THE PRESERVATION OF PLANTS AS FOSSILS.
“The things, we know, are neither rich nor rare,
But wonder how the devil they got there.”
POPE, _Prologue to the Satires_.
The discovery of a fossil, whether as an impression on the surface of a
slab of rock or as a piece of petrified wood, naturally leads us back
to the living plant, and invites speculation as to the circumstances
which led to the preservation of the plant fragment. There is a certain
fascination in endeavouring, with more or less success, to picture
the exact conditions which obtained when the leaf or stem was carried
along by running water and finally sealed up in a sedimentary matrix.
Attempts to answer the question—How came the plant remains to be
preserved as fossils?—are not merely of abstract interest appealing
to the imagination, but are of considerable importance in the correct
interpretation of the facts which are to be gleaned from the records of
plant-bearing strata.
Before describing any specific examples of the commoner methods of
fossilisation; we shall do well to briefly consider how plants are
now supplying material for the fossils of a future age. In the great
majority of cases, an appreciation of the conditions of sedimentation,
and of the varied circumstances attending the transport and
accumulation of vegetable débris, supplies the solution of a problem
akin to that of the fly in amber and the manner in which it came there.
[Sidenote: OLD SURFACE-SOILS.]
Seeing that the greater part of the sedimentary strata have been formed
in the sea, and as the sea rather than the land has been for the most
part the scene of rock-building in the past, it is not surprising
that fossil plants are far less numerous than fossil animals. With
the exception of the algae and a few representatives of other classes
of plants, which live in the shallow-water belt round the coast,
or in inland lakes and seas, plants are confined to land-surfaces;
and unless their remains are swept along by streams and embedded in
sediments which are accumulating on the sea floor, the chance of their
preservation is but small. The strata richest in fossil plants are
often those which have been laid down on the floor of an inland lake
or spread out as river-borne sediment under the waters of an estuary.
Unlike the hard endo- and exo-skeletons of animals, the majority of
plants are composed of comparatively soft material, and are less likely
to be preserved or to retain their original form when exposed to the
wear and tear which must often accompany the process of fossilisation.
The Coal-Measure rocks have furnished numberless relics of a Palaeozoic
vegetation, and these occur in various forms of preservation in
rocks laid down in shallow water on the edge of a forest-covered
land. The underclays or unstratified argillaceous beds which nearly
always underlie each seam of coal have often been described as old
surface-soils, containing numerous remains of roots and creeping
underground stems of forest trees. The overlying coal has been regarded
as a mass of the carbonised and compressed débris of luxuriant forests
which grew on the actual spot now occupied by the beds of coal. There
are, however, many arguments in favour of regarding the coal seams as
beds of altered vegetable material which was spread out on the floor
of a lagoon or lake, while the underclay was an old soil covered by
shallow water or possibly a swampy surface tenanted by marsh-loving
plants[65].
The Jurassic beds of the Yorkshire Coast, long famous as some of the
richest plant-bearing strata in Britain, and the Wealden rocks of the
south coast afford examples of Mesozoic sediments which were laid
down on the floor of an estuary or large lake. Circumstances have
occasionally rendered possible the preservation of old land-surfaces
with the stumps of trees still in their position of growth. One of the
best examples of this in Britain are the so-called dirt-beds or black
bands of Portland and the Dorset Coast. On the cliffs immediately east
of Lulworth Cove, the surface of a ledge of Purbeck limestone which
juts out near the top of the cliffs, is seen to have the form here and
there of rounded projecting bosses or ‘Burrs’ several feet in diameter.
In the centre of each boss there is either an empty depression, or the
remnants of a silicified stem of a coniferous tree. Blocks of limestone
3 to 5 feet long and of about equal thickness may be found lying on
the rocky ledge presenting the appearance of massive sarcophagi in
which the central trough still contains the silicified remains of an
entombed tree. The calcareous sediment no doubt oozed up to envelope
the thick stem as it sank into the soft mud. An examination of the rock
just below the bed bearing these curious circular elevations reveals
the existence of a comparatively narrow band of softer material, which
has been worn away by denuding agents more rapidly than the overlying
limestone. This band consists of partially rounded or subangular stones
associated with carbonaceous material, and probably marks the site of
an old surface-soil. This old soil is well shown in the cliffs and
quarries of Portland, and similar dirt-beds occur at various horizons
in the Lower and Middle Purbeck Series[66]. In this case, then, we
have intercalated in a series of limestone beds containing marine and
freshwater shells two or three plant beds containing numerous and
frequently large specimens of cycadean and coniferous stems, lying
horizontally or standing in their original position of growth. These
are vestiges of an ancient forest which spread over a considerable
extent of country towards the close of the Jurassic period. The trunks
of cycads, long familiar in the Isle of Portland as fossil crows’
nests, have usually the form of round depressed stems with the central
portion somewhat hollowed out. It was supposed by the quarrymen that
they were petrified birds’ nests which had been built in the forks of
the trees which grew in the Portland forest. The beds separating the
surface-soils of the Purbeck Series, as seen in the sections exposed
on the cliffs or quarries, point to the subsidence of a forest-covered
area over which beds of water-borne sediment were gradually deposited,
until in time the area became dry land and was again taken possession
of by a subtropical vegetation, to be once more depressed and sealed up
under layers of sediment[67].
A still more striking example of the preservation of forest trees
rooted in an old surface-soil is afforded by the so-called fossil-grove
in Victoria Park, Glasgow, (Frontispiece). The stumps of several trees,
varying in diameter from about one to three feet, are fixed by long
forking ‘roots’ in a bed of shale. In some cases the spreading ‘roots,’
which bear the surface features of _Stigmaria_, extend for a distance
of more than ten feet from the base of the trunk. The stem surface is
marked by irregular wrinklings which suggest a fissured bark; but the
superficial characters are very imperfectly preserved. In one place a
flattened _Lepidodendron_ stem, about 30 feet long, lies prone on the
shale. Each of the rooted stumps is oval or elliptical in section, and
the long axes of the several stems are approximately parallel, pointing
to some cause operating in a definite direction which gave to the stems
their present form. Near one of the trees, and at a somewhat higher
level than its base, the surface of the rock is clearly ripple-marked,
and takes us back to the time when the sinking forest trees were washed
by waves which left an impress in the soft mud laid down over the
submerged area. The stumps appear to be those of Lepidodendron trees,
rooted in Lower Carboniferous rocks. From their manner of occurrence
it would seem that we have in them a corner of a Palaeozoic forest in
which Lepidodendra played a conspicuous part. The shales and sandstones
containing the fossil trees were originally overlain by a bed of
igneous rock which had been forced up as a sheet of lava into the
hardened sands and clays[68].
