The origin of coal, that combustible which is distributed over the earth
in all latitudes, from the frozen regions of Greenland to Zambesi in the
tropics, utilized by the Chinese from the remotest antiquity for the
baking of pottery and porcelain, employed by the Greeks for working iron,
and now the indispensable element of the largest as well of the smallest
industries, is far from being sufficiently clear. The most varied
hypotheses have been offered to explain its formation. To cite them all
would not be an easy thing to do, and so we shall recall but three: (1) It
has been considered as the result of eruptions of bitumen coming from the
depths, and covering and penetrating masses of leaves, branches, bark,
wood, roots, etc., of trees that had accumulated in shallow water, and
whose most delicate relief and finest impressions have been preserved by
this species of tar solidified by cooling. (2) It has also been considered
as the result of the more or less complete decomposition of plants under
the influence of heat and dampness, which has led them to pass
successively through the following principal stages: _peat, lignite,
bituminous coal, anthracite_. (3) Finally, while admitting that the
decomposition of plants can cause organic matter to assume these different
states, other scientists think that it is not necessary for such matter to
have been peat and lignite in order to become coal, and that at the
carboniferous epoch plants were capable of passing directly to the state
of coal if the conditions were favorable; and, in the same way, in the
secondary and tertiary epochs the alteration of vegetable tissues
generally led to lignite, while now they give rise to peat. In other
words, the nature of the combustible formed at every great epoch depended
upon general climatic conditions and local chemical action. Anthracite and
bituminous coal would have belonged especially to primary times, lignites
to secondary and tertiary times, and peat to our own epoch, without the
peat ever being able to become lignites or the latter coal.
As for the accumulation of large masses of the combustible in certain
regions and its entire absence in others belonging to the same formation,
that is attributed, now to the presence of immense forests growing upon a
low, damp soil, exposed to alternate rising and sinking, and whose debris
kept on accumulating during the periods of upheaval, under the influence
of a powerful vegetation, and now to the transportation of plants of all
sorts, that had been uprooted in the riparian forests by torrents and
rivers, to lakes of wide extent or to estuaries. Not being able to enter
in this place into the details of the various hypotheses, or to thoroughly
discuss them, we shall be content to make known a few facts that have been
recently observed, and that will throw a little light upon certain still
obscure points regarding the formation of coal.
(1) According to the first theory, if the impressions which we often find
in coal (such as the leaves of Cordaites, bark of Sigillarias and
Lepidodendrons, wood of Cordaites, Calamodendrons, etc.) are but simple
and superficial mouldings, executed by a peculiar bitumen, formerly fluid,
now solidified, and resembling in its properties no other bitumen known,
we ought not to find in the interior any trace of preservation or any
evidence of structure. Now, upon making preparations that are sufficiently
thin to be transparent, from coal apparently formed of impressions of the
leaves of Cordaites, we succeed in distinguishing (in a section
perpendicular to the limb) the cuticle and the first row of epidermic
cells, the vascular bundles that correspond to the veins and the bands of
hypodermic libers; but the loose, thin-walled cells of the mesophyllum are
not seen, because they have been crushed by pressure, and their walls
touch each other. The portions of coal that contain impressions of the
bark of Sigillaria and Lepidodendron allow the elongated, suberose tissue
characteristic of such bark to be still more clearly seen.
Were we to admit that the bitumen was sufficiently fluid to penetrate all
parts of the vegetable debris, as silica and carbonates of lime and iron
have done in so many cases, we should meet with one great difficulty. In
fact, the number of fragments of coal _isolated_ in schists and sandstone
is very large, and _without any communication_ with veins of coal or of
bitumen that could have penetrated the vegetable. We cannot, then, for an
instant admit such a hypothesis. Neither can we admit that the penetration
of the plants by bitumen was effected at a certain distance, and that they
have been transported, after the operation, to the places where we now
find them, since it is not rare to find at Commentry trunks of
Calamodendrons, Anthropitus, and ferns which are still provided with roots
from 15 to 30 feet in length, and the carbonized wood of which surrounds a
pith that has been replaced by a stony mould. The fragile ligneous
cylinder would certainly have been broken during such transportation.
