In argillaceous strata, as the Kimmeridge and Oxford clay, London clay, &c., the shells of Cephalopoda are oftentimes beautifully preserved; the chambers are frequently filled with the solid matrix, but in many instances these cavities are lined either with brilliant pyrites or spar. Stony or sparry casts of the cells or chambers, the shell having perished, are another common 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;[104] 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 are 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:--