_THE GROWTH OF COAL._
My early boyhood was spent on the Coal formation rocks and in the
vicinity of collieries; and among my first natural history collections,
in a childish museum of many kinds of objects, were some impressions of
fern leaves from the shales of the coal series. It came to pass in this
way that the Carboniferous rocks were those which I first studied as an
embryo geologist, and much of my later work has consisted in collecting
and determining the plants of that ancient period, and in studying
microscopic sections of coals and fossil woods accompanying them. For
this reason, and because I have published so much on this subject, my
first decision was to leave it out of these Salient Points: but on
second thoughts it seemed that this might be regarded as a dereliction
of duty; more especially as some of the conclusions supposed to be the
best established on this subject have recently been called in question.
Had I been writing a few years ago, I might have referred to the mode
of formation of coal as one of the things most surely settled and
understood. The labours of many eminent geologists, microscopists and
chemists in the old and the new worlds had shown that coal nearly
always rests upon old soil-surfaces penetrated with roots, and that
coal beds have in their roofs erect trees, the remains of the last
forests that grew upon them. Logan and the writer have illustrated
this in the case of the series of more than eighty successive coal
beds exposed at the South Joggins, and of the great thirty feet seam
of the Picton coal series, whose innumerable laminæ have all been
subjected to careful scrutiny, and have shown unequivocal evidence of
land surfaces accompanying the deposition of the coal. Microscopical
examination has proved that these coals are composed of the materials
of the same trees whose roots are found in the underclays, and their
stems and leaves in the roof shales; that much of the material of the
coal has been partially subjected to subaërial decay at the time of its
accumulation; and that in this, ordinary coal differs from bituminous
shale, earthy bitumen and some kinds of cannel, which have been formed
under water; that the matter remaining as coal consists almost entirely
of epidermal tissues, which being suberose or corky in character are
highly carbonaceous, very durable and impermeable by water, and are,
hence, the best fitted for the production of pure coal; and finally,
that the vegetation and the climatal and geographical features of
the coal period were eminently fitted to produce in the vast swamps
of that period precisely the effects observed. All these points and
many others have been thoroughly worked out for both European and
American coal fields, and seemed to leave no doubt on the subject. But
several years ago certain microscopists observed in slices of coal,
thin layers full of spore cases, a not unusual circumstance, since
these were shed in vast abundance by the trees of the coal forests,
and because they contain suberose matter of the same character with
epidermal tissues generally. Immediately we were informed that all coal
consists of spores, and this being at once accepted by the unthinking,
the results of the labours of many years are thrown aside in favour of
this crude and partial theory. A little later, a German microscopist
has thought proper to describe coal as made up of minute algæ, and
tries to reconcile this view with the appearances, devising at the same
time a new and formidable nomenclature of generic and specific names,
which would seem largely to represent mere fragments of tissues.
Still later, some local facts in a French coal field have induced an
eminent observer of that country to revive the drift theory of coal,
in opposition to that of growth _in situ_. Views of this kind have
also recently been advanced in England by some of those younger men
who would earn distinction rather by overthrowing the work of their
seniors than by building on it. These writers base their conclusions
on a few exceptional facts, as the occasional occurrence of seams of
coal without distinct underlays, and the occurrence of clay partings
showing aquatic conditions in the substance of thick coals; and they
fail to discern the broader facts which these exceptions confirm. Let
us consider shortly the essential nature of coal, and some of the
conditions necessary to its formation.
A block of the useful mineral which is so important an element in
national wealth, and so essential to the comfort of our winter homes,
may tell us much as to its history if properly interrogated, and what
we cannot learn from it alone we may be taught by studying it in the
mine whence it is obtained, and in the cliffs and cuttings where the
edges of the coaly beds and their accompaniments are exposed.
