more homogeneous and darker until at the bottom of the marsh ten or
twenty feet from the surface, we have a black, carbonaceous paste,
which, when dried, resembles some varieties of coal, and approaches them
in composition. It has lost half the substance of the original plant,
and shows a marked increase in the relative proportion of carbon.

_Lignite_.--Each inch in vertical thickness of the peat-bog represents a
phase in the progressive change from wood-tissue to lignite, using
this term with its common signification to indicate, not necessarily
carbonized ligneous tissue, but plant-tissue that belongs to a past
though modern geological age--i.e., Tertiary, Cretaceous, Jurassic, or
Triassic. These lignites or modern coals are only peat beds which have
been buried for a longer or shorter time under clay, sand, or solidified
rock, and have progressed farther or less far on the road to coal. As
with peats, so with lignites, we find that at different geological
levels they exhibit different stages of this distillation--the Tertiary
lignites being usually distinguished without difficulty by the presence
of a larger quantity of combined water and oxygen, and a less quantity
of carbon, than the Cretaceous coals, and these in turn differ in the
same respects from the Triassic.

All the coals of the Tertiary and Mesozoic ages are grouped under one
name; but it is evident that they are as different from each other as
the new and spongy from the old and well-rotted peat in the peat-bog.

_Coal_.--By mere convention, we call the peat which accumulated in the
Carboniferous age by the name of bituminous coal; and an examination
of the Carboniferous strata in different countries has shown that the
peat-beds formed in the Carboniferous age, though varying somewhat, like
others, with the kind of vegetation from which they were derived, have a