The carbonized specimens were never fluid or pasty, since there are some
that have left their impressions with the finest details in the schists
and sandstones, but none of the latter that has left its traces upon the
coal. The surface of the isolated specimens is well defined, and their
separation from the gangue (which has never been penetrated) is of the
easiest character.
The facts just pointed out are entirely contrary to the theory of the
formation of coal by way of eruption of bitumen.
(2) The place occupied by peats, lignites, and bituminous and anthracite
coal in sedimentary grounds, and the organic structure that we find less
and less distinct in measure as we pass from one of these combustibles to
one more ancient, have given rise to the theory mentioned above, viz.,
that vegetable matter having, under the prolonged action of heat and
moisture, experienced a greater and greater alteration, passed
successively through the different states whose composition is indicated
in the following table:
H. C. O. N. Coke. Ashes. Density.
Peat 5.63 57.03 29.67 2.09 ---- 5.58 ----
Lignite 5.59 70.49 17.2 1.73 49.1 4.99 1.2
Bitumin. coal 5.14 87.45 4 1.63 68 1.78 1.29
Anthracite 3.3 92.5 2.53 ---- 89.5 1.58 1.3
Aside from the fact that anthracite is not met with solely in the lower
coal measures, but is found in the middle and upper ones, and that
bituminous coal itself is met with quite abundantly in the secondary
formations, and even in tertiary ones, it seems to result from recent
observations that if vegetable matter, when once converted into lignites,
coal, etc., be preserved against the action of air and mineral waters by
sufficient thick and impermeable strata of earth, preserves the chemical
composition that it possessed before burial. The coal measures of
Commentry, as well as certain others, such as those of Bezenet, Swansea,
etc., contain quite a large quantity of coal gravel in sandstone or
argillaceous rocks. These fragments sometimes exhibit a fracture analogous
to that of ordinary coal, with sharp angles that show that they have not
been rolled; and the sandstone has taken their exact details, which are
found in hollow form in the gangue. In other cases these fragments exhibit
the aspect of genuine shingle or rolled pebbles. These pebbles of coal
have not been misshapen under the pressure of the surrounding sandstone,
nor have they shrunk since their burial and the solidification of the
gangue, for their surface is in contact with the internal surface of their
matrix. Everything leads to the belief that they were extracted from
pre-existing coal deposits that already possessed a definite hardness and
bulk, at the same time as were the gravels and sand in which they are
imprisoned. It became of interest, then, to ascertain the age to which the
formation of these fragments might be referred, they being evidently more
ancient than those considered above, which, as we have seen, could not
have been transported in this state on account of their dimensions and the
fragility of made coal. Thanks to the kindness of Mr. Fayol, we have been
enabled to make such researches upon numerous specimens that were still
inclosed in their sandstone gangue and that had been collected in the coal
strata of Commentry. In some of their physical properties they differ from
the more recent isolated fragments and from the ordinary coal of this
deposit. They are less compact, their density is less, and a thin film of
water deposited upon their surface is promptly absorbed, thus indicating a
certain amount of porosity. Their fracture is dull and they are striped
with shining coal, and can be more easily sliced with a razor.
From a fresh fracture, we find by the lens, or microscope, that some of
them are formed of ordinary coal, that is, composed of plates of variable
thickness, brilliant and dull, with or without traces of organization, and
others of divers bits of wood whose structure is preserved. When reduced
to thin, transparent plates, these latter show us the organization of the
wood of _Arthropitus, Cordaites_, and _Calamodendron_, and of the petioles
of _Aulacopteris_, that is to say, of the ligneous and arborescent plants
that we most usually meet with in the coal measures of Commentry in the
state of impression or of coal.
In a certain number of specimens the diminution in volume of the tracheæ
is less than that that we have observed in the same organs of
corresponding genera. The quantity of oxygen and hydrogen that they
contain is greater, and seems to bring them near the lignites.
We cannot attribute these differences to the nature of the plants
converted into coal, since we have just seen that they are the same in the
one case as in the other. Neither does time count for anything here,
since, according to accepted ideas, the burial having been longer, the
carbonization ought to have been more perfect, while the contrary is the
case.
If we admit (1) that vegetable remains alter more and more through
maceration in ordinary water and in certain mineral waters; (2) that,
beginning with their burial in sufficiently thick strata of clay and sand,
their chemical composition scarcely varies any further; and (3) that these
are important changes only as regards their physical properties, due to
loss of water and compression, we succeed quite easily in learning what
has occurred.