Our block of coal, if anthracite, is almost pure carbon. If bituminous
coal, it contains also a certain amount of hydrogen, which in
combination with carbon enables it to yield gas and coal tar, and which
causes it to burn with flame. If, again, we examine some of the more
imperfect and more recent coals, the brown coals, so called, we shall
find that in composition and texture they are intermediate between
coal proper and hardened or compressed peat. Now such coaly rocks can,
under the present constitution of nature, be produced only in one way,
namely, by the accumulation of vegetable matter, for vegetation alone
has the power of decomposing the carbonic acid of the atmosphere, and
accumulating it as carbon. This we see in modern times in the vegetable
soil, in peaty beds, and in vegetable muck accumulated in ponds and
similar places. Such vegetable matter, once accumulated, requires only
pressure and the changes which come of its own slow putrefaction to be
converted into coal.
But in order that it may accumulate at all, certain conditions are
necessary. The first of these includes the climatal and organic
arrangements necessary for abundant vegetable growth. The second is the
facility for the preservation of the vegetable matter, without decay or
intermixture with earthy substances; and this, for a long time, till
a great thickness of it accumulates. The third is its covering up by
other deposits, so as to be compressed and excluded from air. It is
evident that when we have to consider the formation of a bed of coal
several feet in thickness, and spread, perhaps, over hundreds of square
miles, many things must conduce to such a result, and the wonder is
perhaps rather that such conditions should ever have been effectively
combined. Yet this has occurred at different periods of geological
history and in many places, and in some localities it has been so
repeated as to produce many beds of coal in succession.
Let us now question our block of coal as to its origin, supposing it
to be a piece of ordinary bituminous coal, or still better, a specimen
of one of the impure somewhat shaly coals which one sometimes finds
accidentally in the coal bin. In looking at the edge of our specimen
we observe that it has a "reed" or grain, which corresponds with the
lamination or bedding of the seam of coal from which it came. Looking
at this carefully, we shall see that there are many thin layers of
bright shining coal, and the more of these usually the better the
coal. These layers, in tracing them along, we observe often to thin
out and disappear. They are not very continuous. If our specimen
is an impure coal, we will find that it readily splits along the
surfaces of these layers, and that when so split, we can see that each
layer of shining coal has certain markings, perhaps the flattened
ribs and scars of Sigillaria or other coal-formation trees on its
surface. In other words, the layers of fine coal are usually flattened
trunks and branches of trees, or perhaps rather of the imperishable
and impermeable bark of such trees, the wood having perished. A few
very thin layers of shining coal we may also find to consist of the
large-ribbed leaves of the plant known as Cordaites. This kind of coaly
matter then usually represents trunks of trees which in a prostrate and
flattened state may constitute more than half of the bulk of ordinary
coal-formation coal. Under the microscope this variety of coal shows
little structure, and this usually the thickened cells of cortical
tissue. Intervening between these layers we perceive lamina?, more
or less thick and continuous, of what we may call dull coal, black
but not shining; resembling, in fact, the appearance of cannel coal.
If we split the coal along one side of these layers, and examine it
in a strong light, we may see shreds of leaf stalks and occasionally
even of fern leaves, or skeletons of these, showing the veins, and
many flattened disc-like bodies, spore cases and macrospores, shed by
the plants which make up the coal. These layers represent what may be
called compressed vegetable mould or muck, and this is by no means a
small constituent of many coals. This portion of the coal is the most
curious and interesting in microscopic slices, showing a great variety
of tissues and many spores and spore cases. Lastly, we find on the
surface of the coal, when split parallel to the bedding, a quantity
of soft shining fibrous material, known as mineral charcoal or mother
coal, which in some varieties of the mineral is very abundant, in
others much more rare. This is usually too soft and incoherent to be
polished in thin slices for the microscope; but if boiled for a length
of time in nitric acid, so as to separate all the mineral matter
contained in it, the fibres sometimes become beautifully translucent
and reveal the tissues of the wood of various kinds of Carboniferous
trees, more especially of Calamites, Cordaites and Sigillariæ. Fibres
of mineral charcoal prepared in this way are often very beautiful
microscopic objects under high powers; and this material of the coal
is nothing else than little blocks of rotten wood and fibrous bark,
broken up and scattered over the surface of the forming coal bed. All
these materials, it must be observed, have been so compressed that the
fragments of decayed wood have been flattened into films, the vegetable
mould consolidated into a stony mass, and trunks of great trees
converted by enormous pressure into laminæ of shining coal, a tenth of
an inch in thickness, so that the whole material has been reduced to
perhaps one-hundredth of its original volume.