Other examples of old surface-soils occur in different parts of the
world and in rocks of various ages. As an instance of a land surface
preserved in a different manner, reference may be made to the thin
bands of reddish or brown material as well as clays and shale which
occasionally occur between the sheets of Tertiary lava in the Western
Isles of Scotland and the north-east of Ireland. In the intervals
between successive outpourings of basaltic lava in the north-west of
Europe during the early part of the Tertiary period, the heated rocks
became gradually cooler, and under the influence of weathering agents a
surface-soil was produced fit for the growth of plants. In some places,
too, shallow lakes were formed, and leaves, fruits and twigs became
embedded in lacustrine sediments, to be afterwards sealed up by later
streams of lava. In the face of the cliff at Ardtun Head on the coast
of Mull a leaf-bed is exposed between two masses of gravel underlying a
basaltic lava flow; the impressions of the leaves of _Gingko_ and other
plants from the Tertiary sediments of this district are exceptionally
beautiful and well preserved[69]. A large collection obtained by Mr
Starkie Gardner may be seen in the British Museum.
In 1883 the Malayan island of Krakatoa, 20 miles from Sumatra and
Java, was the scene of an exceptionally violent volcanic explosion.
Two-thirds of the island were blown away, and the remnant was left
absolutely bare of organic life. In 1886 it was found that several
plants had already established themselves on the hardened and weathered
crust of the Krakatoan rocks, the surface of the lavas having been to a
large extent prepared for the growth of the higher plants by the action
of certain blue-green algae which represent some of the lowest types
of plant life[70]. We may perhaps assume a somewhat similar state of
things to have existed in the volcanic area in north-west Europe, where
the intervals between successive outpourings of lava are represented by
the thin bands of leaf-beds and old surface-soils.
On the Cheshire Coast at Leasowe[71] and other localities, there is
exposed at low water a tract of black peaty ground studded with old
rooted stumps of conifers and other trees (fig. 6). There is little
reason to doubt that at all events the majority of the trees are in
their natural place of growth. The peaty soil on which they rest
contains numerous flattened stems of reeds and other plants, and is
penetrated by roots, probably of some aquatic or marshy plants which
spread over the site of the forest as it became gradually submerged. A
lower forest-bed rests directly on a foundation of boulder clay. Such
submerged forests are by no means uncommon around the British coast;
many of them belong to a comparatively recent period, posterior to the
glacial age. In many cases, however, the tree stumps have been drifted
from the places where they grew and eventually deposited in their
natural position, the roots of the trees, in some cases aided by stones
entangled in their branches, being heavier than the stem portion.
There is a promising field for botanical investigation in the careful
analysis of the floras of submerged forests; the work of Clement Reid,
Nathorst, Andersson and others, serves to illustrate the value of such
research in the hands of competent students.
[Illustration: FIG. 6. Part of a submerged Forest seen at low water on
the Cheshire Coast at Leasowe. Drawn from a photograph.]
The following description by Lyell, taken from his American travels,
is of interest as affording an example of the preservation of a
surface-soil:
“On our way home from Charleston, by the railway from Orangeburg, I
observed a thin black line of charred vegetable matter exposed in
the perpendicular section of the bank. The sand cast out in digging
the railway had been thrown up on the original soil, on which
the pine forest grew; and farther excavations had laid open the
junction of the rubbish and the soil. As geologists, we may learn
from this fact how a thin seam of vegetable matter, an inch or two
thick, is often the only monument to be looked for of an ancient
surface of dry land, on which a luxuriant forest may have grown
for thousands of years. Even this seam of friable matter may be
washed away when the region is submerged, and, if not, rain water
percolating freely through the sand may, in the course of ages,
gradually carry away the carbon[72].”
[Sidenote: FOSSIL WOOD.]
In addition to the remnants of ancient soils, and the preservation
of plant fragments in rocks which have been formed on the floor of
an inland lake or an estuary, it is by no means rare to find fossil
plants in obviously marine sediments. In fig. 7 we have a piece of
coniferous wood with the shell of an Ammonite (_Aegoceras planicosta_
Sow.) lying on it; the specimen was found in the Lower Lias clay at
Lyme Regis, and illustrates the accidental association of a drifted
piece of a forest tree with a shell which marks at once the age and the
marine character of the beds. Again in fig. 8 we have a block of flint
partially enclosing a piece of coniferous wood in which the internal
structure has been clearly preserved in silica. This specimen was found
in the chalk, a deposit laid down in the clear and deep water of the
Cretaceous sea. The wood must have floated for some time before it
became water-logged and sank to the sea-floor. In the light coloured
wood there occur here and there dark spots which mark the position of
siliceous plugs _b, b_ filling up clean cut holes bored by Teredos
in the woody tissue. The wood became at last enclosed by siliceous
sediment and its tissues penetrated by silica in solution, which
gradually replaced and preserved in wonderful perfection the form of
the original tissue. A similar instance of wood enclosed in flint was
figured by Mantell in 1844 in his _Medals of Creation_[73].
[Illustration: FIG. 7. _Aegoceras planicosta_ Sow. on a piece of
coniferous wood, Lower Lias, Lyme Regis. From a specimen in the
British Museum. Slightly reduced.]
[Illustration: FIG. 8. Piece of coniferous wood in flint, from the
Chalk, Croydon. Drawn from a specimen presented to the British Museum
by Mr Murton Holmes. In the side view, shown above in the figure, the
position of the wood is shown by the lighter portion, with holes, _b,
b_, bored by Teredos or some other wood-eating animal. In the end
view, below, the wood is seen as an irregular cylinder _w, w_,
embedded in a matrix of flint. ⅓ Nat. size.]
The specimen represented in fig. 9 illustrates the almost complete
destruction of a piece of wood by some boring animal. The circular and
oval dotted patches represent the filled up cavities made by a Teredo
or some similar wood-boring animal.
[Illustration: FIG. 9. Piece of wood from the Red Crag of Suffolk,
riddled with holes filled in with mud. From a specimen in the York
Museum. ⅓ Nat. size.]
[Sidenote: CONDITIONS OF FOSSILISATION.]
Before discussing a few more examples of fossils illustrating different
methods of fossilisation, it may not be out of place to quote a few
extracts from travellers’ narratives which enable us to realise more
readily the circumstances and conditions under which plant remains have
been preserved in the Earth’s crust.
