78 THE MEDALS OF CREATION, Cuap. V. 
of many inches, and the same structure be apparent. 
Rarely are any large trunks or branches observable in the 
coal; the appearance of many beds being that of a deposit 
of foliage, shed and accumulated in a forest, (as may be 
observable in existing pine-districts,) and consolidated by 
pressure, while undergoing that peculiar change by which 
vegetable matter is converted into a carbonaceous mass. 
In fine, a gradual transition may be traced from the 
peat-wood and submerged forests of modern times, in which 
leaves, fruits, and trunks of indigenous trees and plants are 
preserved, to those vast deposits of mineral coal, formed by 
the bituminization of the extinct Floras which flourished 
in the paleeozoic ages. 
The geological position of the ancient coal, the manner in 
which it is interstratified with layers of clay, shale, micaceous 
sandstone, grit, and ironstone—in some districts associated 
with beds of fresh-water shells (Si/. Syst. p. 84),—in others 
alternating with strata containing marine remains, —are fully 
treated of in Wond. pp. 729—733, and Sd. p. 525; and it 
is not within the scope of the present work to dwell in detail 
upon what may be termed the physical geology of the car- 
boniferous deposits. But a few observations on the pheno- 
mena presented by these accumulations of bituminized 
vegetables and their associated strata, are necessary to 
render the subsequent remarks on the habits and affinities 
of the plants composing the palzeozoic Flora intelligible to 
the general reader. 
While the essential conditions for the conversion of vege- 
table 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 
Y, 
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