Restoring the mass in imagination to its original state, what do we
find? A congeries of prostate trunks with their interstices filled with
vegetable muck or mould, and occasional surfaces where rotten wood,
disintegrated into fragments, was washed about in local floods or rain
storms, and thus thrown over the surface. Lyell seems very nearly to
have hit the mark when he regarded the conditions of the great dismal
swamp of Virginia as representing those of a nascent coal field. We
have only to realize in the coal period the existence of a dense
vegetation very different from that of modern Virginia, of a humid and
mild climate, and of a vast extension of low swampy plains, to restore
the exact conditions of the coal swamps.
But how does this correspond with the facts observed in mines and
sections? To the late Sir William Logan is due the merit of observing
that in South Wales the underclays or beds of indurated clay and earth
underlying the coal seams are usually filled with the long cylindrical
rootlets and branching roots of a curious plant, very common in the
coal formation, the Stigmaria. He afterwards showed that the same fact
occurs in the very numerous coal beds exposed in the fine section cut
by the tides of the Bay of Fundy, in the coal rocks of Nova Scotia. In
that district I have myself followed up his observations, examining
in detail every one of eighty-one Coal Groups, as I have called them,
each consisting of at least one bed of coal, large or small, with
its accompaniments, and in many cases of several small seams with
intervening clays or shales.[113] In nearly every case the Stigmaria
"underclay" is distinctly recognisable, and often in a single coal
group there are several small seams separated by underclays with roots
and rootlets. These underclays are veritable fossil soils; sometimes
bleached clays or sands, like the subsoils of modern swamps; sometimes
loamy or sandy, or of the nature of hardened vegetable mould. They
rarely contain any remains of aquatic animals, or of animals of any
kind, but are filled with stigmaria roots and rootlets, and sometimes
hold a few prostrate stems of trees.[114] While the underclay is thus
a fossil soil, the roof or bed above the coal, usually of a shaly
character, is full of remains of leaves and stems and fruits, and often
holds erect stumps, the remains of the last trees that grew in the
swamp before it was finally covered up.
[113] For details see _Journal Geol. Society of London_, 1865; and
"Acadian Geology," last edition, 1891.
[114] At the South Joggins, in two or three cases, beds of bituminous
shale full of Naiadites and Cyprids have by elevation and drying become
fit for the growth of trees with stigmaria roots; but this is quite
exceptional, no doubt arising from the accidental draining of lakes or
lagoons on their elevation above the sea level.
Some of the thinnest coals, and some beds so thin and impure that they
can scarcely be called coals at all, are the most instructive. Witness
the following from my section of the South Joggins.
Coal Group 1, of Division 3, is the highest of the series. Its section
is as follows:--
"Grey argillaceous shale.
Coal, 1 inch.
Grey argillaceous underclay, Stigmaria.
"The roof holds abundance of fern leaves (_Alethopteris lonchitica_).
The coal is coarse and earthy, with much epidermal and bast tissue,
spore cases, etc., vascular bundles of ferns and impressions of bark
of Sigillaria and leaves of Cordaites. It may be considered as a
compressed vegetable soil resting on a subsoil full of rootlets of
Stigmaria." In this case the coal is an inch in thickness, but there
are many beds where the coal is a mere film, and supports great
erect stems of Sigillaria, sending downward their roots in the form
of branching Stigmariæ into the underclay, thus proving that the
Stigmariæ of the underclays are the roots of the Sigillariæ of the
coals and their roofs.
Here is another example which may be called a coal group, and is No. 11
of the same division:
"Grey argillaceous shale, erect Calamites.
Coal, 1 inch.
Grey argillaceous underclay, Stigmaria, 1 ft. 6 in.
Coal, 2 inches.
Grey argillaceous underclay, Stigmaria, 4 in.
Coal, 1 inch.
Grey argillaceous underclay, Stigmaria.
"This is an alternation of thin, coarse coals with fossil soils. The
roof shale contains erect Calamites, which seem to have been the last
vegetation which grew on the surface of the upper coal."
