shaly coals and portions of coals, and in the roofs of the
seams. The most marked case I have yet met with is the shaly
coal referred to as containing _Sporangites_ in my paper
on the conditions of accumulation of coal (_Journal of the
Geological Society_, vol. xxii. pp. 115, 139, and 165). The
purer coals certainly consist principally of cubical tissues
with some true woody matter, and the spore-cases, &c.,
are chiefly in the coarse and shaly layers. This is my old
doctrine in my two papers in the _Journal of the Geological
Society_, and I see nothing to modify it. Your observations,
however, make it probable that the frequent _clear spots_ in
the cannels are spore-cases."
Dr. Dawson's results are the more remarkable, as the numerous
specimens of British coal, from various localities, which I have
examined, tell one tale as to the predominance of the spore and
sporangium element in their composition; and as it is exactly in the
finest and purest coals, such as the "Better-Bed" coal of Lowmoor,
that the spores and sporangia obviously constitute almost the entire
mass of the deposit.
Coal, such as that which has been described, is always found in
sheets, or "seams," varying from a fraction of an inch to many feet
in thickness, enclosed in the substance of the earth at very various
depths, between beds of rock of different kinds. As a rule, every seam
of coal rests upon a thicker, or thinner, bed of clay, which is known
as "under-clay." These alternations of beds of coal, clay, and rock
may be repeated many times, and are known as the "coal-measures;"
and in some regions, as in South Wales and in Nova Scotia, the
coal-measures attain a thickness of twelve or fourteen thousand
feet, and enclose eighty or a hundred seams of coal, each with its
under-clay, and separated from those above and below by beds of
sandstone and shale.
The position of the beds which constitute the coal-measures is
infinitely diverse. Sometimes they are tilted up vertically, sometimes
they are horizontal, sometimes curved into great basins; sometimes
they come to the surface, sometimes they are covered up by thousands
of feet of rock. But, whatever their present position, there is
abundant and conclusive evidence that every under-clay was once a
surface soil. Not only do carbonized root-fibres frequently abound in
these under-clays; but the stools of trees, the trunks of which are
broken off and confounded with the bed of coal, have been repeatedly
found passing into radiating roots, still embedded in the under-clay.
On many parts of the coast of England, what are commonly known as
"submarine forests" are to be seen at low water. They consist, for the
most part, of short stools of oak, beech, and fir trees, still fixed
by their long roots in the bed of blue clay in which they originally
grew. If one of these submarine forest beds should be gradually
depressed and covered up by new deposits, it would present just the
same characters as an under-clay of the coal, if the _Sigillaria_ and
_Lepidodendron_ of the ancient world were substituted for the oak, or
the beech, of our own times.
In a tropical forest, at the present day, the trunks of fallen trees,
and the stools of such trees as may have been broken by the violence
of storms, remain entire for but a short time. Contrary to what might
be expected, the dense wood of the tree decays, and suffers from the
ravages of insects, more swiftly than the bark. And the traveller,
setting his foot on a prostrate trunk, finds that it is a mere shell,
which breaks under his weight, and lands his foot amidst the insects,
or the reptiles, which have sought food or refuge within.
The trees of the coal forests present parallel conditions. When the
fallen trunks which have entered into the composition of the bed of
coal are identifiable, they are mere double shells of bark, flattened
together in consequence of the destruction of the woody core; and Sir
Charles Lyell and Principal Dawson discovered, in the hollow stools
of coal trees of Nova Scotia, the remains of snails, millipedes,
and salamander-like creatures, embedded in a deposit of a different
character from that which surrounded the exterior of the trees. Thus,
in endeavouring to comprehend the formation of a seam of coal, we must
try to picture to ourselves a thick forest, formed for the most part
of trees like gigantic club-mosses, mares-tails, and tree ferns, with
here and there some that had more resemblance to our existing yews and
fir-trees. We must suppose that, as the seasons rolled by, the plants
grew and developed their spores and seeds; that they shed these in
enormous quantities, which accumulated on the ground beneath; and
that, every now and then, they added a dead frond or leaf; or, at
longer intervals, a rotten branch, or a dead trunk, to the mass.
