My dear town-dwelling readers, let me tell you now something of a
geological product well known, happily, to all dwellers in towns, and
of late years, thanks to railroad extension, to most dwellers in
country districts: I mean coal.
Coal, as of course you know, is commonly said to be composed of
vegetable matter, of the leaves and stems of ancient plants and
trees--a startling statement, and one which I do not wish you to take
entirely on trust. I shall therefore spend a few pages in showing
you how this fact--for fact it is--was discovered. It is a very good
example of reasoning from the known to the unknown. You will have a
right to say at first starting, "Coal is utterly different in look
from leaves and stems. The only property which they seem to have in
common is that they can both burn." True. But difference of mere
look may be only owing to a transformation, or series of
transformations. There are plenty in nature quite as great, and
greater. What can be more different in look, for instance, than a
green field of wheat and a basket of loaves at the baker's? And yet
there is, I trust, no doubt whatsoever that the bread has been once
green wheat, and that the green wheat has been transformed into
bread--making due allowance, of course, for the bone-dust, or gypsum,
or alum with which the worthy baker may have found it profitable to
adulterate his bread, in order to improve the digestion of Her
Majesty's subjects.
But you may say, "Yes, but we can see the wheat growing, flowering,
ripening, reaped, ground, kneaded, baked. We see, in the case of
bread, the processes of the transformation going on: but in the case
of coal we do not see the wood and leaves being actually transformed
into coal, or anything like it."
Now suppose we laid out the wheat on a table in a regular series,
such as you may see in many exhibitions of manufactures; beginning
with the wheat plant at one end, and ending with the loaf at the
other; and called in to look at them a savage who knew nothing of
agriculture and nothing of cookery--called in, as an extreme case,
the man in the moon, who certainly can know nothing of either; for as
there is neither air nor water round the moon, there can be nothing
to grow there, and therefore nothing to cook--and suppose we asked
him to study the series from end to end. Do you not think that the
man in the moon, if he were half as shrewd as Crofton Croker makes
him in his conversation with Daniel O'Rourke, would answer after due
meditation, "How the wheat plant got changed into the loaf I cannot
see from my experience in the moon: but that it has been changed,
and that the two are the same thing I do see, for I see all the
different stages of the change." And so I think you may say of the
wood and the coal.
The man in the moon would be quite reasonable in his conclusion; for
it is a law, a rule, and one which you will have to apply again and
again in the study of natural objects, that however different two
objects may look in some respects, yet if you can find a regular
series of gradations between them, with all shades of likeness, first
to one of them and then to the other, then you have a fair right to
suppose them to be only varieties of the same species, the same kind
of thing, and that, therefore, they have a common origin.
That sounds rather magniloquent. Let me give you a simple example.
Suppose you had come into Britain with Brute, the grandson of AEneas,
at that remote epoch when (as all archaeologists know who have duly
read Geoffrey of Monmouth and the Arthuric legends) Britain was
inhabited only by a few giants. Now if you had met giants with one
head, and also giants with seven heads, and no others, you would have
had a right to say, "There are two breeds of giants here, one-headed
and seven-headed." But if you had found, as Jack the Giant-Killer
(who belongs to the same old cycle of myths) appears to have found,
two-headed giants also, and three-headed, and giants, indeed, with
any reasonable number of heads, would you not have been justified in
saying, "They are all of the same breed, after all; only some are
more capitate, or heady, than others!"
I hope that you agree to that reasoning; for by it I think we arrive
most surely at a belief in the unity of the human race, and that the
Negro is actually a man and a brother.
If the only two types of men in the world were an extreme white type,
like the Norwegians, and an extreme black type, like the Negros, then
there would be fair ground for saying, "These two types have been
always distinct; they are different races, who have no common
origin." But if you found, as you will find, many types of man
showing endless gradations between the white man and the Negro, and
not only that, but endless gradations between them both and a third
type, whose extreme perhaps is the Chinese--endless gradations, I
say, showing every conceivable shade of resemblance or difference,
till you often cannot say to what type a given individual belongs;
and all of them, however different from each other, more like each
other than they are like any other creature upon earth; then you are
justified in saying, "All these are mere varieties of one kind.