Such facts, with many minor varieties, extend through the whole
eighty-one coal groups of this remarkable section, as any one may see
by referring to the paper and work cited in the preceding note. It
is possibly because in most coal fields the smaller and commercially
useless beds are so little open to observation, that so crude ideas
derived merely from imperfect access to the beds that are worked exist
among geologists. The following summary of facts may perhaps serve to
place the evidence as to the mode of accumulation of coal fairly before
the reader:--
(1) The occurrence of Stigmaria under nearly every bed of coal proves,
beyond question, that the material was accumulated by growth _in situ_,
while the character of the sediments intervening between the beds of
coal proves with equal certainty the abundant transport of mud and sand
by water. In other words, conditions similar to those of the swampy
deltas of great rivers, or the swampy flats of the interiors of great
continents, are implied.
(2) The true coal consists principally of the flattened bark of
sigillaroid and other trees, intermixed with leaves of ferns and
_Cordaites_, and other herbaceous _débris_, including vast numbers of
spores and spore cases, and with fragments of decayed wood constituting
"mineral charcoal," all their materials having manifestly alike grown
and accumulated where we find them.
(3) The microscopical structure and chemical composition of the beds
of cannel coal and earthy bitumen, and of the more highly bituminous
and carbonaceous shales, show them to have been of the nature of the
fine vegetable mud which accumulates in the ponds and shallow lakes
of modern swamps. These beds are always distinct from true subaërial
coal. When such fine vegetable sediment is mixed, as is often the
case, with mud, it becomes similar to the bituminous limestone and
calcareo-bituminous shales of the coal measures.
(4) A few of the underclays 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 in the
chemical condition in which we find such soils under modern bogs,
and must have been sufficiently above water to admit of drainage.
The absence, or small quantity of sulphides, 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 trees present many evidences of subaërial
conditions. Most of the erect and prostrate trees had become hollow
shells of bark before they were finally imbedded, 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 surfaces of
all the larger beds of coal. None of these appearances could have been
produced by subaqueous action.
(6) Though the roots of _Sigillaria_ bear some resemblance to the
rhizomes of certain aquatic plants, yet structurally they have much
resemblance to the roots of Cycads, which the stems also resemble.
Further, the _Sigillariæ_ 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
_Pinnulariæ_ and _Asterophyllites_, and Rhizocarpean spores, there is
a remarkable absence from the coal measures of any form of properly
aquatic vegetation.
(7) The occasional 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. Such facts merely imply that portions of the
areas of coal accumulation were liable to inundation of a character so
temporary as not finally to close the process, as happened when at last
a roof shale was deposited by water over the coal. Cannel coals and
bituminous shales holding mussel-like shells, fish scales, etc., imply
the existence sometimes for long periods of ponds, lakes or lagoons
in the coal swamps, but ordinary coal did not accumulate in these. It
is in the cannels and similar subaqueous coals that the macrospores
which I attribute in great part to aquatic plants, allied to modern
Salvinia, etc., are chiefly found.[115]
[115] "Geological History of Plants," _Bulletin Chicago Academy of
Sciences_, 1886.
For these and other reasons, some of which are more fully stated in the
papers 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 would add the further consideration,
already urged elsewhere, that in the case of the fossil forests
associated with the coal, the conditions of submergence and silting-up
which have preserved the trees as fossils, must have been precisely
those which were fatal to their existence as living plants, a fact
sufficiently evident to us in the case of modern submarine forests, but
often overlooked by the framers of theories of the accumulation of coal.
It seems strange that the occasional inequalities of the floors of
the coal beds, the sand or gravel ridges which traverse them, the
channels cut through the coal, the occurrence of patches of sand, and
the insertion of wedges of such material splitting the beds, have been
regarded by some able geologists as evidences of the aqueous origin of
coal. In truth, these appearances are of constant occurrence in modern
swamps and marshes, more especially near their margins, or where they
are exposed to the effects of ocean storms or river inundations. The
lamination of the coal has also been adduced as a proof of aqueous
deposition; but the microscope shows, as I have elsewhere pointed out,
that this is entirely different from aqueous lamination, and depends
on the superposition of successive generations of more or less decayed
trunks of trees and beds of leaves. The lamination in the truly aqueous
cannels and carbonaceous shales is of a very different character.