A certain proportion of the spores and seeds no doubt fulfilled their
obvious function, and, carried by the wind to unoccupied regions,
extended the limits of the forest; many might be washed away by rain
into streams, and be lost; but a large portion must have remained, to
accumulate like beech-mast, or acorns, beneath the trees of a modern
forest.
But, in this case, it may be asked, why does not our English coal
consist of stems and leaves to a much greater extent than it does?
What is the reason of the predominance of the spores and spore-cases
in it?
A ready answer to this question is afforded by the study of a living
full-grown club-moss. Shake it upon a piece of paper, and it emits a
cloud of fine dust, which falls over the paper, and is the well-known
Lycopodium powder. Now this powder used to be, and I believe still
is, employed for two objects, which seem at first sight to have no
particular connection with one another. It is, or was, employed in
making lightning, and in making pills. The coats of the spores contain
so much resinous matter, that a pinch of Lycopodium powder, thrown
through the flame of a candle, burns with an instantaneous flash,
which has long done duty for lightning on the stage. And the same
character makes it a capital coating for pills; for the resinous
powder prevents the drug from being wetted by the saliva, and thus
bars the nauseous flavour from the sensitive papillae of the tongue.
But this resinous matter, which lies in the walls of the spores and
sporangia, is a substance not easily altered by air and water,
and hence tends to preserve these bodies, just as the bituminized
cerecloth preserves an Egyptian mummy; while, on the other hand, the
merely woody stem and leaves tend to rot, as fast as the wood of the
mummy's coffin has rotted. Thus the mixed heap of spores, leaves,
and stems in the coal-forest would be persistently searched by the
long-continued action of air and rain; the leaves and stems would
gradually be reduced to little but their carbon, or, in other words,
to the condition of mineral charcoal in which we find them; while the
spores and sporangia remained as a comparatively unaltered and compact
residuum.
There is, indeed, tolerably clear evidence that the coal must, under
some circumstances, have been converted into a substance hard enough
to be rolled into pebbles, while it yet lay at the surface of the
earth; for in some seams of coal, the courses of rivulets, which must
have been living water, while the stratum in which their remains are
found was still at the surface, have been observed to contain rolled
pebbles of the very coal through which the stream has cut its way.
The structural facts are such as to leave no alternative but to adopt
the view of the origin of such coal as I have described, which has
just been stated; but, happily, the process is not without analogy at
the present day. I possess a specimen of what is called "white coal"
from Australia. It is an inflammable material, burning with a bright
flame, and having much the consistence and appearance of oat-cake,
which, I am informed, covers a considerable area. It consists, almost
entirely, of a compacted mass of spores and spore-cases. But the fine
particles of blown sand which are scattered through it, show that it
must have accumulated, subaërially, upon the surface of a soil covered
by a forest of cryptogamous plants, probably tree-ferns.