In an account of a journey down the Rawas river in Sumatra, Forbes thus
describes the flooded country:—
“The whole surface of the water was covered, absolutely in a close
sheet, with petals, fruits and leaves, of innumerable species. In
placid corners sometimes I noted a collected mass nearly half a
foot deep, among which, on examination, I could scarcely find a
leaf that was perfect, or that remained attached to its rightful
neighbour, so that were they to become imbedded in some soft muddy
spot, and in after ages to reappear in a fossil form they would
afford a few difficult puzzles to the palaeontologist, both to
separate and to put together[74].”
An interesting example of the mixture of plants and animals in
sedimentary deposits is described by Hooker in his Himalayan Journals:—
“To the geologist the Jheels and Sunderbunds are a most instructive
region, as whatever may be the mean elevation of their waters,
a permanent depression of ten to fifteen feet would submerge an
immense tract, which the Ganges, Burrampooter, and Soormah would
soon cover with beds of silt and sand.
“There would be extremely few shells in the beds thus formed,
the southern and northern divisions of which would present two
very different floras and faunas, and would in all probability
be referred by future geologists to widely different epochs. To
the north, beds of peat would be formed by grasses, and in other
parts temperate and tropical forms of plants and animals would be
preserved in such equally balanced proportions as to confound the
palaeontologist; with the bones of the long-snouted alligator,
Gangetic porpoise, Indian cow, buffalo, rhinoceros, elephant,
tiger, deer, bear, and a host of other animals, he would meet with
acorns of several species of oak, pine-cones and magnolia fruits,
rose seeds, and _Cycas_ nuts, with palm nuts, screw-pines, and
other tropical productions[75].”
In another place the same author writes:
“On the 12th of January, 1848, the _Moozuffer_ was steaming amongst
the low, swampy islands of the Sunderbunds.... Every now and then
the paddles of the steamer tossed up the large fruits of _Nypa
fruticans_, Thunb., a low stemless palm that grows in the tidal
waters of the Indian Ocean, and bears a large head of nuts. It
is a plant of no interest to the common observer, but of much to
the geologist, from the nuts of a similar plant abounding in the
Tertiary formations at the mouth of the Thames, having floated
about there in as great profusion as here, till buried deep in the
silt and mud that now forms the island of Sheppey[76].”
[Sidenote: DRIFTING OF TREES.]
Of the drifting of timber, fruits, &c., we find numerous accounts in
the writings of travellers. Rodway thus describes the formation of
vegetable rafts in the rivers of Northern British Guiana:—
“Sometimes a great tree, whose timber is light enough to float,
gets entangled in the grass, and becomes the nucleus of an immense
raft, which is continually increasing in size as it gathers up
everything that comes floating down the river[77].”
The undermining of river banks in times of flood, and the transport of
the drifted trees to be eventually deposited in the delta is a familiar
occurrence in many parts of the world. The more striking instances of
such wholesale carrying along of trees are supplied by Bates, Lyell and
other writers. In his description of the Amazon the former writes:
“The currents ran with great force close to the bank, especially
when these receded to form long bays or _enseadas_, as they are
called, and then we made very little headway. In such places the
banks consist of loose earth, a rich crumbling vegetable mould,
supporting a growth of most luxuriant forest, of which the currents
almost daily carry away large portions, so that the stream for
several yards out is encumbered with fallen trees, whose branches
quiver in the current[78].”
In another place, Bates writes:
“The rainy season had now set in over the region through which the
great river flows; the sand-banks and all the lower lands were
already under water, and the tearing current, two or three miles in
breadth, bore along a continuous line of uprooted trees and islets
of floating plants[79].”
The rafts of the Mississippi and other rivers described by Lyell afford
instructive examples of the distant transport of vegetable material.
The following passage is taken from the _Principles of Geology_;
“Within the tropics there are no ice-floes; but, as if to
compensate for that mode of transportation, there are floating
islets of matted trees, which are often borne along through
considerable spaces. These are sometimes seen sailing at the
distance of fifty or one hundred miles from the mouth of the
Ganges, with living trees standing erect upon them. The Amazons,
the Orinoco, and the Congo also produce these verdant rafts[80].”
After describing the enormous natural rafts of the Atchafalaya, an arm
of the Mississippi, and of the Red river, Lyell goes on to say:
“The prodigious quantity of wood annually drifted down by the
Mississippi and its tributaries is a subject of geological
interest, not merely as illustrating the manner in which abundance
of vegetable matter becomes, in the ordinary course of nature,
imbedded in submarine and estuary deposits, but as attesting the
constant destruction of soil and transportation of matter to lower
levels by the tendency of rivers to shift their courses.... It
is also found in excavating at New Orleans, even at the depth of
several yards below the level of the sea, that the soil of the
delta contains innumerable trunks of trees, layer above layer, some
prostrate as if drifted, others broken off near the bottom, but
remaining still erect, and with their roots spreading on all sides,
as if in their natural position[81].”
The drifting of trees in the ocean is recorded by Darwin in his
description of Keeling Island, and their action as vehicles for the
transport of boulders is illustrated by the same account.
“In the channels of Tierra del Fuego large quantities of drift
timber are cast upon the beach, yet it is extremely rare to meet a
tree swimming in the water. These facts may possibly throw light
on single stones, whether angular or rounded, occasionally found
embedded in fine sedimentary masses[82].”
Fruits may often be carried long distances from land, and preserved
in beds far from their original source. Whilst cruising amongst the
Solomon Islands, the Challenger met with fruits of _Barringtonia
speciosa_ &c., 130–150 miles from the coast. Off the coast of New
Guinea long lines of drift wood were seen at right angles to the
direction of the river; uprooted trees, logs, branches, and bark, often
floating separately.
“The midribs of the leaves of a pinnate-leaved palm were abundant,
and also the stems of a large cane grass (_Saccharum_), like that
so abundant on the shores of the great river in Fiji. Various
fruits of trees and other fragments were abundant, usually floating
confined in the midst of the small aggregations into which the
floating timber was everywhere gathered.... Leaves were absent
except those of the Palm, on the midrib of which some of the
pinnæ were still present. The leaves evidently drop first to the
bottom, whilst vegetable drift is floating from a shore; thus, as
the débris sinks in the sea water, a deposit abounding in leaves,
but with few fruits and little or no wood, will be formed near
shore, whilst the wood and fruits will sink to the bottom farther
off the land. Much of the wood was floating suspended vertically
in the water, and most curiously, logs and short branch pieces
thus floating often occurred in separate groups apart from the
horizontally floating timber. The sunken ends of the wood were not
weighted by any attached masses of soil or other load of any kind;
possibly the water penetrates certain kinds of wood more easily in
one direction with regard to its growth than the other, hence one
end becomes water-logged before the other.... The wood which had
been longest in the water was bored by a _Pholas_[83].”