However distinct they are now, they were probably like each other at
first, and therefore all probably had a common origin." That seems
to me sound reasoning, and advanced natural science is corroborating
it more and more daily.
Now apply the same reasoning to coal. You may find about the world--
you may see even in England alone--every gradation between coal and
growing forest. You may see the forest growing in its bed of
vegetable mould; you may see the forest dead and converted into peat,
with stems and roots in it; that, again, into sunken forests, like
those to be seen below high-water mark on many coasts of this island.
You find gradations between them and beds of lignite, or wood coal;
then gradations between lignite and common or bituminous coal; and
then gradations between common coal and culm, or anthracite, such as
is found in South Wales. Have you not a right to say, "These are all
but varieties of the same kind of thing--namely, vegetable matter?
They have a common origin--namely, woody fibre. And coal, or rather
culm, is the last link in a series of transformations from growing
vegetation?"
This is our first theory. Let us try to verify it, as scientific men
are in the habit of doing, by saying, If that be true, then something
else is likely to be true too.
If coal has all been vegetable soil, then it is likely that some of
it has not been quite converted into shapeless coal. It is likely
that there will be vegetable fibre still to be seen here and there;
perhaps leaves, perhaps even stems of trees, as in a peat bog. Let
us look for them.
You will not need to look far. The coal, and the sands and shales
which accompany the coal, are so full of plant-remains, that three
hundred species were known to Adolphe Brongniart as early as 1849,
and that number has largely increased since.
Now one point is specially noticeable about these plants of the coal;
namely, that they may at least have grown in swamps.
First, you will be interested if you study the coal flora, with the
abundance, beauty, and variety of the ferns. Now ferns in these
islands grow principally in rocky woods, because there, beside the
moisture, they get from decaying vegetable or decaying rock,
especially limestone, the carbonic acid which is their special food,
and which they do not get on our dry pastures, and still less in our
cultivated fields. But in these islands there are two noble species,
at least, which are true swamp-ferns; the Lastraea Thelypteris, which
of old filled the fens, but is now all but extinct; and the Osmunda,
or King-fern, which, as all know, will grow wherever it is damp
enough about the roots. In Hampshire, in Devon, and Cornwall, and in
the southwest of Ireland, the King-fern too is a true swamp fern.
But in the Tropics I have seen more than once noble tree-ferns
growing in wet savannahs at the sea-level, as freely as in the
mountain-woods; ferns with such a stem as some of the coal ferns had,
some fifteen feet in height, under which, as one rode on horseback,
one saw the blazing blue sky, as through a parasol of delicate lace,
as men might have long ages since have seen it, through the plumed
fronds of the ferns now buried in the coal, had there only been a man
then created to enjoy its beauty.
Next we find plants called by geologists Calamites. There is no
doubt now that they are of the same family as our Equiseta, or horse-
tails, a race which has, over most parts of the globe, dwindled down
now from twenty or thirty feet in height, as they were in the old
coal measures, to paltry little weeds. The tallest Equisetum in
England--the beautiful E. Telmateia--is seldom five feet high. But
they, too, are mostly mud and swamp plants; and so may the Calamites
have been.
The Lepidodendrons, again, are without doubt the splendid old
representatives of a family now dwindled down to such creeping things
as our club-mosses, or Lycopodiums. Now it is a certain fact, which
can be proved by the microscope, that a very great part of the best
coal is actually made up of millions of the minute seeds of club-
mosses, such as grow--a few of them, and those very small--on our
moors; a proof, surely, not only of the vast amount of the vegetation
in the coal-making age, but also of the vast time during which it
lasted. The Lepidodendra may have been fifty or sixty feet high.
There is not a Lycopodium in the world now, I believe, five feet
high. But the club-mosses are now, in these islands and elsewhere,
lovers of wet and peaty soils, and so may their huger prototypes have
been, in the old forests of the coal.
Of the Sigillariae we cannot say as much with certainty, for
botanists are not agreed as to what low order of flowerless plants
they belong. But that they rooted in clay beds there is proof, as
you will hear presently.