It is scarcely necessary to remark that in the above summary I have
had reference principally to my own observations in the coal formation
of Nova Scotia; but similar facts have been detailed by many other
observers in other districts.[116]
[116] Especially Brongniart, Goeppert, Hawkshaw, Lyell, Logan, De la
Boche, Beaumont, Binney, Rogers, Lesquereux, Williamson, Grand' Eury.
A curious point in connection with the origin of coal is the question
how could vegetable matter be accumulated in such a pure condition?
There is less difficulty in regard to this if we consider the coal
as a swamp accumulation _in situ_. It is in this way that the purest
vegetable accumulations take place at present, whereas in lakes and at
the mouths of rivers vegetable matter is always mixed up with mud. Coal
swamps, however, must have been liable to submergences or to temporary
inundations, and it is no doubt to these that we have to attribute
the partings of argillaceous matter often found in coal beds, as well
as the occasional gulches cut into the coal and filled with sand and
lenticular masses of earthy matter. To a similar cause we must also
attribute the association of cannel with ordinary coal. The cannel is
really a pulpy, macerate mass of vegetable matter accumulated in still
water, surrounded and perhaps filled with growing aquatic herbage.
Hence it is in such beds that we find the greatest accumulations of
macrospores, derived, probably, in great part from aquatic plants.
Buckland long ago compared the matter of cannel to the semi-fluid
discharge of a bursting bog, and Alex. Agassiz has more recently shown
that in times of flood the vegetable muck of the Everglades of Florida
flows out in thick inky streams, and may form large beds of vegetable
matter having the character of the materials of cannel. It is evident
that in swamps of so great extent as those of the coal formation, there
must have been shallow lakes and ponds, and wide sluggish streams,
forming areas for the accumulation of vegetable _débris_ and this
readily accounts for the association of ordinary beds of coal with
those of cannel, and with bituminous shales or earthy bitumen, as well
as for the occurrence of scales of fish and other aquatic animals in
such beds. Lyell's interesting observation of the submerged areas at
New Madrid, keeping free of Mississippi mud, because fringed with a
filter of cane-brake, shows that the areas of coal accumulation might
often be inundated without earthy deposit, if, as seems probable, they
were fringed with dense brakes of calamites, sheltering them from the
influx of muddy water. It seems also certain that the water of the coal
areas would be brown and laden with imperfect vegetable acids, like
that of modern bogs, and such water has usually little tendency to
deposit any mineral matter, even in the pores of vegetable fragments.
The only exception to this is one which also occurs in modern swamps,
namely, the tendency to deposit iron, either as carbonate (Clay
Ironstone), or sulphide (Iron Pyrite), both of which are products of
modern bogs, and equally characteristic of the coal swamps.
Where great accumulations of sediment are going on, as at the mouths
of modern rivers, there is a tendency to subsidence of the area of
the deposit, owing to its weight. This applies, perhaps, to a greater
extent to coal areas. Thus the area of a coal swamp would ultimately
sink so low as to be overflowed, and a roof shale would be deposited to
bury up the bed of coal, and transmit it to future ages, chemically,
and mechanically changed by pressure and by that slow decomposition
which gradually converts vegetable matter into carbon and hydrocarbons.
The long continuance and great extent of these alternations of growth
and subsidence is perhaps the most extraordinary fact of all. At the
South Joggins, if we include the surfaces having erect trees with those
having beds of coal, the process of growth of a forest or bog, and
its burial by subsidence and deposition must have been repeated about
a hundred times before the final burial of the whole under the thick
sandstones of the Upper Carboniferous and Permian.
