942

THE COLLIERY GUARDIAN.

May 18, 1917.

any connection with the natural outlines of the plant
structure concerned, each, as a rule, being merely a
torn fragment.

The nature of those bright lenticles was clearly
described by Potonie, Jeffrey, Thiessen, and Lomax.
Jeffrey described them under the title of “lignitoid,”
while Lomax referred to the same structures as
“ jettonised wood.” Either term may be used for the
purpose, but both are open to the objection that they
convey a false impression as to the nature of the
vegetable structures preserved in that fashion. Cer-
tainly, among the numerous examples the author has
examined, the great majority represent the soft
cortical tissues of the plants, and not the woody axes.

Returning to the consideration of the microscopic
structure of the “ dull layers,” the details of the struc-
ture of that part of the coal vary enormously in
different seams, and generally in different portions of
a single seam. The same elements are present in most
cases, but in widely varying proportions. Firstly,
there are the more minute fragments of lignitoid
vegetable tissue, varying in form from very elongate
lenticular strips, which may in some cases be less than
in. in thickness, to small elliptical or even irre-
gular patches. These usually exhibit a moderate
intensity of colour like the larger lenticles, but it is
rarely that any cellular structure is discernible in
them. Secondly, there are frequently to be seen
minute ribbon-like toothed strips, always of a rela-
tively very pale yellow colour, which represent the
glossy cuticles from the epidermis of stems or leaves.
Occasionally those cuticles are seen in their original
position edging or enclosing a patch of lignitoid. The
third element almost invariably present represents the
hard coats of the spores of various plants, usually both
the large megaspores, which may attain a millimetre
or more in diameter, and the relative minute mega-
spores only to y^ mm. in diameter, are present.
Like the cuticles, those spore coats are invariably of a
relatively pale colour, and nearly always pressed quite
flat. A fourth element is nearly always distinguish-
able, which may be described as a matrix intervening
between the other elements. It is usually somewhat
dark coloured, and, with the ordinary magnification,
appears structureless or somewhat granular; though,
with very high powers, even that tends to resolve itself
simply into a moss of more minutely comminuted
debris. The above four elements may be said to be
represented in every coal. In addition, there are
several others of less universal occurrence. All pre-
vious writers have been struck by the sharp, regular
oval outlines of certain bright, somewhat pale coloured
bodies, which have been interpreted by the American
writers as resin globules, or strands. These bodies
vary in size from less than V^mm. to 1 mm. or more.
In many cases there would seem to be little doubt that
the regular form of those bodies is of a purely physical,
rather than “ organic ” origin, though in other very
similar bodies definite traces of organic structure are
clearly distinguishable. There would appear to be no
serious reason why, at any rate, the more definitely
homogeneous of these oval bodies, should not repre-
sent actual fossil resin, beyond the circumstance that
few examples of structures which may be regarded as
resin secreting glands have been met with among
carboniferous plants, and that the presence of exuded
resins in large quantity is not, therefore, to be anti-
cipated.

What would seem to be essentially similar substances
are found elsewhere forming irregular bands or patches
of flocculent or “curdled” appearance. In those
instances, the form of the masses still more definitely
suggested that they are congealed liquids.

Lignitoid Tissue.

The next element is the most sharply distinguished
among the varied constituents of a coal seam, namely,
the fragments of lignitoid tissue preserved in'the well-
known form of “mineral charcoal.” or “ mother-of
coal.” Those fragments represented woody tissues
preserved in a manner entirely different from that
which characterised all the other elements of the coal.
In this case, the tissues have not been impregnated by
any organic fluids; instead, the cavities of the vessels
either remained empty, or are filled ^vith purely
inorganic mineral matter. There would seem to be no
doubt that the walls of those vessels, as now existing,
consisted of the altered substance of the original
walls, without any addition of other material. That
is probably the only instance among the plant remains
to be seen in coal where that simple, direct relationship
between the original and the present substance can be
said to exist. The walls of those vessels are now found
to be extremely opaque. As is well known, those
fragments are usually thickly ’ clustered on the surface
of certain layers of the coal, constituting planes of
weakness along which the coal very readily parts.
Where the cavities of the vessels have remained empty,
the fragments are usually flattened to mere plates,
that being the most frequent condition. In these
cases, however, where the fragments have been ren-
dered more resistant by infilling with mineral matter,
they retain their original form, and their resemblance
in such cases to charred masses of wood is extremely
striking. Mr. Lomax has suggested that they actually
represent the relics of forest fires. Another view of
their origin is, however, possible. If we assume that
the cavities of the wood vessels are filled with air, as
very possibly may have been the case to some extent
in the living plant, and may easily have been increased
by drying at a later stage, then the presence of the air
would effectively prevent the entry of fluids into the
wood, and thus determine that it should be preserved
differently from the more moist tissues. Since those
masses of dried and more or less altered wood must
have become somewhat brittle, it is very intelligible
that they should have broken into small fragments
under the influence of the movements which must have
taken place in the plastic accumulations of vegetable
debris in which they are embedded.