As regards this important point of the subaërial region of coal, I am
glad to find myself in entire accordance with Principal Dawson,
who bases his conclusions upon other, but no less forcible,
considerations. In a passage, which is the continuation of that
already cited, he writes:--
"(3) The microscopical structure and chemical composition of
the beds of cannel coal and earthy bitumen, and of the more
highly bituminous and carbonaeceous shale, show them to have
been of the nature of the fine vegetable mud which accumulates
in the ponds and shallow lakes of modern swamps. When such
fine vegetable sediment is mixed, as is often the case, with
clay, it becomes similar to the bituminous limestone and
calcareo-bituminous shales of the coal-measures. (4) A few of
the under-clays, which support beds of coal, are of the nature
of the vegetable mud above referred to; but the greater part
are argillo-arenaceous in composition, with little vegetable
matter, and bleached by the drainage from them of water
containing the products of vegetable decay. They are, in
short, loamy or clay soils, and must have been sufficiently
above water to admit of drainage. The absence of sulphurets,
and the occurrence of carbonate of iron in connection with
them, prove that, when they existed as soils, rain-water, and
not sea-water, percolated them. (5) The coal and the fossil
forests present many evidences of subaërial conditions. Most
of the erect and prostrate trees had become hollow shells of
bark before they were finally embedded, and their wood had
broken into cubical pieces of mineral charcoal. Land-snails
and galley-worms _Xylobius_ crept into them, and they became
dens, or traps, for reptiles. Large quantities of mineral
charcoal occur on the surface of all the large beds of
coal. None of these appearances could have been produced by
subaqueous action. (6) Though the roots of the _Sigillaria_
bear more resemblance to the rhizomes of certain aquatic
plants; yet, structurally, they are absolutely identical with
the roots of Cycads, which the stems also resemble. Further,
the _Sigillariae_ grew on the same soils which supported
Conifers, _Lepidodendra, Cordaites_, and Ferns--plants which
could not have grown in water. Again, with the exception
perhaps of some _Pinnulariae_ and _Asterophyllites_, there
is a remarkable absence from the coal measures of any form of
properly aquatic vegetation. (7) The occurrence of marine, or
brackish-water animals, in the roofs of coal-beds, or even
in the coal itself, affords no evidence of subaqueous
accumulation, since the same thing occurs in the case of
modern submarine forests. For these and other reasons, some of
which are more fully stated in the papers already referred
to, while I admit that the areas of coal accumulation were
frequently submerged, I must maintain that the true coal is a
subaërial accumulation by vegetable growth on soils, wet and
swampy it is true, but not submerged."
I am almost disposed to doubt whether it is necessary to make the
concession of "wet and swampy;" otherwise, there is nothing that I
know of to be said against this excellent conspectus of the reasons
for believing in the subaërial origin of coal.
But the coal accumulated upon the area covered by one of the great
forests of the carboniferous epoch would, in course of time, have
been wasted away by the small, but constant, wear and tear of rain and
streams, had the land which supported it remained at the same level,
or been gradually raised to a greater elevation. And, no doubt, as
much coal as now exists has been destroyed, after its formation, in
this way. What are now known as coal districts owe their importance to
the fact that they were areas of slow depression, during a greater or
less portion of the carboniferous epoch; and that, in virtue of this
circumstance, Mother Earth was enabled to cover up her vegetable
treasures, and preserve them from destruction.
Wherever a coal-field now exists, there must formerly have been free
access for a great river, or for a shallow sea, bearing sediment in
the shape of sand and mud. When the coal-forest area became slowly
depressed, the waters must have spread over it, and have deposited
their burden upon the surface of the bed of coal, in the form of
layers, which are now converted into shale, or sandstone. Then
followed a period of rest, in which the superincumbent shallow waters
became completely filled up, and finally replaced, by fine mud, which
settled down into a new under-clay, and furnished the soil for a fresh
forest growth. This flourished, and heaped up its spores and wood into
coal, until the stage of slow depression recommenced. And, in some
localities, as I have mentioned, the process was repeated until the
first of the alternating beds had sunk to near three miles below its
original level at the surface of the earth.
In reflecting on the statement, thus briefly made, of the main facts
connected with the origin of the coal formed during the carboniferous
epoch, two or three considerations suggest themselves.
In the first place, the great phantom of geological time rises before
the student of this, as of all other, fragments of the history of our
earth--springing irrepressibly out of the facts, like the Djin from
the jar which the fisherman so incautiously opened; and like the Djin
again, being vaporous, shifting, and indefinable, but unmistakably
gigantic. However modest the bases of one's calculation may be,
the minimum of time assignable to the coal period remains something
stupendous.