Mention has been made of Sigillaria and other trees of the coal
formation period. These trees and others allied to them, of which there
were many kinds, may be likened to gigantic club mosses, which they
resembled in fruit and foliage, though vastly more complex in structure
of stem and branch. Some of them, perhaps, were of much higher rank
than any of the modern plants most nearly allied to them. One of their
most remarkable features was that of their roots--those Stigmariæ, to
which so frequent reference has been made. They differed from modern
roots, not only in some points of structure, but in their regular
bifurcation, and in having huge root fibres articulated to the roots,
and arranged in a regular spiral manner, like leaves. They radiate
regularly from a single stem, and do not seem to have sent up buds or
secondary stems. They thus differed from the botanical definition of a
root, and also from that of a rhizoma, or root stock; being, in short,
a primitive and generalized contrivance, suited to trees themselves
primitive and generalized, and to special and peculiar circumstances
of growth. Some botanists have imagined that they were aquatic plants,
growing at the bottom of lakes, but their mode of occurrence negatives
this. I have elsewhere stated this as follows:--[117]
[117] _Natural Science_, May, 1892.
"It is quite certain that Stigmariæ are not 'rhizomes which floated
in water, or spread themselves out on the surface of mud.' Whether
rhizomes or not, they grew in the soil, or in the upper layers of peaty
deposits since changed into coal. The late Richard Brown and the writer
have shown that they grew in the underclays or fossil soils, and that
their rootlets radiated in these soils in all directions.[118] In one
of my papers I have figured a Stigmarian root penetrating through an
erect _Sigillaria_, and Logan, in his Report of 1845, had already
figured a similar example. The penetration of decaying stems by the
rootlets of _Stigmaria_ is a fact well known to all who have studied
slices of Carboniferous plants,[119] while _Stigmariæ_ are often found
creeping inside the bark of erect and prostrate trunks. Besides this,
as I have shown in 'Acadian Geology,' in the section of 5,000 feet
of coal measures at the South Joggins (including eighty-one distinct
coal groups, and a larger number of soils with _Stigmaria_, or erect
trees), _Sigillaria_ and _Stigmaria_ occur together, and the latter
nearly always either in argillaceous soils, or sands hardened into
'Gannister,' which are often filled with roots or rootlets, or on the
surfaces of coal beds. On the other hand, the numerous bituminous
limestones, and calcareous and other shales holding remains of fishes,
crustaceans, and bivalve shells do not contain Stigmaria _in situ_--the
only exceptions being two beds of bituminous limestone, the upper
parts of which have been converted into underclays. This section, and
that of North Sydney--two of the most complete and instructive in
the world--have afforded conclusive proof of this mode of growth of
_Sigillaria_ and _Stigmaria_.
[118] _Quart. Journ. Geol. Soc._, vol. ii. p. 394 (1846); _Ibid._, vol.
iv. p. 47 (1847); _Ibid._, vol. v. p. 355 (1849); _Ibid._, vol. v. pp.
23, 30.
[119] Williamson has noticed this in his excellent Memoirs in the
_Phil. Trans._
"The objection to calling the Stigmariæ roots and their processes
rootlets, appears to me a finical application of modern botanical
usages to times for which they do not hold. We might equally object
to the application of the term roots to those which spring from the
earthed-up stems of Calamites, radiating as they do from nodes which,
in the air, would produce branchlets. Grand' Eury's figures show
abundant instances of this. We might also object to the exogenous stems
described by Williamson, which belong to cryptogamous plants; and,
unlike anything modern, are made up exclusively of scalariform tissue.
If the articulation and regular arrangement of those gigantic root
hairs, the rootlets, or 'leaves' of _Stigmaria_, are to be regarded
as depriving them of the name which clearly describes their function,
we may call them underground branches, though, by so doing, we set at
nought both their function and their mode of growth."
Dr. Williamson, in a recent paper, expresses the same view in the
following terms[120]:--"At that period (the Carboniferous age) no
Angiosperms existed on the earth, and even the Gymnosperms were very
far from reaching their modern development. Under these circumstances
the Cryptogams chiefly became the giant forest trees of that remote
age. To become such, they required an organization very different in
some respects from that of their degraded living representatives. Hence
we must not appeal to these degenerate types for illustrations and
explanations of structures no longer existing. Still less must we turn
to what we find in the Angiosperms, that wholly distinct race which has
taken the place of the primæval Cryptogams in our woods. The primeval
giants of the swampy forests had doubtless a morphology assigned to
them, adapted to the physical conditions by which they were surrounded;
but if even their dwarfed and otherwise modified descendants fail to
throw light upon morphological details once so common, still less must
we expect to obtain that light from the living and wholly different
flowering plants."