There remain, finally, certain bodies which have
been the subject of much controversy, and which are

restricted in their occurrence to cannel coals and bog-
heads, of which latter mineral they form the essential
constituents. Those bodies were studied with great
care by MM. Bertrand and Renault, who regarded
them as gelatinous algse, and distinguished among them
several genera. It would be beyond the scope of the
communications to attempt any critical analysis of
these objects, but it should be mentioned that Prof.
E. C. Jeffrey recently recorded observations which, in
his opinion, demonstrated that the supposed algse are
merely more or less decomposed spores. It is worth
noting, however, that when those objects occur in
cannel coals along with ordinary spore coats, the
general appearance of the two objects is markedly dis-
similar. The “ algse,” as seen in vertical section, are
much less compressed than the spores, giving the
impression of much more “solid” objects. They are
also of a much paler colour. When examined with
high powers, they show, moreover, a complex honey-
combed or cellular structure. It must further be
regarded as very significant that the presence of those
objects in large quantities is invariably associated with
a paraffin-yielding quality of the mineral—a feature
not associated with the presence of ordinary spores.
The true nature of those “ algse ” must be regarded as
still unsettled, but, whatever they may have been,
their presence serves to mark one of the most distinc-
tive varieties of coal.

In the foregoing paragraphs, the chief definitely
recognisable elements which may be regarded as
normal constituents of coals have been briefly
described. No reference has been made to the acces-
sory inorganic minerals, such as pyrites and the
various sulphates, and other compounds, commonly
included in the coal, since the nature and history of
those constituents is not a matter which calls for
discussion at this meeting.

Plant Tissues and Coal Substance.

With regard to the essential components of the coal,
of organic origin, we must now consider somewhat
carefully the relationship between the plant tissues
whose form is visible under the microscope and the
actual coal substance of which those visible structures
are now composed. When the minute structure of a
fragment of bark or woody tissue is distinguished in_
the coal, there is a natural temptation to assume that
the substance, as well as the form of the plant, is actu-
ally under observation; to believe, that is to say, that
the portion of coal in question represents the actual
materials of the original plant in a more or less
altered condition. Were that the case, it would repre-
sent a very exceptional type of preservation of organic
remains. Fossils, whether they represent shells, corals,
bones, or plants, in the vast majority of cases are pre-
cise replicas of the form of the original objects, often
reproducing the minute details of microscopic struc-
ture, yet with rare exceptions the substance of the
original organism has been almost completely removed
and replaced by fresh material, sometimes of a
similar, more often of a totally distinct nature. To
quote an example, particularly appropriate to the
matter in hand, every pakeobotanist has admired the
perfect preservation of the vegetable tissues in those
portions of certain coal seams where the original
organic material has been replaced by carbonate of
lime or other inorganic minerals; yet in those cases
only the minutest fraction of the original substance
of the plant remains. It is abundantly clear, there-
fore, that the preservation of the form must never be
taken as proof of preservation of the substance. It is
convenient to deal first with those well-marked frag-
ments of tissue which have been described in the paper
under Jeffrey’s term “ lignitoid,” and it would further
facilitate discussion to remark that certain common
types of coal are mainly composed of that material.
That being the case, we are clearly justified in describ-
ing the material simply as normal coal substance.
From an examination of a coloured illustration of
typical “lignitoid,” the following facts are evident:
Firstly, that the whole of the coal fragment there
represented is a nearly homogeneous mass of material,
the visible structure being due merely to varying
intensity of coloration which distinguishes “cell walls”
from the substance contained within them; secondly,
that the difference of coloration fades out in certain
areas, which thus become visibly homogeneous; thirdly,
that nearly all “ cell walls ” are unbroken ; and finally,
that the tissues in that case are not very greatly com-
pressed. A similar example, where extremely delicate
tissues are preserved without any compression, is
shown in another plate, where the small fragment of
“ lignitoid ” apparently represents the corte of a
stigmarian rootlet. Reference is also made to a plate
where a small portion of a strip of lignitoid is shown,
in which the relations as to coloration are the reverse
of those in the two previous examples, the “ cell walls”
being paler than the substance within. That example
also illustrates very well the fadings out of the distinc-
tion between “walls” and “contents.” That strip
of “ lignitoid ” can be traced for a considerable length
through the section, most of it appearing absolutely
homogeneous.