In fact, when, as a consequence of the aforesaid alteration, the vegetable
matter had taken the chemical composition that we find in the less
advanced coal of the pebbles, it was in the first place covered with sand
and protected against further destruction, and it gradually acquired the
physical properties that we now find in it. At the period that channels
were formed, the coal was torn from the beds in fragments, and these
latter were rolled about for a time, sometimes being broken, and then
covered anew, and this too at the same time as were the plants less
advanced in composition that we meet with at the same level. These latter,
being like them protected against ulterior alteration, we now find less
advanced in carbonization (notwithstanding their more ancient origin) than
the other vegetable fragments that were converted into coal after them,
but that were more thoroughly altered at the time of burial.
There are yet a few other important deductions to be made from the
foregoing facts: (1) the same coal basin may, at the same level, contain
fragments of coal of very different ages; (2) its contour may have been
much modified owing to the ravines made by the water which transported the
ancient parts into the lowest regions of the basin; and (3) finally, since
the most recent sandstones and schists of the same basin may contain coal
which is more ancient, but which is formed from the same species of plants
that we find at this more recent level, we must admit that the conversion
of the vegetable tissues into coal was relatively rapid, and far from
requiring an enormous length of time, as we are generally led to believe.
If, then, lignites have not become soft coal, and if the latter has not
become anthracite, it is not that time was wanting, but climatic
conditions and environment. Most analyses of specimens of coal have been
made up to the present with fragments so selected as to give a mean
composition of the mass; it is rare that trouble has been taken to select
bits of wood, bark, etc., of the same plant, determined in advance by
means of thin and transparent sections in order to assure the chemist of
the sole origin and of the absolute purity of the coal submitted to
analysis. This void has been partially fitted, and we give in the
following table the results published by Mr. Carnot of analyses made of
different portions of plants previously determined by us:
Carbon Hydrogen Oxygen Nitrogen
1. Calamodendron (5 specimens) 82.95 4.78 11.89 0.48
2. Cordaites (4 specimens) 82.94 4.88 11.84 0.44
3. Lepidodendron (3 specimens) 83.28 4.88 11.45 0.39
4. Psaronius (4 specimens) 81.64 4.80 13.11 0.44
\----v----/
5. Ptychopteris (1 specimen) 80.62 4.85 14.53
6. Megaphyton (1 specimen) 83.37 4.40 12.23
As seen from this table, the elementary composition of the various
specimens is nearly the same, notwithstanding that the selection was made
from among plants that are widely separated in the botanical scale, or
from among very different parts of plants. In fact, with Numbers 1 and 2
the analysis was made solely of the wood, and with No. 3 only of the
prosenchymatous and suberose parts of the bark. Here we remark a slight
increase in carbon, as should be the case. With No. 4 the analysis was of
the roots and the parenchymatous tissue that descends along the stem, and
with No. 6 of the bark and small roots. One will remark here again a
slight increase in the proportion of carbon, as was to be foreseen. The
elementary composition found nearly corresponds with that of the coal
taken from the large Commentry deposit.
Carbon. Hydrogen. Oxygen and
Nitrogen.
Regnault 82.92 5.39 11.78
Mr Carnot 83.21 5.57 11.22
Although the chemical composition is nearly the same, the manner in which
the different species or fragments of vegetables behave under distillation
is quite different.
In fact, according to Mr. Carnot, the plants already cited furnish the
following results on distillation:
Volatile Fixed Coke.
matters. residue.
Calamodendron 35.5 64.7 Well agglomerated.
Cordaites 42.1 57.8 Quite porous.
Lepidodendron 34.7 55.3 Well agglomerated.
Psaronius 29.4 60.5 Slightly porous.
Ptychopteris 39.4 60.5
Megaphyton 35.5 64.5 Well agglomerated.
Coal of the Great Bed 40.5 59.5 Slightly porous.
These differences in the proportions of volatile substances, of fixed
residua, and of density in the coke obtained seem to be in harmony with
the primitive organic nature of the carbonized tissues. We know, in fact,
that the wood of the Calamodendrons is composed of alternately radiating
bands formed of ligneous and thick walled prosenchymatous tissue, while
the wood of Cordaites, which is less dense, recalls that of certain
coniferæ of the present day (Araucariæ).