[120] _Natural Science_, July, 1892.
With the remarkable trees above referred to, there coëxisted a vast
multitude of ferns, some arborescent, others herbaceous, tall,
reed-like plants, the Calamites, allied to modern Mare's-tails, a
very remarkable family of plants allied to modern Cycads and Pines;
the Cordaites, which seem to have grown plentifully in certain parts
of the coal areas--probably the drier parts, so that their remains
sometimes constitute the greater part of small seams of coal. There
were also true pine-like trees, though these would seem to have grown
most abundantly on the higher levels. Nor was strictly aquatic
vegetation wanting. We find, both in the preceding Devonian and the
Carboniferous, that the little aquatic plants now known as Rhizocarps,
and structurally allied to the Ferns--such plants as the floating
Salvinia, and the Pillworts of our swamps, were vastly abundant, and
they may have filled and choked up with their exuberant growth many of
the lakes and slow streams of the period, furnishing layers of cannel
and "macrospore" coal, and earthly bitumen or Torbanite.
We have hitherto confined our attention to the great Carboniferous
period, so called, as emphatically the age of coal; but this mineral,
and allied forms of carbon, were produced both before and after. Even
in that old Laurentian age, which includes the oldest rocks that we
know, formed when the first land had just risen out of the waters,
there are thick beds of graphite, or plumbago, chemically the same with
anthracite coal, and which must have been produced by the agency of
plants, whether terrestrial or aquatic. We may suppose that the plants
of this remote age were of very humble type as much lower than those
of the coal formation as these are lower than those of the present
day; but if so, then, on the analogy of the Carboniferous, they would
be high and complex representatives of those low types. But there is
another and more startling possibility; that the Laurentian may have
been a period when vegetable life culminated on the earth, and existed
in its most complete and grandest forms in advance of the time when
it was brought into subordination to the higher life of the animal.
In the meantime, the Laurentian rocks are in a state of so extreme
metamorphism that they have afforded no certain indication of the forms
or structures of the vegetation of the period.
We find indications of plant life through all the Palæozoic groups
succeeding the Laurentian; but it is not till we reach the Devonian,
the system immediately preceding the Carboniferous, that we find
an abundance of forms not essentially different from those of the
Carboniferous, though similar in details. Only a few and very small
beds of coal were accumulated in this age; but there was an immense
abundance of bituminous shale enriched with the macrospores of
Rhizocarps. The Ohio black shale, which is said to extend its outcrop
across that state with a breadth of ten to twenty miles, and a
thickness of 550 feet, is filled with macrospores of Protosalvinia, as
is its continuation in Canada.
Above the great coal formation the Permian and Jurassic contain beds
of coal, though of limited extent, and formed in the case of the two
latter of very different plants from those of the Carboniferous. In
the Cretaceous and Tertiary ages, after the abundant introduction
of species of forest trees still living, coal making seems to have
obtained a new impulse, so that in China and the western part of
America there are coals of great extent and value, all made of plants
of genera still existing. In the Cretaceous coal of Vancouver Island
there are remains of such modern trees as the Poplars, Magnolias,
Palmettos, Sequoias, and a great variety of other genera still living
in America. Out of the remains of these, under favouring conditions,
quite as good coal as that of the coal formation has been made,
although the plants are so different. There is, indeed, reason to
believe that those now rare trees, the Sequoias, represented at the
present time only by the big trees of California, and their companion,
the redwood, were then spread universally over the northern hemisphere,
and formed dense forests on swampy flats which led to the accumulation
of coal beds in which the trunks and leaves of the Sequoias formed
main ingredients, so that Sequoia and its allies in this later age
take the place of the Sigillariæ of the coal formation. Last of all,
coal accumulation is still going on in the Everglades of Florida,
the dismal swamp of Virginia, and the peat-bogs of the more northern
regions. So the vegetable kingdom has, throughout its long history,
been continually depriving the atmosphere of its carbon dioxide, and
accumulating this in beds of coal. In the earlier ages indeed, this
would seem to us to have been its main use.
