COAL. 79 
carbonized vegetables, like that observable when new-mown 
hay undergoes spontaneous combustion, from bituminous 
fermentation in the atmosphere (Wond. p. 701. Org. Rem. 
fp, 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 occu- 
pied 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;* and from drifted forests, like the 
rafts of the American rivers, transported into the sea, and 
engulfed in the abyss of the ocean;t and the remains of 
* Tn this coal-field are beds of limestone several feet thick, abound- 
ing in cyprides, fresh-water mollusks, &c.—*Stl. Syst. p. 84. 
+ The immense thicknesss of some coal-beds, without any inter- 
calations 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 
