day ; 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 Stigmarice (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 tlian 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 tipper 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 exnlored. 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. 



' See Medals of Creation, vol. ii • chap. i. ; and Thoughts on a Pebble, pp. 20, 69. 