We have remarked above that the portions of Lepidodendron analyzed
belonged to that part of the bark that was considerably thickened and
lignefied. So too the portion of the Megaphyton that was submitted to
distillation was the external part of the hard bark, formed of hypodermic
fibers and traversed by small roots. The Psaronius, on the contrary, was
represented by a mixture of roots and of parenchymatous tissue in which
they descend along the trunk.
It results from these remarks that we may admit that those parts of the
vegetable that are ordinarily hard, compact, and profoundly lignefied
furnish a compact coke and relatively less volatile matter, while the
tissues that are usually not much lignefied, or are parenchymatous, give a
bubbly, porous coke and a larger quantity of gas. The influence of the
varied mode of grouping of the elements in the primitive tissues is again
found, then, even after carbonization, and is shown by the notable
differences in the quantities and physical properties of the products of
distillation.
The elementary chemical composition, which is perceptibly the same in the
specimens isolated in the sandstones and in those taken from the great
deposit, demonstrates that the difference in composition of the
environment serving as gangue did not have a great influence upon the
definitive state of the coal, a conclusion that we had already reached
upon examining the structure and properties of the coal pebbles.
We may get an idea of the nearly similar composition of the coal produced
by very different plants or parts thereof, in remarking that as the cells,
fibers, and vessels are formed of cellulose, and some of them isomeric,
the difference in composition is especially connected with the contents of
the cells, canals, etc., such as protoplasm, oils, resins, gums, sugars,
and various acids, various incrustations, etc. After the prolonged action
of water that was more or less mineralized and of multiple organisms,
matters that were soluble, or that were rendered so by maceration, were
removed, and the organic skeletons of the different plants were brought to
a nearly similar centesimal composition representing the carbonized
derivatives of the cellulose and its isomers. The vegetable debris thus
transformed, but still resistant and elastic, were the ones that were
petrified in the mineral waters or covered with sand and clay. Under the
influence of gradual pressure, and of a desiccation brought about by it,
and by a rising of the ground, the walls of the organic elements came into
contact, and the physical properties that we now see gradually made their
appearance.
The waters derived from a prolonged steeping of vegetables, and charged
with all the soluble principles extracted therefrom, have, after their
sojourn in a proper medium, deposited the carbonized residua that have
themselves become soluble, and have there formed masses of combustibles of
a different composition from that resulting from the skeletons of plants,
such as _cannel coal, pitch coal, boghead_, etc.
A thin section of a piece of Commentry cannel coal shows that this
substance consists of a yellowish-brown amorphous mass holding here and
there in suspension very different plant organs, such as fragments of
Cordaites, leaves, ferns, microspores, macrospores, pollen grains,
rootlets, etc., exactly as would have done a gelatinous mass that upon
coagulating in a liquid had carried along with it all the solid bodies
that had accidentally fallen into it and that were in suspension.
It is evident (as we have demonstrated) that other cannel coals may show
different plant organs, or even contain none at all, their presence
appearing to be accidental. The composition itself of cannel coal must be,
in our theory, connected with the chemical nature of the materials from
whence it is derived, and that were first dissolved and then became
insoluble through carbonization. Several preparations made from Australian
(New South Wales), Autun, etc., boghead have shown us merely a
yellowish-brown amorphous mass holding in suspension lens-shaped or
radiating floccose masses which it is scarcely possible to refer to any
known vegetable organism.
Among the theories that we have cited in the beginning, the one that best
agrees with the facts that we have pointed out is the third, which would
admit, then, two things in the formation of coal. The first would include
the different chemical reactions which cannot yet be determined, but which
would have brought the vegetable matter now to the state of soft coal
(with its different varieties), and now to the state of anthracite. The
second would comprehend the preservation, through burial, of the organic
matter in the stage of carbonization that it had reached, and as the
result of compression and gradual desiccation, the development of the
physical properties that we now find in the different carbonized
substances.
We annex to this article a number of figures made from preparations of
various coals. These preparations were obtained by making the fragments
sufficiently thin without the aid of any chemical reagent, so as to avoid
the reproach that things were made to appear that the coal did not
contain. This slow and delicate method is not capable of revealing all the
organisms That the carbonaceous substance contains, but, per contra, one
is riot absolutely sure of the pre-existence of everything that resembles
organs or fragments of such that he distinguishes therein by means of the
microscope.