Principal Dawson is the last person likely to be guilty of
exaggeration in this matter, and it will be well to consider what he
has to say about it:--
"The rate of accumulation of coal was very slow. The climate
of the period, in the northern temperate zone, was of such
a character that the true conifers show rings of growth, not
larger, nor much less distinct, than those of many of their
modern congeners. The _Sigillariae_ and _Calamites_ were not,
as often supposed, composed wholly, or even principally, of
lax and soft tissues, or necessarily short-lived. The former
had, it is true, a very thick inner bark; but their dense
woody axis, their thick and nearly imperishable outer bark,
and their scanty and rigid foliage, would indicate no very
rapid growth or decay. In the case of the _Sigillariae_, the
variations in the leaf-scars in different parts of the trunk,
the intercalation of new ridges at the surface representing
that of new woody wedges in the axis, the transverse marks
left by the stages of upward growth, all indicate that several
years must have been required for the growth of stems of
moderate size. The enormous roots of these trees, and the
condition of the coal-swamps, must have exempted them from the
danger of being overthrown by violence. They probably fell in
successive generations from natural decay; and making every
allowance for other materials, we may safely assert that every
foot of thickness of pure bituminous coal implies the
quiet growth and fall of at least fifty generations of
_Sigillariae_, and therefore an undisturbed condition of
forest growth enduring through many centuries. Further, there
is evidence that an immense amount of loose parenchymatous
tissue, and even of wood, perished by decay, and we do not
know to what extent even the most durable tissues may have
disappeared in this way; so that, in many coal-seams, we may
have only a very small part of the vegetable matter produced."
Undoubtedly the force of these reflections is not diminished when the
bituminous coal, as in Britain, consists of accumulated spores and
spore-cases, rather than of stems. But, suppose we adopt Principal
Dawson's assumption, that one foot of coal represents fifty
generations of coal plants; and, further, make the moderate
supposition that each generation of coal plants took ten years to come
to maturity--then, each foot-thickness of coal represents five hundred
years. The superimposed beds of coal in one coal-field may amount to
a thickness of fifty or sixty feet, and therefore the coal alone, in
that field, represents 500 x 50 = 25,000 years. But the actual coal is
but an insignificant portion of the total deposit, which, as has been
seen, may amount to between two and three miles of vertical thickness.
Suppose it be 12,000 feet--which is 240 times the thickness of the
actual coal--is there any reason why we should believe it may not have
taken 240 times as long to form? I know of none. But, in this case,
the time which the coal-field represents would be 25,000 x 240
=6,000,000 years. As affording a definite chronology, of course such
calculations as these are of no value; but they have much use in
fixing one's attention upon a possible minimum. A man may be puzzled
if he is asked how long Rome took a-building; but he is proverbially
safe if he affirms it not to have been built in a day; and our
geological calculations are all, at present, pretty much on that
footing.
A second consideration which the study of the coal brings prominently
before the mind of anyone who is familiar with palaeontology is, that
the coal Flora, viewed in relation to the enormous period of time
which it lasted, and to the still vaster period which has elapsed
since it flourished, underwent little change while it endured, and in
its peculiar characters, differs strangely little from that which at
present exists.
The same species of plants are to be met with throughout the whole
thickness of a coal-field, and the youngest are not sensibly different
from the oldest. But more than this. Notwithstanding that the
carboniferous period is separated from us by more than the whole time
represented by the secondary and tertiary formations, the great types
of vegetation were as distinct then as now. The structure of the
modern club-moss furnishes a complete explanation of the fossil
remains of the _Lepidodendra_, and the fronds of some of the ancient
ferns are hard to distinguish from existing ones. At the same time,
it must be remembered, that there is nowhere in the world, at present,
any _forest_ which bears more than a rough analogy with a coal-forest.
The types may remain, but the details of their form, their relative
proportions, their associates, are all altered. And the tree-fern
forest of Tasmania, or New Zealand, gives one only a faint and remote
image of the vegetation of the ancient world.
Once more, an invariably-recurring lesson of geological history,
at whatever point its study is taken up: the lesson of the almost
infinite slowness of the modification of living forms. The lines of
the pedigrees of living things break off almost before they begin to
converge.
Finally, yet another curious consideration. Let us suppose that one of
the stupid, salamander-like Labyrinthodonts, which pottered, with
much belly and little leg, like Falstaff in his old age, among the
coal-forests, could have had thinking power enough in his small brain
to reflect upon the showers of spores which kept on falling through
years and centuries, while perhaps not one in ten million fulfilled
its apparent purpose, and reproduced the organism which gave it birth:
surely he might have been excused for moralizing upon the thoughtless
and wanton extravagance which Nature displayed in her operations.