A little reflection would convince anyone that the
substance filling the “ cells ” in those cases could have
little or no connection with any substance which may
have been present in the living cell itself. The latter
material was principally water, the total quantity of
“organic” substance having been very small. Even
if the whole of the organic substance had remained, it
could only constitute a small part of the material
which now fills the cell. What, then, was the nature
and history of that material? Since it may form the
main bulk of an ordinary coal, its general chemical
composition cannot differ greatly from that of any
other normal constituent of coal, and there can be no
doubt that it is of organic origin. Since, again, it is
found within the closed membranes, which are the
cell walls of the original plant, it must have passed
through those walls by a process of diffusion, and was
therefore a liquid substance at the time of its entry.
These indisputable facts make the general history

clear. The plant fragment in question became satur-
ated with liquid decomposition products of other
vegetation; these passed through the cell walls, and
so reached their present situation; subsequent physico-
chemical changes resulted in the solidification of the
mass. The cell walls themselves had doubtless been
impregnated, if not ultimately replaced by, the same
material, and since in the large majority of fragments
of “.lignitoid” no distinction between “cell walls”
and “contents” is now visible, it would seem pro-
bable that considerable replacement of the walls has
usually taken place. How much of the original sub-
stance of either the cell walls or their contents remains
in the existing lignitoid, it is impossible to say; but
everything would suggest that it constitutes only a
small fraction of the mass. The impregnating liquid
has certainly contributed the greater part. Of all
ordinary coals, a great part, usually the greater part,
consists of fragments of “ vegetable tissue ” preserved
in the same manner, whence it follows that the liquid
decomposition products of the carboniferous plants
have been the most important contributors to the sub-
stance of coal. That fact must clearly have important
bearings on any discussion of the nature and distribu-
tion of the chemical compounds entering into the con-
stitution of coal.

Apart from the larger lenticles of “ lignitoid ” (con-
stituting the “bright bands”), the remaining consti-
tuents of coal (forming the “dull bands”) may be
grouped under four headings in relation to their essen-
tial character, so far as this can be judged from their
appearance under the microscope: —

(1)	The “ matrix ” or “ ground mass ” of the dull
bands. When the section is sufficiently thin to render
that part transparent enough for satisfactory exami-
nation with high powers (a V12in. immersion lens is
often useful), it appears usually to consist of very
minute fragments of “ lignitoid ” character, so that
what has been said in the preceding paragraphs may
be applied in large measure to that important element
of the coal.

(2)	Cuticles of stems or leaves, spore coals, and the
algse of Bertrand and Renault. These all agreed in
the fact that wherever they occur they are much paler
in colour than any other of the “ organic ” constituents
of the coal. The first two are much the most common
of the definitely recognisable plant fragments in coal,
though it may be doubted if they often form more
than 20 per cent, of the whole mass. Both may be
isolated from the coal by the action of macerating
fluids, which remove the other constituents by destruc-
tive solution. They are similar in original composi-
tion, and both were very resistant to decay. Those
facts together make it probable that in those bodies a
considerable part of the original substance remains,
though some replacement may have occurred. The
case of the “ algse ” cannot be profitably discussed in
the present state of the knowledge of those bodies.

(3)	The oval or flocculent bodies which have been
described as masses of resin. Whatever their chemical
nature, there can be no doubt that those masses were
originally liquid substances.

(4)	The fragments of “ mother-of-coal.” These may
(in exceptional cases) make up 5 per cent., or possibly
more, of a seam, but usually much less. As already
stated, that material presents the one clear case in
which the present substance would appear to be the
result of direct alteration of the original plant tissue,
without the addition of extraneous material.

Derivation of the Coal Substance.

The tendency of the foregoing observations is to sug-
gest that only in the case of cuticles, spore coats, and
“mother-of-coal,” is the substance, which now consti-
tutes the coal, derived in any large measure from the
original substance of the plant fragments which
entered into its composition. In most cases more than
75 per cent, of the coal appears to have been built up
of vegetable fragments, in most of which little or no
structure is now visible, and regarding which one has
attempted to show that the present substance is in
the main derived from impregnating liquid material.
That being the case, it would be obviously unwise to
assume any very intimate relationship between the
chemical composition of the coal and the character of
the plants whose remains may be distinguished in it.
Even in those cases, however, where the replacement
of the original plant substance by impregnating
materials has been most complete, they may neverthe-
less be an important relationship between the original
plant and the replacing substance. The latter has
reached its present position by diffusion through the
tissues of the original plant, and it may therefore be
anticipated that the nature of the plant membranes
have exercised an .important selective action in deter-
mining what substance should pass through it. For
that reason, it may perhaps be expected that the char-
acter of the tissues rather than the species of the
plant will be found to have influenced the quality of
the resulting coal.

Various writers who have dealt with the microscopic
structure of coal have dealt considerably on the part
played by resins in its formation. It is sufficient
merely to mention the controversy initiated by
Huxley’s reference (Contemporary Review, 1870) to
the important part played by spores as a principal
element of many coals, and as imparting to such a
resinous quality, and also to the recent description by
White and Thiessen of various coals in their recent
work. While there is no reason to doubt that the
spore coats of the carboniferous plants may have been
more or less resinous, and while some of the oval and
flocculent bodies which have been described may have
been actual globules of liquid resin, it would seem
desirable to emphasise again the fact that nearly all
the transparent substances which collectively make up
by far the greater part of an ordinary coal seam have
inevitably an amber-like or resinous appearance, and
that it is, therefore, very dangerous to base any con-
clusions as to chemical constitution on that fact. That
statement seems the more desirable, in view of the fact
that, so far as was known, resin secreting organs are