The bearing of this account on the manner of preservation of fossils,
and the differential sorting so frequently seen in plant beds, is
sufficiently obvious.
As another instance of the great distance to which land plants may be
carried out to sea and finally buried in marine strata, an observation
by Bates may be cited. When 400 miles from the mouth of the main
Amazons, he writes:
“We passed numerous patches of floating grass mingled with tree
trunks and withered foliage. Amongst these masses I espied many
fruits of that peculiar Amazonian tree the Ubussú Palm; this was
the last I saw of the great river[84].”
The following additional extract from the narrative of the Cruise of
H.M.S. Challenger illustrates in a striking degree the conflicting
evidence which the contents of fossiliferous beds may occasionally
afford; it describes what was observed in an excursion from Sydney to
Berowra Creek, a branch of the main estuary or inlet into which flows
the Hawkesbury river. It was impossible to say where the river came
to an end and the sea began. The Creek is described as a long tortuous
arm of the sea, 10 to 15 miles long, with the side walls covered with
orchids and _Platycerium_. The ferns and palms were abundant in the
lateral shady glens; marine and inland animals lived in close proximity.
“Here is a narrow strip of the sea water, twenty miles distant
from the open sea; on a sandy shallow flat close to its head are
to be seen basking in the sun numbers of sting-rays.... All over
these flats, and throughout the whole stretch of the creek, shoals
of Grey Mullet are to be met with; numerous other marine fish
inhabit the creek. Porpoises chase the mullet right up to the
commencement of the sand-flat. At the shores of the creek the rocks
are covered with masses of excellent oysters and mussel, and other
shell-bearing molluscs are abundant, whilst a small crab is to be
found in numbers in every crevice. On the other hand the water is
overhung by numerous species of forest trees, by orchids and ferns,
and other vegetation of all kinds; mangroves grow only in the
shallow bays. The gum trees lean over the water in which swim the
_Trygon_ and mullet, just as willows hang over a pool of carp. The
sandy bottom is full of branches and stems of trees, and is covered
in patches here and there by their leaves. Insects constantly fall
in the water, and are devoured by the mullet. Land birds of all
kinds fly to and fro across the creek, and when wounded may easily
be drowned in it. Wallabies swim across occasionally, and may add
their bones to the débris at the bottom. Hence here is being formed
a sandy deposit, in which may be found cetacean, marsupial, bird,
fish, and insect remains, together with land and sea shells, and
fragments of a vast land flora; yet how restricted is the area
occupied by this deposit, and how easily might surviving fragments
of such a record be missed by future geological explorers![85]”
[Sidenote: MEANING OF THE TERM ‘FOSSIL.’]
The term ‘fossil’ suggests to the lay mind a petrifaction or a
replacement by mineral matter of the plant tissues. In the scientific
sense, a fossil plant, that is a plant or part of a plant whether
in the form of a true petrifaction or a structureless mould or
cast, which has been buried in the earth by natural causes, may be
indistinguishable from a piece of recent wood lately fallen from the
parent tree. In the geologically recent peat beds such little altered
fossils (or sub-fossils) are common enough, and even in older rocks
the more resistant parts of plant fragments are often found in a
practically unaltered state. In the leaf impressions on an impervious
clay, the brown-walled epidermis shows scarcely any indication of
alteration since it was deposited in the soft mud of a river’s delta.
Such fossil leaves are common in the English Tertiary beds, and even in
Palaeozoic rocks it is not uncommon to find an impression of a plant
on a bed of shale from which the thin brown epidermis may be peeled
off the rock, and if microscopically examined it will be found to have
retained intact the contours of the cuticularised epidermal cells. A
striking example of a similar method of preservation is afforded by
the so-called paper-coal of Culm age from the Province of Toula in
Russia[86]. In the Russian area the Carboniferous or Permian rocks have
been subjected to little lateral pressure, and unlike the beds of the
same age in Western Europe, they have not been folded and compressed by
widespread and extensive crust-foldings. Instead of the hard seams of
coal there occur beds of a dark brown laminated material, made up very
largely of the cuticles of Lepidodendroid plants.
From such examples we may naturally pass to fossils in which the plant
structure has been converted into carbonaceous matter or even pure
coal. This form of preservation is especially common in plant-bearing
beds at various geological horizons. In other cases, again, some
mineral solution, oxide of iron, talc, and other substances, has
replaced the plant tissues. From the Coal-Measures of Switzerland Heer
has figured numerous specimens of fern fronds and other plants in which
the leaf form has been left on the dark coloured rock surface as a thin
layer of white talcose material[87]. In the Buntersandstone of the
Vosges and other districts the red imperfectly preserved impressions
of plant stems and leaves are familiar fossils[88]; the carbonaceous
substance of the tissues has been replaced by a brown or red oxide of
iron.
[Sidenote: INCRUSTATIONS.]
Plants frequently occur in the form of incrustations; and in fact
incrustations, which may assume a variety of forms, are the commonest
kind of fossil. The action of incrusting springs, or as they are
often termed petrifying springs, is illustrated at Knaresborough, in
Yorkshire, and many other places where water highly charged with
carbonate of lime readily deposits calcium carbonate on objects placed
in the path of the stream.
The travertine deposited in this manner forms an incrustation on plant
fragments, and if the vegetable substance is subsequently removed by
the action of water or decay, a mould of the embedded fragment is
left in the calcareous matrix. An instructive example of this form of
preservation was described in 1868[89] by Sharpe from an old gravel pit
near Northampton. He found in a section eight feet high (fig. 10), a
mass of incrusted plants of _Chara_ (_a_) resting on and overlain by a
calcareous paste (_c_) and (_d_) made up of the decomposed material of
the overlying rock, and this again resting on sand. The place where the
section occurred was originally the site of a pool in which Stoneworts
grew in abundance. Large blocks of these incrusted Charas may be seen
in the fossil-plant gallery of the British Museum.
[Illustration: FIG. 10. Section of an old pool filled up with a mass of
_Chara_. (From the _Geol. Mag._ vol. v. 1868, p. 563.)]
In the Natural History Museum in the Jardin des Plantes, Paris, one
of the table-cases contains what appear to be small models of flowers
in green wax. These are in reality casts in wax of the moulds or
cavities left in a mass of calcareous travertine, on the decay and
disappearance of the encrusted flowers and other plant fragments[90].