Our researches, as we have above stated, have been confined to different
cannel coals, anthracite, boghead, and coal plants isolated either in coal
pebbles, or in schists and sandstones.
[Illustration: 12a: FIG. 1.--Lancashire cannel coal; longitudinal section,
X200.]
[Illustration: 12b: FIG. 2.--Lancashire cannel coal; transverse section,
X200.]
Figs. 1 and 2 (magnified two hundred times) represent two sections, made
in rectangular planes, of fragments of Lancashire cannel coal. In a
certain measure, they remind one of Figs. 4 and 5, Pl 11, of Witham's
"Internal Structure of Fossil Vegetables," and which were drawn from
specimens of cannel coal derived likewise from Lancashire, but which are
not so highly magnified. There is an interesting fact to note in this
coincidence, and that is that this structure, which is so difficult to
explain in its details, is not accidental, but a consequence of the
nature of the materials that served to produce the coal of this region.
In the midst of a mass of blackish debris, _a_, organic and inorganic,
and immersed in an amorphous and transparent gangue, we find a few
recognizable fragments, such as thick-walled macrospores, _b_, of
various sizes, bits of flattened petioles, _c_, pollen grains, _d_,
debris of bark, etc. In Fig. 2 all these different remains are cut
either obliquely or longitudinally, and are not very recognizable. It is
not rare to meet with a sort of vacuity, _e_, filled with clearer matter
of resinoid aspect, without organization.
[Illustration: 12c: FIG. 3.--Commentry cannel coal, X200.]
In Fig. 3, which represents a section made from Commentry cannel coal,
the number of recognizable organs in the midst of the mass of debris is
much larger. Thus, at _a_ we see a macrospore, at _b_ a fragment of the
coat of a macrospore, at _c_ another macrospore having a silicified
nucleus, such as has been found in no other case, at _d_ we have a
transverse section of a vascular bundle, at _e_ a longitudinal section
of a rootlet traversed by another one, at _f_ we have a transverse
section of another rootlet, at _g_ an almost entire portion of the
vascular bundle of a root, and at _h_ we see large pollen grains
recalling those that we meet with in the silicified seeds from Saint
Etienne.
Cannel coal, then, shows that it is formed of a sort of dark brown gangue
of resinoid aspect (when a thin section of it is examined) holding in
suspension indeterminable black organic and inorganic debris, which are
arranged in layers, and in the midst of which (according to the locality
and the fragment studied) is found a varying number of easily recognized
vegetable organs.
[Illustration: 12d: FIG. 4.--Pennsylvania anthracite, X200.]
It is very rare that anthracite offers any discernible trace of
organization. Preparations made from fragments of Sable and Lamore coal
could not be made sufficiently thin to be transparent; the mass remained
very opaque, and the clearest parts exhibited merely amorphous,
irregular granulations. Still, fragments of anthracite from Pennsylvania
furnished, amid a dominant mass of dark, yellow-brown, structureless
substance, a few organized vegetable debris, such as a fragment of a
vascular bundle with radiating elements (Fig. 4, _a_), a macrospore,
_b_, and a few pollen grains or microspores, _c_.
[Illustration: 12e: FIG. 5.--Boghead from New South Wales, X500.]
From what precedes it seems to result, then, that anthracite is in a much
less appreciable state of preservation than cannel coal, and that it is
only rarely, and according to locality, that we can discover vegetable
organs in it. Soft coal comes nearer to amorphous carbon. Boghead appears
to be of an entirely different character (Fig. 5, magnified X300). It is
easily reduced to a thin transparent plate, and shows itself to be formed
of a multitude of very small lenses, differing in size and shape, and much
more transparent than the bands that separate them. In the interior of
these lenses we distinguish very fine lines radiating from the center and
afterward branching several times. The ramifications are lost in the
periphery amid fine granulations that resemble spores. We might say that
we here had to do with numerous mycelia moulded in a slightly colored
resin. Preparations made from New South Wales and Autun boghead presented
the same aspect.
If boghead was derived from the carbonization of parts that were soluble,
or that became so through maceration, and were made insoluble at a given
moment by carbonization, we can understand the very peculiar aspect that
this combustible presents when it is seen under the microscope.
