Coal, as we have said, is only the result of a partial decomposition of
the plants which covered the earth during a geological period of immense
duration. No one, now, has any doubt that this is its origin. In
coal-mines it is not unusual to find fragments of the very plants whose
trunks and leaves characterise the Coal-measures, or Carboniferous era.
Immense trunks of trees have also been met with in the middle of a seam
of coal. In the coal-mines of Treuil,[44] at St. Etienne, for instance,
vertical trunks of fossil trees, resembling bamboos or large Equiseta,
are not only mixed with the coal, but stand erect, traversing the
overlying beds of micaceous sandstone in the manner represented in the
engraving, which has been reproduced from a drawing by M. Ad. Brongniart
(Fig. 69).
[44] “Elements of Geology,” p. 480.
In England it is the same; entire trees are found lying across the
coal-beds. Sir Charles Lyell tells us[45] that in Parkfield Colliery,
South Staffordshire, there was discovered in 1854, upon a surface of
about a quarter of an acre, a bed of coal which has furnished as many as
seventy-three stumps of trees with their roots attached, some of the
former measuring more than eight feet in circumference; their roots
formed part of a seam of coal ten inches thick, resting on a layer of
clay two inches thick, under which was a second forest resting on a band
of coal from two to five feet thick. Underneath this, again, was a third
forest, with large stumps of _Lepidodendra_, _Calamites_, and other
trees.[46]
[45] Ibid, p. 479.
[46] Ibid, p. 479.
In the lofty cliffs of the South Joggins, in the Bay of Fundy, in Nova
Scotia, Sir Charles Lyell found in one portion of the coal-field 1,500
feet thick, as many as sixty-eight different surfaces, presenting
evident traces of as many old soils of forests, where the trunks of the
trees were still furnished with roots.[47]
[47] Ibid, p. 483.
We will endeavour to establish here the true geological origin of coal,
in order that no doubt may exist in the minds of our readers on a
subject of such importance. In order to explain the presence of coal in
the depths of the earth, there are only two possible hypotheses. This
vegetable débris may either result from the burying of plants brought
from afar and transported by river or maritime currents, forming immense
rafts, which may have grounded in different places and been covered
subsequently by sedimentary deposits; or the trees may have grown on the
spot where they perished, and where they are now found. Let us examine
each of these theories.
[Illustration: Fig. 69.--Treuil coal-mine, at St. Etienne.]
Can the coal-beds result from the transport by water, and burial
underground, of immense rafts formed of the trunks of trees? The
hypothesis has against it the enormous height which must be conceded to
the raft, in order to form coal-seams as thick as some of those which
are worked in our collieries. If we take into consideration the specific
gravity of wood, and the amount of carbon it contains, we find that the
coal-deposits can only be about seven-hundredths of the volume of the
original wood and other vegetable materials from which they are formed.
If we take into account, besides, the numerous voids necessarily arising
from the loose packing of the materials forming the supposed raft, as
compared with the compactness of coal, this may fairly be reduced to
five-hundredths. A bed of coal, for instance, sixteen feet thick, would
have required a raft 310 feet high for its formation. These
accumulations of wood could never have arranged themselves with
sufficient regularity to form those well-stratified coal-beds,
maintaining a uniform thickness over many miles, and that are seen in
most coal-fields to lie one above another in succession, separated by
beds of sandstone or shale. And even admitting the possibility of a slow
and gradual accumulation of vegetable débris, like that which reaches
the mouth of a river, would not the plants in that case be buried in
great quantities of mud and earth? Now, in most of our coal-beds the
proportion of earthy matter does not exceed fifteen per cent. of the
entire mass. If we bear in mind, finally, the remarkable parallelism
existing in the stratification of the coal-formation, and the state of
preservation in which the impressions of the most delicate vegetable
forms are discovered, it will, we think, be proved to demonstration,
that those coal-seams have been formed in perfect tranquillity. We are,
then, forced to the conclusion that coal results from the mineralisation
of plants which has taken place on the spot; that is to say, in the very
place where the plants lived and died.
It was suggested long ago by Bakewell, from the occurrence of the same
peculiar kind of fireclay under each bed of coal, that it was the soil
proper for the production of those plants from which coal has been
formed.[48]
[48] “Introduction to Geology,” by Robert Bakewell, 5th ed., p. 179.
1838.
It has, also, been pointed out by Sir William Logan, as the result of
his observations in the South Wales coal-field, and afterwards by Sir
Henry De la Beche, and subsequently confirmed by the observations of Sir
Charles Lyell in America, that not only in this country, but in the
coal-fields of Nova Scotia, the United States, &c., every layer of true
coal is co-extensive with and invariably underlaid by a marked stratum
of arenaceous clay of greater or less thickness, which, from its
position relatively to the coal has been long known to coal-miners,
among other terms, by the name of _under-clay_.
The clay-beds, “which vary in thickness from a few inches to more than
ten feet, are penetrated in all directions by a confused and tangled
collection of the roots and leaves, as they may be, of the _Stigmaria
ficoides_, these being frequently traceable to the main stem
(_Sigillaria_), which varies in diameter from about two inches to half a
foot. The main stems are noticed as occurring nearer the top than the
bottom of the bed, as usually of considerable length, the leaves or
roots radiating from them in a tortuous irregular course to considerable
distances, and as so mingled with the under-clay that it is not possible
to cut out a cubic foot of it which does not contain portions of the
plant.” (Logan “On the Characters of the Beds of Clay immediately below
the Coal-seams of South Wales,” Geol. Transactions, Second Series, vol.
vi., pp. 491-2. An account of these beds had previously been published
by Mr. Logan in the Annual Report of the Royal Institution of South
Wales for 1839.)