And as to the Conifers, or pine-like trees--the Dadoxylon, of which
the pith goes by the name of Sternbergia, and the uncertain tree
which furnishes in some coal-measures bushels of a seed connected
with that of the yew--we may suppose that they would find no more
difficulty in growing in swamps than the cypress, which forms so
large a portion of the vegetation in the swamps of the Southern
United States.
I have given you these hints, because you will naturally wish to know
what sort of a world it was in which all these strange plants grew
and turned into coal.
My answer is, that it was most probably just like the world in which
we are living now, with the one exception that the plants and animals
are different.
It was the fashion a few years since to explain the coal--like other
phenomena of geology--by some mere hypothesis of a state of things
quite unlike what we see now. We were brought up to believe that in
the Carboniferous, or coal-bearing era, the atmosphere was intensely
moist and hot, and overcharged with carbonic acid, which had been
poured out from the interior of the planet by volcanic eruptions, or
by some other convulsion. I forget most of it now: and really there
is no need to remember; for it is all, I verily believe, a dream--an
attempt to explain the unknown not by the known, but by the still
more unknown. You may find such theories lingering still in
sensational school-books, if you like to be unscientific. If you
like, on the other hand, to be scientific you will listen to those
who tell you that instead of there having been one unique
carboniferous epoch, with a peculiar coal-making climate, all epochs
are carboniferous if they get the chance; that coal is of every age,
from that of the Scotch and English beds, up to the present day. The
great coal-beds along the Rocky Mountains, for instance, are
tertiary--that is, later than the chalk. Coal is forming now, I
doubt not, in many places on the earth, and would form in many more,
if man did not interfere with the processes of wild nature, by
draining the fens, and embanking the rivers.
Let me by a few words prove this statement. They will give you,
beside, a fresh proof of Sir Charles Lyell's great geological rule--
that the best way to explain what we see in ancient rocks is to take
for granted, as long as we can do so fairly, that things were going
on then very much as they are going on now.
When it was first seen that coal had been once vegetable, the
question arose--How did all these huge masses of vegetable matter get
there? The Yorkshire and Derbyshire coal-fields, I hear, cover 700
or 800 square miles; the Lancashire about 200. How large the North
Wales and the Scotch fields are I cannot say. But doubtless a great
deal more coal than can be got at lies under the sea, especially in
the north of Wales. Coal probably exists over vast sheets of England
and France, buried so deeply under later rocks, that it cannot be
reached by mining. As an instance, a distinguished geologist has
long held that there are beds of coal under London itself, which
rise, owing to a peculiar disturbance of the strata, to within 1,000
or 1,200 feet of the surface, and that we or our children may yet see
coal-mines in the marshes of the Thames. And more, it is a provable
fact that only a portion of the coal measures is left. A great part
of Ireland must once have been covered with coal, which is now
destroyed. Indeed, it is likely that the coal now known of in Europe
and America is but a remnant of what has existed there in former
ages, and has been eaten away by the inroads of the sea.
Now whence did all that enormous mass of vegetable soil come? Off
some neighbouring land, was the first and most natural answer. It
was a rational one. It proceeded from the known to the unknown. It
was clear that these plants had grown on land; for they were land-
plants. It was clear that there must have been land close by, for
between the beds of coal, as you all know, the rock is principally
coarse sandstone, which could only have been laid down (as I have
explained to you already) in very shallow water.
It was natural, then, to suppose that these plants and trees had been
swept down by rivers into the sea, as the sands and muds which buried
them had been. And it was known that at the mouths of certain
rivers--the Mississippi, for instance--vast rafts of dead floating
trees accumulated; and that the bottoms of the rivers were often full
of snags, etc.; trees which had grounded, and stuck in the mud; and
why should not the coal have been formed in the same way?
Because--and this was a serious objection--then surely the coal would
be impure--mixed up with mud and sand, till it was not worth burning.