To the modern naturalist, vegetable life, with regard to its uses,
is the great accumulator of pabulum for the sustenance of the higher
forms of vital energy manifested in the animal. In the Palæozoic this
consideration sinks in importance. In the Coal period we know few land
animals, and these not vegetable feeders, with the exception of some
insects, millipedes, and snails. But the Carboniferous forests did not
live in vain, if their only use was to store up the light and heat of
those old summers in the form of coal, and to remove the excess of
carbonic acid from the atmosphere. In the Devonian period even these
utilities fail, for coal does not seem to have been accumulated to
any great extent, though the abundant petroleum of the Devonian is,
no doubt, due to the agency of aquatic vegetation. In addition to
scorpions, a few insects are the only known tenants of the Devonian
land, and these are of kinds whose lame probably lived in water, and
were not dependent on land plants. We may have much yet to learn of the
animal life of the Devonian; but for the present, the great plan of
vegetable nature goes beyond our measures of utility; and there remains
only what is perhaps the most wonderful and suggestive correlation
of all, namely, that our minds are able to trace in these perished
organisms structures similar to those of modern plants, and thus to
reproduce in imagination the forms and habits of growth of living
things which so long preceded us on the earth.
In another way Huxley has put the utilitarian aspect of the case so
admirably, that I cannot refrain from quoting his clever apotheosis of
nature in connection with the production of coal.
"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 for millions of years without being
able to find a use for them; she has sent them beneath the sea, and the
sea beasts could make nothing of them; she had 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 Cæsar was good
enough to deal with Britain as we have dealt with New Zealand, the
primæval Briton, blue with cold and woad, may have known that the
strange black stone which he found 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 return for the capital
she had invested in 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 of this growth and development as carbonic
acid is of a club moss. Wanting the coal, we could not have smelted
the iron needed to make our engines; nor have worked our engines
when we 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 could live where now ten
thousand are amply supported.
"Thus all this abundant wealth of money and of vivid life is Nature's
investment in club mosses and the like so long ago. But what becomes
of the coal which is burnt in yielding the 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 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
the most notable of housekeepers."[121]
[121] _Contemporary Review_, 1871.
All this is true and well told; but who is "Nature," this goddess who,
since the far-distant Carboniferous age, has been planning for man? Is
this not another name for that Almighty Maker who foresaw and arranged
all things for His people "before the foundation of the world."
References:--On Structures in Coal, _Journal Geological Society
of London_, xv., 1853. Contains results of microscopic study of
Nova Scotia coals. Conditions of Accumulation of Coal, _Ibid._,
xxii., 1866. Contains South Joggins section. Spore-cases in Coal,
_Am. Journal of Science_, 3rd series, vol. I, 1871. Rhizocarps in
the Devonian, _Bulletin Chicago Academy_, vol. I, 1886. "Acadian
Geology and Supplement," 3rd edition, 1891, Cumberland Coal Field.
"Geological History of Plants," chap, iv., London and New York, 2nd
edition, 1892.
_THE OLDEST AIR-BREATHERS._
DEDICATED TO THE MEMORY OF
MY FRIEND AND EARLY PATRON AND GUIDE
SIR CHARLES LYELL,
To whom we are Indebted for so much
of the Scientific Basis of Modern Geology.
Earliest Discoveries--Footprints of Batrachians--Labyrinthodents
of the Carboniferous--Microsauria of the Carboniferous--Other
Types--Discoveries in Erect Trees--Invertebrate Air-breathers, Land
Snails, Millipedes, Insects, Spiders And Scorpions--General Conclusions
[Illustration: Remains of Hylonomus Lyelli, Dawson, 1859. Coal
Measures, South Joggins; Nova Scotia.
Photograph of Type specimen somewhat enlarged, _Geol. Magazine_, 1891
(p. 279). (1) Cranial bones and mandibles; (1_a_) Sternal and shoulder
bones; (2) Mandible; (3) Humerus, ribs and vertebræ; (4) Hind limb; (5)
Pelvis; (6) Caudal vertebræ.]