_Peat_.--Dry plant-tissue consists of about 50 per cent, of carbon,
44 per cent, of oxygen, with a little nitrogen, and 6 per cent. of
hydrogen. In a peat-bog, we find the upper part of the scale represented
above very well shown: plants are growing on the surface with the normal
composition of cellulose. The first stratum of peat consists of browned
and partially decomposed plant-tissue, which is found to have lost
perhaps 20 per cent. of the components of wood, and to have acquired an
increasing percentage of carbon. As we descend in the peat, it becomes
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
common character by which they may be distinguished from the more modern
coals; containing less water, less oxygen, and more carbon, and usually
exhibiting the property of coking, which is rare in coals of later date.
Though there is great diversity in the Carboniferous coals, and it would
be absurd to express their composition by a single formula, it may be
said that, over the whole world, these coals have characteristics, as
a group, by which they can be recognized, the result of the slow
decomposition of the tissue of plants which lived in the Carboniferous
age, and which have, by a broad and general change, approximated to
a certain phase in the spontaneous distillation of plant-tissue. An
experienced geologist will not fail to refer to their proper horizon
a group of coals of Carboniferous age any more than those of the
Cretaceous or Tertiary.
_Anthracite_--In the ages anterior to the Carboniferous, the quantity
of land vegetation was apparently not sufficient to form thick and
extensive beds of peat; but the remains of plant-tissue are contained
in all the older formations, though there only as anthracite or
graphite--the last two groups of residual products. Of these we have
examples in the beds of graphite in the Laurentian rocks of Canada,
and of anthracite of the lower Silurian strata of Upper Church and
Kilnaleck, Ireland.
From these facts it is apparent that the carbon series is graded
geologically, that is, by the lapse of time during which plant-tissue
has been subjected to this natural and spontaneous distillation. But we
have better evidence than this of the derivation of one from another
of the groups of residual products which have been enumerated. In many
localities, the coals and lignites of different ages have been exposed
to local influences--such as the outbursts of trap-rock, or the
metamorphism of mountain chains--which have hastened the distillation,
and out of known earlier groups have produced the last. For example,
trap outbursts have converted Tertiary lignites in Alaska into good
bituminous coals; on Queen Charlotte's Island, on Anthracite Creek, in
southwestern Colorado, and at the Placer Mountains, near Santa Fe,
New Mexico, Cretaceous lignites into anthracite; those from Queen
Charlotte's Island and southwestern Colorado are as bright, hard, and
valuable as any from Pennsylvania. At a little distance from the focus
of volcanic action, the Cretaceous coals of southwestern Colorado have
been made bituminous and coking, while at the Placer Mountains the same
stratum may be seen in its anthracitic and lignitic stages.
A still better series, illustrating the derivation of one form of carbon
solids from another, is furnished by the coals of Ohio, Pennsylvania,
and Rhode Island. These are of the same age; in Ohio, presenting the
normal composition and physical characters of bituminous coals, that
is, of plant tissue generally and uniformly descending the scale in
the lapse of time from the Carboniferous age to the present. In the
mountains of Pennsylvania the same coal beds, somewhat affected by the
metamorphism which all the rocks of the Alleghanies have shared, have
reached the stage of _semi-bituminous_ coals, where half the volatile
constituents have been driven off; again, in the anthracite basins of
eastern Pennsylvania, the distillation further effected has formed from
these coals _anthracite_, containing only from three to ten per cent. of
volatile matter; while in the focus of metamorphic action, at Newport,
Rhode Island, the Carboniferous coals have been changed to _graphitic
anthracite_, that is, are half anthracite and half graphite. Here,
traveling from west to east, a progressive change is noted, similar to
that which may be observed in making a vertical section of a peat bog,
or in comparing the coals of Tertiary, Mesozoic, and Carboniferous age,
only the latter is the continuation and natural sequence of the former
series of changes.
In the Laurentian rocks of Canada are large accumulations of
carbonaceous matter, all of which is graphite, and that which is
universally conceded to be derived from plant-tissue. The oxidation of
graphite is artificially difficult, and in nature's laboratory slow; but
it is inevitable, as we see in the decomposition of its outcrops and the
blanching of exposed surfaces of clouded marbles, where the coloring is
graphite. Thus the end is reached, and by observations in the field,
the origin and relationship of the different carbon solids derived from
organic tissue are demonstrated.
It only remains to be said, in regard to them, that all the changes
enumerated may be imitated artificially, and that the stages of
decomposition which we have designated by the names graphite,
anthracite, coal, lignite, are not necessary results of the
decomposition of plant-tissue. A fallen tree may slowly consume away,
and all its carbonaceous matter may be oxidized and dissipated without
exhibiting the phases of lignite, coal, etc.; and lignite and coal,
when exposed to air and moisture, are burned away to ashes in the same
manner, simply because in these cases complete oxidation of the carbon
takes place, particle by particle, and the mass is not affected as a
whole in such a way as to assume the intermediate stages referred to.
Chemical analysis, however, proves that the process is essentially the
same, although the physical results are different.