This porous calcareous rock occurs near Sézanne in Southern France,
and is of Eocene age[91]. The plants were probably blown on to the
freshly deposited carbonate of lime, or they may have simply fallen
from the tree on to the incrusting matrix; more material was afterwards
deposited and the flowers were completely enclosed. Eventually the
plant substance decayed, and as the matrix hardened moulds were left
of the vegetable fragments. Wax was artificially forced into these
cavities and the surrounding substance removed by the action of an
acid, and thus perfect casts were obtained of Tertiary flowers.
Darwin has described the preservation of trees in Van Diemen’s land
by means of calcareous substances. In speaking of beds of blown sand
containing branches and roots of trees he says:
“The whole became consolidated by the percolation of calcareous
matter; and the cylindrical cavities left by the decaying of the
wood were thus also filled up with a hard pseudo-stalactitical
stone. The weather is now wearing away the softer parts, and in
consequence the hard casts of the roots and branches of the trees
project above the surface, and, in a singularly deceptive manner,
resemble the stumps of a dead thicket[92].”
As a somewhat analogous method of preservation to that in travertine,
the occurrence of plants in amber should be mentioned. In Eocene times
there existed over a region, part of which is now the North-east
German coast, an extensive forest of conifers and other trees. Some
of the conifers were rich in resinous secretions which were poured
out from wounded surfaces or from scars left by falling branches.
As these flowed as a sticky mass over the stem or collected on the
ground, flowers, leaves, and twigs blown by the wind or falling from
the trees, became embedded in the exuded resin. Evaporation gradually
hardened the resinous substance until the plant fragments became
sealed up in a mass of amber, in precisely the same manner in which
objects are artificially preserved in Canada balsam. In many cases the
amber acts as a petrifying agent, and by penetrating the tissues of a
piece of wood it preserves the minute structural details in wonderful
perfection[93]. Dr Thomas in an account of the amber beds of East
Prussia in 1848, refers to the occurrence of large fossil trees; he
writes:
“The continuous changes to which the coast is exposed, often bring
to light enormous trunks of trees, which the common people had
long regarded as the trunks of the amber tree, before the learned
declared that they were the stems of palm trees, and in consequence
determined the position of Paradise to be on the coast of East
Prussia[94].”
[Sidenote: CASTS OF TREES.]
In 1887 an enormous fossil plant was discovered in a sandstone
quarry at Clayton near Bradford[95]. The fossil was in the form of a
sandstone cast of a large and repeatedly branched _Stigmaria_, and it
is now in the Owens College Museum, where it was placed through the
instrumentality of Prof. Williamson. The plant was found spread out
in its natural position on the surface of an arenaceous shale, and
overlain by a bed of hard sandstone identical with the material of
which the cast is composed. Williamson has thus described the manner of
formation of the fossil:
“It is obvious that the entire base of the tree became encased in a
plastic material, which was firmly moulded upon these roots whilst
the latter retained their organisation sufficiently unaltered to
enable them to resist all superincumbent pressure. This external
mould then hardened firmly, and as the organic materials decayed
they were floated out by water which entered the branching cavity;
at a still later period the same water was instrumental in
replacing the carbonaceous elements by the sand of which the entire
structure now consists[96].”
Although the branches have not been preserved for their whole length,
they extend a distance of 29 feet 6 inches from right to left, and 28
feet in the opposite direction.
The fossil represented in fig. 1 (p. 10), from the collection of Dr
John Woodward, affords a good example of a well-defined impression.
The surface of the specimen, of which a cast is represented in fig. 1,
shows very clearly the characteristic leaf-cushions and leaf-scars
of a _Lepidodendron_. The stem was embedded in soft sand, and as the
latter became hard and set, an impression was obtained of the external
markings of the _Lepidodendron_. Decay subsequently removed the
substance of the plant.
[Illustration: FIG. 11. _Equisetites columnaris_ Brongn. From a
specimen in the Woodwardian Museum, Cambridge. ⅓ nat. size.]
In fig. 11 some upright stems of a fossil Horse-tail (_Equisetites
columnaris_) from the Lower Oolite rocks near Scarborough, are seen in
a vertical position in sandstone. On the surface of the fossils there
is a thin film of carbonaceous matter, which is all that remains of the
original plant substance; the stems were probably floated into their
present position and embedded vertically in an arenaceous matrix. The
hollow pith-cavity was filled with sand, and as the tissues decayed
they became in part converted into a thin coaly layer. The vertical
position of such stems as those in fig. 11 naturally suggests their
preservation _in situ_, but in this as in many other cases the erect
manner of occurrence is due to the settling down of the drifted plants
in this particular position.
[Sidenote: FOSSIL CASTS.]
An example of _Stigmaria_ drawn in fig. 12 further illustrates the
formation of casts[97]. The outer surface with the characteristic
spirally arranged circular depressions, represents the wrinkled bark of
the dried plant; the smaller cylinder, on the left side of the upper
end (fig. 12, 2, _p_) marks the position of the pith surrounded by the
secondary wood, which has been displaced from its axial position. The
pith decayed first, and the space was filled in with mud; somewhat
later the wood and cortex were partially destroyed, and the rod of
material which had been introduced into the pith-cavity dropped towards
one side of the decaying shell of bark.
[Illustration: FIG. 12. _Stigmaria ficoides_ Brongn. 1. Side view,
showing wrinkled surface and the scars of appendages. 2. End view
(upper) showing the displaced central cylinder; _p_, pith, _x_,
xylem, _r_, medullary rays. 3. End view (lower). From a specimen in
the Woodwardian Museum. ½ nat. size.]
As the parenchymatous medullary rays readily decayed, the mud in
the pith extended outwards between the segments of wood which still
remained intact, and so spokes of argillaceous material were formed
which filled the medullary ray cavities. The cortical tissues were
decomposed, and their place taken by more argillaceous material. At one
end of the specimen (fig. 12, 3) we find the wood has decayed without
its place being afterwards filled up with foreign material. At the
opposite end of the specimen, the woody tissue has been partially
preserved by the infiltration of a solution containing carbonate of
lime (fig. 12, 2).