The following figures were made in order to show the details of anatomical
structure that are still visible in coal, and to permit of estimating the
shrinkage that the organic substance has undergone in becoming converted
into coal.
It is not rare in coal mines to find fragments of wood, of which a portion
has been preserved by carbonates of iron and lime, and another portion
converted into coal. This being the case, it was considered of interest to
ascertain whether the carbonized portion had preserved a structure that
was still recognizable, and, in such an event, to compare this structure
with that of the portion of the specimen that was preserved in all its
details by mineralization.
[Illustration: 12f: FIG. 6.--_Arthropitus gallica_, St. Etienne; transverse
section, X200.]
Fig. 6 shows a transverse section of a specimen of _Arthropitus Gallica_
found under such conditions. The region marked c is carbonized; the
organic elements of the wood-cells, tracheæ, etc., have undergone but
little change in shape. Moreover, no change at all exists in the
internal parts of another specimen (Fig. 8), where we easily distinguish
by their form and dimensions the ligneous cells, _aa_, and the elements,
_bb_, of the wood itself.
[Illustration: 12h: FIG. 8.--_Arthropitus gallica_, St. Etienne; transverse
section through the carbonized part.]
In the region, _b_, of Fig. 6, the ligneous elements have undergone an
evident change of form, and the walls have been broken. This region,
already filled by petrifying salts, but not completely hardened, has not
been able to resist, as the region, _a_, an external pressure, and has
become more or less misshapened. As for the not yet mineralized external
portion, _c_, it has completely given way under the pressure, the walls of
the different organic elements have come into contact, the calcareous or
other salts have been expressed, and this region exhibits the aspect of
ordinary coal, while at the same time preserving a little more hardness on
account of the small quantity of mineral salts that has remained in them
despite the compression.
From the standpoint of carbonization there seems to us but little
difference between the organic elements that occupy the region, _a_, and
those that occupy _b_. If the former had not been filled with hardened
petrifying matter, they would have been compressed and flattened like
those of region _c_, and would have given a compact and brilliant coal,
having very likely before petrifaction reached the same degree of
carbonization as the latter. The layer of coal in contact with the
carbonized or silicified part of the specimens is due, then, to a
compression of the organic elements already chemically carbonized, but in
which the mineral matter was not yet hardened and was able to escape.
[Illustration: 12g: FIG. 7.--_Arthropitus gallica_, St. Etienne; tangential
longitudinal section.]
If this be so, we ought to find the remains of organic structure in this
region _c_. In fact, on referring to Fig. 7, which represents a
tangential, longitudinal section of the same specimen, we perceive at _ab_
a ligneous duct and some unchanged tracheæ situated in the carbonized
region, and then at _c_ the same elements, though flattened, in which,
however, we still clearly distinguish the bands of the tracheæ; at _d_ is
found a trachea whose contents were already solidified, and which has not
been flattened; then, near the surface, in the region, _e_, the pressure
having been greater, it is no longer possible to recognize traces of
organization in a tangential section. In a large number of cases, the fact
that the coal does not seem to be organized must be due to the too great
compression that the carbonized cells and vessels have undergone when yet
soft and elastic, at the time this slow but continuous pressure was being
exerted.
It also became of interest to find out whether, through the very fact of
carbonization, the dimensions of the organic elements had perceptibly
varied--a sort of research that presents certain difficulties. At present
we have no living plant that is comparable, even remotely, with those that
grew during the coal epoch. Moreover, the organic elements have absolutely
nothing constant in their dimensions.
Still, if we limit ourselves to a comparison of the same carbonized wood,
preserved on the one hand by petrifaction, and on the other hand
non-mineralized, we find a very perceptible diminution in bulk. The
elements have contracted in length, breadth, and thickness, but
principally in the direction of the compression that they have undergone
in the purely carbonized specimens.
In the vicinity of the carbonized portions, those of the tracheæ that have
not done so have perceptibly preserved their primitive length, which has,
so to speak, been maintained by their neighbors, but their other
dimensions have become much smaller--a quarter in thickness and half in
length.
[Illustration: 12i: FIG. 9.--_Calamodendron,_ Commentry; prosenchymatous
portion of the wood carbonized, X200.]
If the two fragments of the same wood are, one of them silicified and the
other simply carbonized and preserved in sandstone, the diminution in
volume will have occurred in all directions in the latter of the two.