But we have the advantage over our shovel-headed predecessor--or
possibly ancestor--and can perceive that a certain vein of thrift runs
through this apparent prodigality. Nature is never in a hurry, and
seems to have had always before her eyes the adage, "Keep a thing long
enough, and you will find a use for it." She has kept her beds of coal
many millions of years without being able to find much use for them;
she has sent them down beneath the sea, and the sea-beasts could make
nothing of them; she has raised them up into dry land, and laid the
black veins bare, and still, for ages and ages, there was no living
thing on the face of the earth that could see any sort of value in
them; and it was only the other day, so to speak, that she turned a
new creature out of her workshop, who by degrees acquired sufficient
wits to make a fire, and then to discover that the black rock would
burn.
I suppose that nineteen hundred years ago, when Julius Caesar was good
enough to deal with Britain as we have dealt with New Zealand, the
primaeval Briton, blue with cold and woad, may have known that the
strange black stone, of which he found lumps here and there in his
wanderings, would burn, and so help to warm his body and cook his
food. Saxon, Dane, and Norman swarmed into the land. The English
people grew into a powerful nation, and Nature still waited for a full
return of the capital she had invested in the ancient club-mosses. The
eighteenth century arrived, and with it James Watt. The brain of that
man was the spore out of which was developed the steam-engine, and all
the prodigious trees and branches of modern industry which have grown
out of this. But coal is as much an essential condition of this growth
and development as carbonic acid is for that of a club-moss. Wanting
coal, we could not have smelted the iron needed to make our engines,
nor have worked our engines when we had got them. But take away the
engines, and the great towns of Yorkshire and Lancashire vanish like a
dream. Manufactures give place to agriculture and pasture, and not ten
men can live where now ten thousand are amply supported.
Thus, all this abundant wealth of money and of vivid life is Nature's
interest upon her investment in club-mosses, and the like, so long
ago. But what becomes of the coal which is burnt in yielding this
interest? Heat comes out of it, light comes out of it, and if we could
gather together all that goes up the chimney; and all that remains in
the grate of a thoroughly-burnt coal-fire, we should find ourselves in
possession of a quantity of carbonic acid, water, ammonia, and mineral
matters, exactly equal in weight to the coal. But these are the very
matters with which Nature supplied the club-mosses which made the
coal. She is paid back principal and interest at the same time; and
she straightway invests the carbonic acid, the water, and the ammonia
in new forms of life, feeding with them the plants that now live.
Thrifty Nature! Surely no prodigal, but most notable of housekeepers!
VI.
ON CORAL AND CORAL REEFS.
The marine productions which are commonly known by the names of
"Corals" and "Corallines," were thought by the ancients to be
sea-weeds, which had the singular property of becoming hard and
solid, when they were fished up from their native depths and came into
contact with the air.
"Sic et curalium, quo primum contigit auras Tempore durescit:
mollis fuit herba sub undis,"
says Ovid (Metam. xv.); and it was not until the seventeenth century
that Boccone was emboldened, by personal experience of the facts, to
declare that the holders of this belief were no better than "idiots,"
who had been misled by the softness of the outer coat of the living
red coral to imagine that it was soft all through.
Messer Boccone's strong epithet is probably undeserved, as the
notion he controverts, in all likelihood, arose merely from the
misinterpretation of the strictly true statement which any coral
fisherman would make to a curious inquirer; namely, that the outside
coat of the red coral is quite soft when it is taken out of the sea.
At any rate, he did good service by eliminating this much error from
the current notions about coral. But the belief that corals are plants
remained, not only in the popular, but in the scientific mind; and
it received what appeared to be a striking confirmation from the
researches of Marsigli in 1706. For this naturalist, having the
opportunity of observing freshly-taken red coral, saw that its
branches were beset with what looked like delicate and beautiful
flowers, each having eight petals. It was true that these "flowers"
could protrude and retract themselves, but their motions were hardly
more extensive, or more varied, than those of the leaves of the
sensitive plant; and therefore they could not be held to militate
against the conclusion so strongly suggested by their form and their
grouping upon the branches of a tree-like structure.