Numerous instances have been recorded from rocks of various geological
ages of casts of stems standing erect and at right angles to the
bedding of the surrounding rock. These vertical trees occasionally
attain a considerable length, and have been formed by the filling
in by sand or mud of a pipe left by the decay of the stem. It is
frequently a matter of some difficulty to decide how far such fossils
are in the position of growth of the tree, or whether they are merely
casts of drifted stems, which happen to have been deposited in an
erect position. The weighting of floating trees by stones held in the
roots, added to the greater density of the root wood, has no doubt
often been the cause of this vertical position. In attempting to
determine if an erect cast is in the original place of growth of the
tree, it is important to bear in mind the great length of time that
wood is able to resist decay, especially under water. The wonderful
state of preservation of old piles found in the bed of a river, and
the preservation of wooden portions of anchors of which the iron has
been completely removed by disintegration, illustrate this power of
resistance. In this connection, the following passage from Lyell’s
travels in America is of interest. In describing the site of an old
forest, he writes[98]:
“Some of the stumps, especially those of the fir tribe, take fifty
years to rot away, though exposed in the air to alternations of
rain and sunshine, a fact on which every geologist will do well
to reflect, for it is clear that the trees of a forest submerged
beneath the water, or still more, if entirely excluded from the
air, by becoming imbedded in sediment, may endure for centuries
without decay, so that there may have been ample time for the slow
petrifaction of erect fossil trees in the Carboniferous and other
formations, or for the slow accumulation around them of a great
succession of strata.”
In another place, in speaking of the trees in the Great Dismal Swamp,
Lyell writes:—“When thrown down, they are soon covered by water,
and keeping wet they never decompose, except the sap wood, which is
less than an inch thick[99].” We see, then, that trees may have
resisted decay for a sufficiently long time to allow of a considerable
deposition of sediment. It is very difficult to make any computation of
the rate of deposition of a particular set of sedimentary strata, and,
therefore, to estimate the length of time during which the fossil stems
must have resisted decay.
[Sidenote: PLANTS AND COAL.]
The protective qualities of humus acids, apart from the almost complete
absence of Bacteria[100] from the waters of Moor- or Peat-land, is a
factor of great importance in the preservation of plants against decay
for many thousands of years.
From examples of fossil stems or leaves in which the organic material
has been either wholly or in part replaced by coal, we may pass by a
gradual transition to a mass of opaque coal in which no plant structure
can be detected. It is by no means uncommon to notice on the face of a
piece of coal a distinct impression of a plant stem, and in some cases
the coal is obviously made up of a number of flattened and compressed
branches or leaves of which the original tissues have been thoroughly
carbonised. A block of French coal, represented in fig. 13, consists
very largely of laminated bands composed of the long parallel veined
leaves of the genus _Cordaites_ and of the bark of _Lepidodendron_,
_Sigillaria_, and other Coal-Measure genera. The long rhizomes and
roots below the coal are preserved as casts in the underclay.
In examining thin sections of coal, pieces of pitted tracheids or
crushed spores are frequently met with as fragments of plant structures
which have withstood decay more effectually than the bulk of the
vegetable débris from which the coal was formed.
The coaly layer on a fossil leaf is often found to be without any trace
of the plant tissues, but not infrequently such carbonised leaves, if
treated with certain reagents and examined microscopically, are seen
to retain the outlines of the epidermal cells of the leaf surface. If
a piece of the Carbonaceous film detached from a fossil leaf is left
for some days in a small quantity of nitric acid containing a crystal
of chlorate of potash, and, after washing with water, is transferred
to ammonia, transparent film often shows very clearly the outlines
of the epidermal cell and the form of the stomata. Such treatment has
been found useful in many cases as an aid to determination[101]. Prof.
Zeiller informs me that he has found it particularly satisfactory in
the case of cycadean leaves.
[Illustration: FIG. 13. Part of a coal seam largely made up of
_Cordaites_ leaves. _Stigmaria_ and _Stigmariopsis_ shown in the rock
(underclay) underlying the coal. (After Grand’Eury [82] Pl. I. fig.
3.)]
[Sidenote: FOSSILS IN HALF-RELIEF.]
It is sometimes possible to detach the thin lamina representing the
carbonised leaf or other plant fragment from the rock on which it lies
and to mount it whole on a slide. Good examples of plants treated in
this way may be seen in the Edinburgh and British Museums, especially
_Sphenopteris_ fronds from the Carboniferous oil shales of Scotland.
In the excellent collection of fossil plants in Stockholm there are
still finer examples of such specimens, obtained by Dr Nathorst from
some of the Triassic plants of Southern Sweden. In a few instances the
tissues of a plant have been converted into coal in such a manner as
to retain the form of the individual cells, which appear in section
as a black framework in a lighter coloured matrix. Examples of such
carbonised tissues were figured by some of the older writers, and
Solms-Laubach has recently[102] described sections of Palaeozoic plants
preserved in this manner. The section represented in fig. 70 is that
of a Calamite stem (8 × 9·5 cm.) in which the wood has been converted
into carbonaceous material, but the more delicate tissues have been
almost completely destroyed. The thin and irregular black line a
little distance outside the ring of wood, and forming the limit of the
drawing, probably represents the cuticle. The whole section is embedded
in a homogeneous matrix of calcareous rock, in which the more resistant
tissues of the plant have been left as black patches and faint lines.
Mention should be made of a special form of preservation which has been
described as fossilisation in half-relief. If a stem is imbedded in
sand or mud, the matrix receives an impression of the plant surface,
and if the hollow pith-cavity is filled with the surrounding sediment,
the surface of the medullary cast will exhibit markings different from
those seen on the surface in contact with the outside of the stem. The
space separating the pith-cast from the mould bearing the impression of
the stem surface may remain empty, or it may be filled with sedimentary
material. In half-relief fossils, on the other hand, we have projecting
from the under surface of a bed a more or less rounded and prominent
ridge with certain surface markings, and fitting into a corresponding
groove in the underlying rock on which the same markings have been
impressed. It is conceivable that such a cast might be obtained if
soft plant fragments were lying on a bed of sand, and were pressed into
it by the weight of superincumbent material. The plant fragment would
be squeezed into a depression, and its substance might eventually be
removed and leave no other trace than the half-relief cast and hollow
mould. A twig lying on sand would by its own weight gradually sink a
little below the surface; if it were then blown away or in some manner
removed, the depression would show the surface features of the twig.
When more sand came to be spread out over the depression, it would
find its way into the pattern of the mould, and so produce a cast. If
at a later period when the sand had hardened, the upper portion were
separated from the lower, from the former there would project a rounded
cast of the hollow mould. The preservation of soft algae as half-relief
casts has been doubted by Nathorst[103] and others as an unlikely
occurrence in nature. They prefer to regard such ridges on a rock face
as the casts of the trails or burrows of animals. This question of the
preservation of the two sides of a mould showing the same impression
of a plant has long been a difficult problem; it is discussed by
Parkinson in his _Organic Remains_. In one of the letters (No. XLVI.),
he quotes the objection of a sceptical friend, who refuses to believe
such a manner of preservation possible, “until,” says Parkinson, “I
can inform him if, by involving a guinea in plaster of Paris, I could
obtain two impressions of the king’s head, without any impression of
the reverse[104].”