[Illustration: 12j: FIG. 10.--_Calamodendron,_ fragment of the vascular
portion of the wood carbonized.]
Figs. 9 and 11, which represent a portion of the _fibrous_ region of
Calamodendron wood, may give an idea of the shrinkage that has taken place
therein. In Figs. 11 and 12, which show a few tracheæ and medullary rays
of the ligneous bands of the same plant, we observe the same phenomenon.
We might cite a large number of analogous examples, but shall be content
to give the following: Figs. 13 and 15 represent radial and tangential
sections of the bark of _Syringodendron pes-capræ_. This is the first time
that one has had before his eyes the anatomical structure of the bark of a
_Syringodendron_, a plant which has not yet been found in a petrified
state. It is coal, then, with its structure preserved, that allows of a
verification of the theory advanced by several scientists that the often
bulky trunks of _Syringodendron_ are bases of _Sigillariæ_.
[Illustration: 12k: FIG. 11.--_Calamodendron,_ from Autun; prosenchymatous
portion of the wood silicified, X200.]
[Illustration: 12l: FIG. 12.--_Calamodendron,_ from Autun; vascular portion
of the wood silicified.]
If we refer to Fig. 13, which represents a radial vertical section running
through the center of one of the scars that permitted the specimen to be
determined, we shall observe, in fact, a tissue formed of rectangular
cells, longer than wide, arranged in horizontal series, and very analogous
in their aspect to those that we have described in the suberose region of
the bark of Sigillariæ. Fig. 15 shows in tangential section the fibrous
aspect of this tissue, which has been rendered denser through compression.
Fig. 14 shows it restored. In Fig. 13, the external part of the bark is
occupied by a thick layer of cellular tissue that exists over the entire
surface of the trunk, but particularly thick near the scars, exactly as in
the barks of the Sigillariæ that we have formerly described. Finally, at
_b_, we recognize the undoubted traces of a vascular bundle running to the
leaves. If the bundle appears to be larger than that of the Sigillariæ,
this is due to the flattening that the trunk has undergone, the effect of
this having been to spread the bundle out in a vertical plane, although
its greatest width in the first place was in a horizontal one.
[Illustration: 12m: FIG. 13.--_Syringodendron pes-capræ_; from Saarbruck;
radial vertical section, X200.]
[Illustration: 12n: FIG. 14.--Suberose cells restored.]
In anatomical structure, the barks of the Syringodendrons are, then,
analogous to those of the Sigillariæ. If, now, we compare the dimensions
of the tissues of these barks with the same silicified tissues of the
barks of Sigillariæ, we shall find that there was likewise a diminution in
the dimensions, but yet a less pronounced one than in the woods that we
have previously spoken of. The corky nature of this region of the bark was
likely richer in carbonizable elements than the wood properly so called,
and had, in consequence, to undergo much less shrinkage.--_Dr. B. Renault
(of Paris Museum) in Le Genie Civil_.
[Illustration: 12o: FIG. 15.--_Syringodendron pes-capræ;_ tangential
vertical section in the corky part of the bark, X200.]
DESCRIPTION OF THE FIGURES.--Fig. 1, Lancashire cannel coal; longitudinal
section, X200. Fig. 2, Lancashire cannel coal; transverse section, X200.
Fig. 3. Commentry cannel coal, X200. Fig. 4, Pennsylvania anthracite,
X200. Fig. 5, Boghead from New South Wales, X500. Fig. 6, _Arthropitus
gallica_, St. Etienne; transverse section, X200. Fig. 7, same; tangential
longitudinal section. Fig. 8, same; transverse section through the
carbonized part. Fig. 9. _Calamodendron_, Commentry; prosenchymatous
portion of the wood carbonized, X200. Fig. 10, same; fragment of the
vascular portion of the wood carbonized. Fig. 11, same, from Autun;
prosenchymatous portion of the wood silicified, X200. Fig. 12, same,
Autun; vascular portion of the wood silicified. Fig. 13, _Syringodendron
pes-capræ_; from Saarbruck; radial vertical section, X200. Fig. 14,
Suberose cells restored. Fig. 15. _Syringodendron pes-capræ_; tangential
vertical section in the corky part of the bark, X200.
* * * * *