Instead of which, the coal is usually pure vegetable, parted sharply
from the sandstone which lies on it. The only other explanation was,
that the coal vegetation had grown in the very places where it was
found. But that seemed too strange to be true, till that great
geologist, Sir W. Logan--who has since done such good work in Canada-
-showed that every bed of coal had a bed of clay under it, and that
that clay always contained fossils called Stigmaria. Then it came
out that the Stigmaria in the under clay had long filaments attached
to them, while when found in the sandstones or shales, they had lost
their filaments, and seemed more or less rolled--in fact, that the
natural place of the Stigmaria was in the under clay. Then Mr.
Binney discovered a tree--a Sigillaria, standing upright in the coal-
measures with its roots attached. Those roots penetrated into the
under clay of the coal; and those roots were Stigmarias. That seems
to have settled the question. The Sigillarias, at least, had grown
where they were found, and the clay beneath the coal-beds was the
original soil on which they had grown. Just so, if you will look at
any peat bog you will find it bottomed by clay, which clay is pierced
everywhere by the roots of the moss forming the peat, or of the
trees, birches, alders, poplars, and willows, which grow in the bog.
So the proof seemed complete, that the coal had been formed out of
vegetation growing where it was buried. If any further proof for
that theory was needed, it would be found in this fact, most
ingeniously suggested by Mr. Boyd Dawkins. The resinous spores, or
seeds of the Lepidodendra make up--as said above--a great part of the
bituminous coal. Now those spores are so light, that if the coal had
been laid down by water, they would have floated on it, and have been
carried away; and therefore the bituminous coal must have been
formed, not under water, but on dry land.
I have dwelt at length on these further arguments, because they seem
to me as pretty a specimen as I can give my readers of that regular
and gradual induction, that common-sense regulated, by which
geological theories are worked out.
But how does this theory explain the perfect purity of the coal? I
think Sir C. Lyell answers that question fully in p. 383 of his
"Student's Elements of Geology." He tells us that the dense growths
of reeds and herbage which encompass the margins of forest-covered
swamps in the valley and delta of the Mississippi, in passing through
them, are filtered and made to clear themselves entirely before they
reach the areas in which vegetable matter may accumulate for
centuries, forming coal if the climate be favourable; and that in the
cypress-swamps of that region no sediment mingles with the vegetable
matter accumulated from the decay of trees and semi-aquatic plants;
so that when, in a very dry season, the swamp is set on fire, pits
are burnt into the ground many feet deep, or as far as the fire can
go down without reaching water, and scarcely any earthy residuum is
left; just as when the soil of the English fens catches fire, red-hot
holes are eaten down through pure peat till the water-bearing clay
below is reached. But the purity of the water in peaty lagoons is
observable elsewhere than in the delta of the Mississippi. What can
be more transparent than many a pool surrounded by quaking bogs,
fringed, as they are in Ireland, with a ring of white water-lilies,
which you dare not stoop to pick, lest the peat, bending inward,
slide you down into that clear dark gulf some twenty feet in depth,
bottomed and walled with yielding ooze, from which there is no
escape? Most transparent, likewise, is the water of the West Indian
swamps. Though it is of the colour of coffee, or rather of dark
beer, and so impregnated with gases that it produces fever or cholera
when drunk, yet it is--at least when it does not mingle with the salt
water--so clear, that one might see every marking on a boa-
constrictor or alligator, if he glided along the bottom under the
canoe.
But now comes the question--Even if all this be true, how were the
forests covered up in shale and sandstone, one after another?
By gradual sinking of the land, one would suppose.
If we find, as we may find in a hundred coal-pits, trees rooted as
they grew, with their trunks either standing up through the coal, and
through the sandstone above the coal; their bark often remaining as
coal while their inside is filled up with sandstone, has not our
common-sense a right to say--The land on which they grew sank below
the water-line; the trees were killed; and the mud and sand which
were brought down the streams enveloped their trunks? As for the
inside being full of sandstone, have we not all seen hollow trees?