It would occupy too much space to attempt even a brief reference to the
various materials in which impressions of plants have been preserved.
Carbonaceous matter is the most usual substance, and in some cases
it occurs in the form of graphite which on dark grey or black rocks
has the appearance of a plant drawn in lead pencil. The impressions
of plants on the Jurassic (Kimeridgian) slates of Solenhofen[105] in
Bavaria, like those on the Triassic sandstones of the Vosges, are
usually marked out in red iron oxide.
[Sidenote: PETRIFIED TREES.]
So far we have chiefly considered examples of plants preserved in
various ways by _incrustation_, that is, by having been enclosed in
some medium which has received an impression of the surface of the
plant in contact with it. By far the most valuable fossil specimens
from a botanical point of view are however those in which the internal
structure has been preserved; that is in which the preserving medium
has not served merely as an encasing envelope or internal cast, but has
penetrated into the body of the plant fragment and rendered permanent
the organization of the tissues. In almost every Natural History or
Geological Museum one meets with specimens of petrified trees or
polished sections of fossil palm stems and other plants, in which
the internal structure has been preserved in siliceous material, and
admits of detailed investigation in thin sections under the microscope.
Silica, calcium carbonate, with usually a certain amount of carbonate
of iron and magnesium carbonate, iron pyrites, amber, and more rarely
calcium fluoride or other substances have taken the place of the
original cell-walls. Of silicified stems, those from Antigua, Egypt,
Central France, Saxony, Brazil, Tasmania[106], and numerous other
places afford good examples. Darwin records numerous silicified stems
in Northern Chili, and the Uspallata Pass. In the central part of the
Andes range, 7000 feet high, he describes the occurrence of “Snow-white
projecting silicified columns.... They must have grown,” he adds, “in
volcanic soil, and were subsequently submerged below sea-level, and
covered with sedimentary beds and lava-flows[107].” A striking example
of the occurrence of numerous petrified plant stems has been described
by Holmes from the Tertiary forests of the Yellowstone Park. From
the face of a cliff on the north side of Amethyst mountain “rows of
upright trunks stand out on the ledges like the columns of a ruined
temple. On the more gentle slopes farther down, but where it is still
too steep to support vegetation, save a few pines, the petrified
trunks fairly cover the surface, and were at first supposed by us to
be the shattered remains of a recent forest[108].” Marsh[109] and
Conwentz[110] have described silicified trees more than fifty feet
in length from a locality in California where several large forest
trees of Tertiary age have been preserved in volcanic strata. In South
Africa on the Drakenberg hills there occur numerous silicified trunks,
occasionally erect and often lying on the ground, probably of Triassic
age[111]. In some instances the specimens measure several feet in
length and diameter. Some of the coniferous stems seen in Portland,
and occasionally met with reared up against a house side, illustrate
the silicification of plant structure on a large scale. These are of
Upper Jurassic (Purbeck) age. From Grand’Croix in France a silicified
stem of _Cordaites_ of Palaeozoic age has been recorded with a length
of twenty meters. The preservation of plants by siliceous infiltrations
has long been known. One of the earliest descriptions of this form
of petrifaction in the British Isles is that of stems found in Lough
Neagh, Ireland. In his lectures on Natural Philosophy, published at
Dublin in 1751, Barton gives several figures of Irish silicified wood,
and records the following occurrence in illustration of the peculiar
properties erroneously attributed to the waters of Lough Neagh.
Describing a certain specimen (No. XXVI), he writes:—
“This is a whetstone, which as Mr Anthony Shane, apothecary, who
was born very near the lake, and is now alive, relates, he made by
putting a piece of holly in the water of the lake near his father’s
house, and fixing it so as to withstand the motion of the water,
and marking the place so as to distinguish it, he went to Scotland
to pursue his studies, and seven years after took up a stone
instead of holly, the metamorphosis having been made in that time.
This account he gave under his handwriting. The shore thereabouts
is altogether loose sand, and two rivers discharge themselves into
the lake very near that place[112].”
The well-known petrified trees from the neighbourhood of Lough Neagh
are probably of Pliocene age, but their exact source has been a matter
of dispute[113].
[Sidenote: PETRIFIED WOOD.]
In 1836 Stokes described certain stems in which the tissues had been
partially mineralised. In describing a specimen of beech from a Roman
aqueduct at Eibsen in Lippe Bückeburg], he says:—
“The wood is, for the most part, in the state of very old dry wood,
but there are several insulated portions, in which the place of the
wood has been taken by carbonate of lime. These portions, as seen
on the surface of the horizontal section, are irregularly circular,
varying in size, but generally a little less or more than ⅛ inch in
diameter, and they run through the whole thickness of the specimen
in separate, perpendicular columns. The vessels of the wood are
distinctly visible in the carbonate of lime, and are more perfect
in their form and size in those portions of the specimen than in
that which remains unchanged[114].”
[Illustration: FIG. 14.
A. _Araucarioxylon Withami_ (L. and H.). Radiating lines of
crystallisation in secondary wood, as seen in transverse
section.
B. _Lepidodendron_ sp. Concentric lines of crystallisation, and
scalariform tracheids, as seen in longitudinal section.]
This partial petrifaction of the structure in patches is often met with
in fossil stems, and may be seriously misleading to those unfamiliar
with the appearance presented by the crystallisation of silica from
scattered centres in a mass of vegetable tissue. A good example of
this is afforded by the gigantic stems discovered in 1829 in the
Craigleith Quarry near Edinburgh[115]. Of those two large stems found
in the Sandstone rock, the longest, originally 11 meters long and
3·3–3·9 meters in girth, is now set up in the grounds of the British
Museum, and a large polished section (1 m. × 87 cm.) is exhibited in
the Fossil-plant Gallery. The other stem is in the Botanic Garden,
Edinburgh. Transverse sections of the wood of the London specimen show
scattered circular patches (fig. 14 A) in the mineralised wood in which
the tracheids are very clearly preserved; while in the other portion
the preservation is much less perfect. The patch of tissue in fig. 14
A shows a portion of the wood of the Craigleith tree [_Araucarioxylon
Withami_ (L. and H.)] in which the mineral matter, consisting of
dolomite with a little silica here and there, has crystallised in such
a manner as to produce what is practically a cone-in-cone structure
on a small scale, which has partially obliterated the structural
features. This minute cone-in-cone structure is not uncommon in
petrified tissues; it is precisely similar in appearance to that
described by Cole[116] in certain minerals. The crystallisation has
been set up along lines radiating from different centres, and the
particles of the tissue have been pushed as it were along these lines.
