22 PETRIFACTIONS AND THEIR TEACHINGS. CHAP. I. 



2cL Carbonized Vegetables : The other state in which ve- 

 getable substances are found is that of carbonization that 

 peculiar transmutation which dead vegetable structures un- 

 dergo when buried in the earth, and subjected to heat and 

 moisture : a specific fermentation or putrefaction (the bitu- 

 minous) then takes place, and either bog-wood, lignite, brown 

 coal, jet, or true mineral coal, containing combustible 

 oils, is the result, accordingly as the necessary conditions are 

 more or less perfectly fulfilled ; for the formation of coal ap- 

 pears to depend on the engulfing of large quantities of recent 

 vegetable substances beneath deposits of clay, mud, silt, and 

 sand, which shall exclude the air, and prevent the escape of 

 the gaseous elements, when released by decomposition from 

 their organic combination. Such has been the origin of the 

 immense accumulations of fossil fuel, or coal, in various 

 parts of the world ; and of the delicate fern-leaves and 

 other foliage, which appear as pellicles or films of carbona- 

 ceous matter adherent to the surfaces of the slabs of 

 slate and stone in the cases A, and B, before us : these are 

 the leaves of vegetables converted into carbon or charcoal ; 

 some of these leaves even retain their flexibility, and may 

 be removed from the surface, like the specimens in a hortus 

 siccus. 



COAL. But though the vegetable origin of all coal is un- 

 questionable, yet evidence of the original structure of the 

 plants or trees whence it was derived is not always attainable. 

 The most perfect coal seems to have undergone a complete 

 liquefaction, and if any portions of the vegetable tissues 

 remain, they appear as if imbedded in a bituminous mass. 

 The slaty coal generally preserves traces of cellular or vascu- 

 lar tissue, and the spiral vessels and dotted cells of coniferous 

 trees may often be detected by the microscope. In many 

 instances the cells are filled with an amber-coloured resinous 

 substance ; in others the organization is so well preserved, that 

 on the surface of a block of coal cracked by heat, the dotted 

 glands may be observed. Some beds of coal are wholly com- 

 posed of minute leaves or disintegrated foliage, and if a mass 

 recently extracted from the mine be split asunder, the ex- 

 posed surfaces will be found covered with delicate lamina? of 

 carbonized leaves and fibres matted together, and flake after 

 flake may be peeled off through a thickness of many inches. 