Do we not all know that when a tree dies its wood decays first, its
bark last? It is so, especially in the Tropics. There one may see
huge dead trees with their bark seemingly sound, and their inside a
mere cavern with touchwood at the bottom; into which caverns one used
to peep with some caution. For though one might have found inside
only a pair of toucans, or parrots, or a whole party of jolly little
monkeys, one was quite as likely to find a poisonous snake four or
five feet long, whose bite would have very certainly prevented me
having the pleasure of writing this book.
Now is it not plain that if such trees as that sunk, their bark would
be turned into lignite, and at last into coal, while their insides
would be silted up with mud and sand? Thus a core or pillar of hard
sandstone would be formed, which might do to the collier of the
future what they are too apt to do now in the Newcastle and Bristol
collieries. For there, when the coal is worked out below, the
sandstone stems--"coal-pipes" as the colliers call them--in the roof
of the seam, having no branches, and nothing to hold them up but
their friable bark of coal, are but too apt to drop out suddenly,
killing or wounding the hapless men below.
Or again, if we find--as we very often find--as was found at
Parkfield Colliery, near Wolverhampton, in the year 1814--a quarter
of an acre of coal-seam filled. with stumps of trees as they grew,
their trunks broken off and lying in every direction, turned into
coal, and flattened, as coal-fossils so often are, by the weight of
the rock above--should we not have a right to say--These trees were
snapped off where they grew by some violent convulsion; by a storm,
or by a sudden inrush of water owing to a sudden sinking of the land,
or by the very earthquake shock itself which sank the land?
But what evidence have we of such sinkings? The plain fact that you
have coal-seam above coal-seam, each with its bed of under-clay; and
that therefore the land MUST have sunk ere the next bed of soil could
have been deposited, and the next forest have grown on it.
In one of the Rocky Mountain coal-fields there are more than thirty
seams of coal, each with its under-clay below it. What can that mean
but thirty or more subsidences of the land, and the peat of thirty or
more forests or peat-mosses, one above the other? And now if any
reader shall say, Subsidence? What is this quite new element which
you have brought into your argument? You told us that you would
reason from the known to the unknown. What do we know of subsidence?
You offered to explain the thing which had gone on once by that which
is going on now. Where is subsidence going on now upon the surface
of our planet? And where, too, upheaval, such as would bring us
these buried forests up again from under the sea-level, and make
them, like our British coal-field, dry land once more?
The answer is--Subsidence and elevation of the land are common now,
probably just as common as they were in any age of this planet's
history.
To give two instances, made now notorious by the writings of
geologists. As lately as 1819 a single earthquake shock in Cutch, at
the mouth of the Indus, sunk a tract of land larger than the Lake of
Geneva in some places to a depth of eighteen feet, and converted it
into an inland sea. The same shock raised, a few miles off, a
corresponding sheet of land some fifty miles in length, and in some
parts sixteen miles broad, ten feet above the level of the alluvial
plain, and left it to be named by the country-people the "Ullah
Bund," or bank of God, to distinguish it from the artificial banks in
the neighbourhood.
Again: in the valley of the Mississippi--a tract which is now, it
would seem, in much the same state as central England was while our
coal-fields were being laid down--the earthquakes of 1811-12 caused
large lakes to appear suddenly in many parts of the district, amid
the dense forests of cypress. One of these, the "Sunk Country," near
New Madrid, is between seventy and eighty miles in length, and thirty
miles in breadth, and throughout it, as late as 1846, "dead trees
were conspicuous, some erect in the water, others fallen, and strewed
in dense masses over the bottom, in the shallows, and near the
shore." I quote these words from Sir Charles Lyell's "Principles of
Geology" (11th edit.), vol. i. p. 453. And I cannot do better than
advise my readers, if they wish to know more of the way in which coal
was formed, to read what is said in that book concerning the Delta of
the Mississippi, and its strata of forests sunk where they grew, and
in some places upraised again, alternating with beds of clay and
sand, vegetable soil, recent sea-shells, and what not, forming, to a
depth of several hundred feet, just such a mass of beds as exists in
our own coal-fields at this day.
