While the essential conditions for the conversion of vegetable substances into coal appear to be the imbedding of large quantities of recent trees and plants in a deposit which shall exclude the air, and prevent the escape of the gaseous elements when released by decomposition from their organic combination, so, according to the more or less perfect manner in which these conditions are fulfilled, will result coal, jet, lignite, brown-coal, or peat-wood; or a mass of partially carbonized vegetables, like that observable when new-mown hay undergoes spontaneous combustion, from bituminous fermentation in the atmosphere (_Wond._ p. 701. _Org. Rem._ I. p. 181). The manner in which the carboniferous strata have been deposited, has been a subject of much discussion. Some contend that the coal-measures were originally in the state of peat-bogs, and that the successive layers were formed by the subsidences of forests which grew on the sites now occupied by their carbonized remains; others suppose that the vegetable matter originated from rafts, like those of the Mississippi, which floated out to sea, and became engulfed; while many affirm that the coal-measures were accumulated in inland seas or lakes, the successive beds of vegetable matter being supplied by periodical land-floods; and the supporters of each hypothesis bring numerous facts in corroboration of their respective opinions. There can, I think, be no doubt that the production of coal has taken place under each of these conditions, and that at different periods, and in various localities, all these causes have been in operation; in some instances singly, in others in combination. Coal may have been formed at the bottom of fresh-water lakes, as in those instances where it is associated with fresh-water shells and crustaceans, as at Burdie House (_Wond._ p. 693), and in some of the Derbyshire and Yorkshire deposits; in the beds of rivers and estuaries, as in the Wealden, and in the Shrewsbury coal-field;[55] and from drifted forests, like the rafts of the American rivers, transported into the sea, and engulfed in the abyss of the ocean;[56] and the remains of terrestrial, lacustrine, and marine animals, may accordingly be found associated with it.[BE] But though many coal-fields (or basins, as they are termed, because they occupy depressions) 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. [55] In this coal-field are beds of limestone several feet thick, abounding in cyprides, fresh-water mollusks, &c.--_Sil. Syst._ p, 84.