From the circumstance of the main stem of the Sigillaria, of which the
_Stigmaria ficoides_ have been traced to be merely a continuation, it
was inferred by the above-mentioned authors, and has subsequently been
generally recognised as probably the truth, that the roots found in the
underclay are merely those of the plant (_Sigillaria_), the stem of
which is met with in the overlying coal-beds--in fact, that the
_Stigmaria ficoides_ is only the root of the _Sigillaria_, and not a
distinct plant, as was once supposed to be the case.
This being granted, it is a natural inference to suppose that the
present indurated under-clay is only another condition of that soft,
silty soil, or of that finely levigated muddy sediment--most likely of
still and shallow water--in which the vegetation grew, the remains of
which were afterwards carbonised and converted into coal.[49]
[49] For the opinions respecting the _Stigmaria ficoides_, see a
Memoir on “The Formation of the Rocks in South Wales and
South-Western England,” by Sir Henry T. De la Beche, F.R.S., in
the “Memoirs of the Geological Survey of Great Britain,” vol. i.,
p. 149.
In order thoroughly to comprehend the phenomena of the transformation
into coal of the forests and of the herbaceous plants which filled the
marshes and swamps of the ancient world, there is another consideration
to be presented. During the coal-period, the terrestrial crust was
subjected to alternate movements of elevation and depression of the
internal liquid mass, under the impulse of the solar and lunar
attractions to which they would be subject, as our seas are now, giving
rise to a sort of subterranean tide, operating at intervals, more or
less widely apart, upon the weaker parts of the crust, and producing
considerable subsidences of the ground. It might, perhaps, happen that,
in consequence of a subsidence produced in such a manner, the vegetation
of the coal-period would be submerged, and the shrubs and plants which
covered the surface of the earth would finally become buried under
water. After this submergence new forests sprung up in the same place.
Owing to another submergence, the second forests were depressed in their
turn, and again covered by water. It is probably by a series of
repetitions of this double phenomenon--this submergence of whole regions
of forest, and the development upon the same site of new growths of
vegetation--that the enormous accumulations of semi-decomposed plants,
which constitute the Coal-measures, have been formed in a long series of
ages.
But, has coal been produced from the larger plants only--for example,
from the great forest-trees of the period, such as the Lepidodendra,
Sigillariæ, Calamites, and Sphenophylla? That is scarcely probable, for
many coal-deposits contain no vestiges of the great trees of the period,
but only of Ferns and other herbaceous plants of small size. It is,
therefore, presumable that the larger vegetation has been almost
unconnected with the formation of coal, or, at least, that it has played
a minor part in its production. In all probability there existed in the
coal-period, as at the present time, two distinct kinds of vegetation:
one formed of lofty forest-trees, growing on the higher grounds; the
other, herbaceous and aquatic plants, growing on marshy plains. It is
the latter kind of vegetation, probably, which has mostly furnished the
material for the coal; in the same way that marsh-plants have, during
historic times and up to the present day, supplied our existing peat,
which may be regarded as a sort of contemporaneous incipient coal.
To what modification has the vegetation of the ancient world been
subjected to attain that carbonised state, which constitutes coal? The
submerged plants would, at first, be a light, spongy mass, in all
respects resembling the peat-moss of our moors and marshes. While under
water, and afterwards, when covered with sediment, these vegetable
masses underwent a partial decomposition--a moist, putrefactive
fermentation, accompanied by the production of much carburetted hydrogen
and carbonic acid gas. In this way, the hydrogen escaping in the form of
carburetted hydrogen, and the oxygen in the form of carbonic acid gas,
the carbon became more concentrated, and coal was ultimately formed.
This emission of carburetted hydrogen gas would, probably, continue
after the peat-beds were buried beneath the strata which were deposited
and accumulated upon them. The mere weight and pressure of the
superincumbent mass, continued at an increasing ratio during a long
series of ages, have given to the coal its density and compact state.
The heat emanating from the interior of the globe would, also, exercise
a great influence upon the final result. It is to these two causes--that
is to say, to pressure and to the central heat--that we may attribute
the differences which exist in the mineral characters of various kinds
of coal. The inferior beds are _drier_ and more compact than the upper
ones; or less bituminous, because their mineralisation has been
completed under the influence of a higher temperature, and at the same
time under a greater pressure.
An experiment, attempted for the first time in 1833, at Sain-Bel,
afterwards repeated by M. Cagniard de la Tour, and completed at
Saint-Etienne by M. Baroulier in 1858, fully demonstrates the process by
which coal was formed. These gentlemen succeeded in producing a very
compact coal artificially, by subjecting wood and other vegetable
substances to the double influence of heat and pressure combined.
The apparatus employed for this experiment by M. Baroulier, at
Saint-Etienne, allowed the exposure of the strongly compressed vegetable
matter enveloped in moist clay, to the influence of a long-continued
temperature of from 200° to 300° Centigrade. This apparatus, without
being absolutely closed, offered obstacles to the escape of gases or
vapours in such a manner that the decomposition of the organic matters
took place in the medium saturated with moisture, and under a pressure
which prevented the escape of the elements of which it was composed. By
placing in these conditions the sawdust of various kinds of wood,
products were obtained which resembled in many respects, sometimes
brilliant shining coal, and at others a dull coal. These differences,
moreover, varied with the conditions of the experiment and the nature of
the wood employed; thus explaining the striped appearance of coal when
composed alternately of shining and dull veins.
When the stems and leaves of ferns are compressed between beds of clay
or pozzuolana, they are decomposed by the pressure only, and form on
these blocks a carbonaceous layer, and impressions bearing a close
resemblance to those which blocks of coal frequently exhibit. These
last-mentioned experiments, which were first made by Dr. Tyndall, leave
no room for doubt that coal has been formed from the plants of the
ancient world.