If, therefore, the reader wishes to picture to himself the scenery of
what is now central England, during the period when our coal was
being laid down, he has only, I believe, to transport himself in
fancy to any great alluvial delta, in a moist and warm climate,
favourable to the growth of vegetation. He has only to conceive
wooded marshes, at the mouth of great rivers, slowly sinking beneath
the sea; the forests in them killed by the water, and then covered up
by layers of sand, brought down from inland, till that new layer
became dry land, to carry a fresh crop of vegetation. He has thus
all that he needs to explain how coal-measures were formed. I myself
saw once a scene of that kind, which I should be sorry to forget; for
there was, as I conceived, coal, making, or getting ready to be made,
before my eyes: a sheet of swamp, sinking slowly into the sea; for
there stood trees, still rooted below high-water mark, and killed by
the waves; while inland huge trees stood dying, or dead, from the
water at their roots. But what a scene--a labyrinth of narrow
creeks, so narrow that a canoe could not pass up, haunted with
alligators and boa-constrictors, parrots and white herons, amid an
inextricable confusion of vegetable mud, roots of the alder-like
mangroves, and tangled creepers hanging from tree to tree; and
overhead huge fan-palms, delighting in the moisture, mingled with
still huger broad-leaved trees in every stage of decay. The drowned
vegetable soil of ages beneath me; above my head, for a hundred feet,
a mass of stems and boughs, and leaves and flowers, compared with
which the richest hothouse in England was poor and small. But if the
sinking process which was going on continued a few hundred years, all
that huge mass of wood and leaf would be sunk beneath the swamp, and
covered up in mud washed down from the mountains, and sand driven in
from the sea; to form a bed many feet thick, of what would be first
peat, then lignite, and last, it may be, coal, with the stems of
killed trees standing up out of it into the new mud and sand-beds
above it, just as the Sigillariae and other stems stand up in the
coal-beds both of Britain and of Nova Scotia; while over it a fresh
forest would grow up, to suffer the same fate--if the sinking process
went on--as that which had preceded it.
That was a sight not easily to be forgotten. But we need not have
gone so far from home, at least, a few hundred years ago, to see an
exactly similar one. The fens of Norfolk and Cambridgeshire, before
the rivers were embanked, the water pumped off, the forests felled,
and the reed-beds ploughed up, were exactly in the same state. The
vast deposits of peat between Cambridge and the sea, often filled
with timber-trees, either fallen or upright as they grew, and often
mixed with beds of sand or mud, brought down in floods, were formed
in exactly the same way; and if they had remained undrained, then
that slow sinking, which geologists say is going on over the whole
area of the Fens, would have brought them gradually, but surely,
below the sea-level, to be covered up by new forests, and converted
in due time into coal. And future geologists would have found--they
may find yet, if, which God forbid, England should become barbarous
and the trees be thrown out of cultivation--instead of fossil
Lepidodendra and Sigillariae, Calamites and ferns, fossil ashes and
oaks, alders and poplars, bulrushes and reeds. Almost the only
fossil fern would have been that tall and beautiful Lastraea
Thelypteris, once so abundant, now all but destroyed by drainage and
the plough.
We need not, therefore, fancy any extraordinary state of things on
this planet while our English coal was being formed. The climate of
the northern hemisphere--Britain, at least, and Nova Scotia--was
warmer than now, to judge from the abundance of ferns; and especially
of tree-ferns; but not so warm, to judge from the presence of
conifers (trees of the pine tribe), as the Tropics. Moreover, there
must have been, it seems to me, a great scarcity of animal-life.
Insects are found, beautifully preserved; a few reptiles, too, and
land-shells; but very few. And where are the traces of such a
swarming life as would be entombed were a tropic forest now sunk;
which is found entombed in many parts of our English fens? The only
explanation which I can offer is this--that the club-mosses, tree-
ferns, pines, and other low-ranked vegetation of the coal afforded
little or no food for animals, as the same families of plants do to
this day; and if creatures can get nothing to eat, they certainly
cannot multiply and replenish the earth. But, be that as it may, the
fact that coal is buried forest is not affected.
Meanwhile, the shape and arrangements of sea and land must have been
utterly different from what they are now. Where was that great land,
off which great rivers ran to deposit our coal-measures in their
deltas? It has been supposed, for good reasons, that north-western
France, Belgium, Holland, and Germany were then under the sea; that
Denmark and Norway were joined to Scotland by a continent, a tongue
of which ran across the centre of England, and into Ireland, dividing
the northern and southern coal-fields. But how far to the west and
north did that old continent stretch? Did it, as it almost certainly
did long ages afterwards, join Greenland and North America with
Scotland and Norway? Were the northern fields of Nova Scotia, which
are of the same geological age as our own, and contain the same
plants, laid down by rivers which ran off the same continent as ours?
Who can tell now? That old land, and all record of it, save what
these fragmentary coal-measures can give, are buried in the dark
abyss of countless ages; and we can only look back with awe, and
comfort ourselves with the thought--Let Time be ever so vast, yet
Time is not Eternity.
One word more. If my readers have granted that all for which I have
argued is probable, they will still have a right to ask for further
proof.
They will be justified in saying: "You say that coal is transformed
vegetable matter; but can you show us how the transformation takes
place? Is it possible according to known natural laws?"
The chemist must answer that. And he tells us that wood can become
lignite, or wood-coal, by parting with its oxygen, in the shape of
carbonic acid gas, or choke-damp; and then common or bituminous coal,
by parting with its hydrogen, chiefly in the form of carburetted
hydrogen--the gas with which we light our streets. That is about as
much as the unscientific reader need know. But it is a fresh
corroboration of the theory that coal has been once vegetable fibre,
for it shows how vegetable fibre can, by the laws of nature, become
coal. And it certainly helps us to believe that a thing has been
done, if we are shown that it can be done.
This fact explains, also, why in mines of wood-coal carbonic acid,
i.e. choke-damp, alone is given off. For in the wood-coal a great
deal of the hydrogen still remains. In mines of true coal, not only
is choke-damp given off, but that more terrible pest of the miners,
fire-damp, or explosive carburetted hydrogen and olefiant gases. Now
the occurrence of that fire-damp in mines proves that changes are
still going on in the coal: that it is getting rid of its hydrogen,
and so progressing toward the state of anthracite or culm--stone-coal
as it is sometimes called. In the Pennsylvanian coal-fields some of
the coal has actually done this, under the disturbing force of
earthquakes; for the coal, which is bituminous, like our common coal,
to the westward where the strata are horizontal, becomes gradually
anthracite as it is tossed and torn by the earthquake faults of the
Alleghany and Appalachian mountains.
And is a further transformation possible? Yes; and more than one.
If we conceive the anthracite cleared of all but its last atoms of
oxygen, hydrogen, and nitrogen, till it has become all but pure
carbon, it would become--as it has become in certain rocks of immense
antiquity, graphite--what we miscall black-lead. And, after that, it
might go through one transformation more, and that the most startling
of all. It would need only perfect purification and crystallisation
to become--a diamond; nothing less. We may consider the coal upon
the fire as the middle term of a series, of which the first is live
wood, and the last diamond; and indulge safely in the fancy that
every diamond in the world has probably, at some remote epoch, formed
part of a growing plant.
A strange transformation; which will look to us more strange, more
truly poetical, the more steadily we consider it.
The coal on the fire; the table at which I write--what are they made
of? Gas and sunbeams; with a small percentage of ash, or earthy
salts, which need hardly be taken into account.
Gas and sunbeams. Strange, but true.
The life of the growing plant--and what that life is who can tell?--
laid hold of the gases in the air and in the soil; of the carbonic
acid, the atmospheric air, the water--for that too is gas. It drank
them in through its rootlets: it breathed them in through its leaf-
pores, that it might distil them into sap, and bud, and leaf, and
wood. But it has to take in another element, without which the
distillation and the shaping could never have taken place. It had to
drink in the sunbeams--that mysterious and complex force which is for
ever pouring from the sun, and making itself partly palpable to our
senses as heat and light. So the life of the plant seized the
sunbeams, and absorbed them, buried them in itself--no longer as
light and heat, but as invisible chemical force, locked up for ages
in that woody fibre.