[Illustration: FIG. 15. Transverse section of the central cylinder of a
Carboniferous Lepidodendroid stem in the collection of Mr Kidston.
From Dalmeny, Scotland. _s._ Silica filling up the central portion of
the pith. _p._ Remains of the pith tissue. _x_¹. Primary xylem. _x_².
Secondary xylem. _c._ Innermost cortex.]
[Sidenote: PRESERVATION OF TISSUES.]
A somewhat different crystallisation phenomenon is illustrated by
the extremely fine section of a Lepidodendroid plant shown in fig.
15. The tissues of the primary and secondary wood (_x_¹ and _x_²)
are well preserved throughout in silica, but scattered through the
siliceous matrix there occur numerous circular patches, as seen in the
figure. One of these is more clearly shown in fig. 14 B drawn from
a longitudinal section through the secondary wood, _x_²; it will be
noticed that where the concentric lines of the circular patch occur,
the scalariform thickenings of the tracheids are sharply defined, but
immediately a tracheid is free of the patch these details are lost.
It would appear that in this case silicification was first completed
round definite isolated centres, and the secondary crystallisation in
the matrix partially obliterated some of the more delicate structural
features. The same phenomenon has been observed in oolitic rocks[117],
in which the oolitic grains have resisted secondary crystallisation and
so retained their original structure.
Among the most important examples of silicified plants are those from
a few localities in Central France. In the neighbourhood of Autun
there used to be found in abundance loose nodules of siliceous rock
containing numerous fragments of seeds, twigs, and leaves of different
plants. The rock of which the broken portions are found on the surface
of the ground was formed about the close of the Carboniferous period.
At the hands of French investigators the microscopic examination of
these fragments of a Palaeozoic vegetation have thrown a flood of
light on the anatomical structure of many extinct types. Sometimes the
silica has penetrated the cavities of the cells and vessels, and the
walls have decayed without their substance being replaced by mineral
material. Sections of tissues preserved in this manner, if soaked in a
coloured solution assume an appearance almost identical with that of
stained sections of recent plants. The spaces left by the decayed walls
act as fine capillaries and suck up the coloured solution[118].
[Illustration: FIG. 16. Internal cast of a sclerenchymatous cell from
the root of a Cretaceous fern (_Rhizodendron oppoliense_ Göpp.).
After Stenzel (86) Pl. III. fig. 29. × 240 and reduced to one-half.]
In the Coal-Measure sandstones of England large pieces of woody
stems are occasionally met with in which the mineralisation has been
incomplete. A brown piece of fossil stem lying in a bed of sandstone
shows on the surface a distinct woody texture, and the lines of wood
elements are clearly visible. The whole is, however, very friable and
falls to pieces if an attempt is made to cut thin sections of it;
the tracheids of the wood easily fall apart owing to the walls being
imperfectly preserved, and the absence of a connecting framework
such as would have been formed had the membranes been thoroughly
silicified. It is occasionally possible to obtain from petrified plant
stems perfect casts in silica or other substances of the cavity of
a sclerenchymatous fibre, in which the mineral has been deposited
not only in the cavity but in the fine pit-canals traversing the
lignified walls. Such a cast is represented in fig. 16, the fine
lateral projections are the delicate casts of the pit canals. Numerous
instances of minute and delicate tissues preserved in silica are
recorded in later chapters. A somewhat unusual type of silicification
is met with in some of the Gondwana rocks of India, in which cycadean
fronds occur as white porcellaneous specimens showing a certain amount
of internal structure in a siliceous matrix. Specimens of such leaves
may be seen in the British Museum.
[Sidenote: COAL-BALLS.]
In the Coal-Measures of England, especially in the neighbourhood of
Halifax in Yorkshire, and in South Lancashire, the seams of coal
occasionally contain calcareous nodules varying in size from a nut to
a man’s head, and consisting of about 70% of carbonate of calcium and
magnesium, and 30% of oxide of iron, sulphide of iron, &c.[119] The
nodules, often spoken of by English writers as ‘coal-balls,’ contain
numerous fragments of plants in which the minute cellular structure is
preserved with remarkable perfection. It should be noted that the term
coal-ball is also applied to rounded or subangular pieces of coal which
are occasionally met with in coal seams, and especially in certain
French coal fields. To avoid confusion it is better to speak of the
plant-containing nodules as calcareous nodules, restricting the term
coal-ball to true coal pebbles. A section of a calcareous nodule, when
seen under the microscope, presents the appearance of a matrix of a
crystalline calcareous substance containing a heterogeneous mixture of
all kinds of plant tissues, usually in the form of broken pieces and in
a confused mass.
[Illustration: FIG. 17. A thin section of a calcareous nodule from the
Coal-Measures. Binney collection, Woodwardian Museum, Cambridge. Very
slightly reduced.]
A large section of one of these nodules (12·5 cm. × 8·5 cm.) is
shown in fig. 17. It illustrates the manner of occurrence of various
fragments of different plants in which the structure has been
more or less perfectly preserved. In this particular example we
see sections of _Myeloxylon_ (I), _Calamites_ (II), Fern petioles
(_Rachiopteris_) (III), Stigmarian appendages (IV), Lepidodendroid
leaves (V), _Myeloxylon_ pinnules (VI), Gymnospermous seeds (VII),
Twig of a _Lepidodendron_, showing the central xylem cylinder and
large leaf-bases on the outer cortex, (VIII), Sporangia and spores
of a strobilus (IX), Tangential section of a _Myeloxylon_ petiole
(X), _Rachiopteris_ sp. (XI), _Rachiopteris_ sp. (XII), Band of
sclerenchymatous tissue (XIII), _Rachiopteris_ sp. (XIV).
The general appearance of a calcareous plant-nodule suggests a soft
pulpy mass of decaying vegetable débris, through which roots were able
to bore their way, as in a piece of peat or leafy mould. Overlying
this accumulation of soft material there was spread out a bed of muddy
sediment containing numerous calcareous shells, which supplied the
percolating water with the material which was afterwards deposited in
portions of the vegetable débris. According to this view the calcareous
nodules of the coal seams represent local patches of a widespread
mass of débris which were penetrated by a carbonated solution, and so
preserved as samples of a decaying mass of vegetation, of which by far
the greater portion became eventually converted into coal[120].
[Sidenote: FOSSIL NUCLEI.]
In such nodules, we find that not only has the framework of the
tissues been preserved, but frequently the remains of cell contents
are clearly seen. In some cases the cells of a tissue may contain in
each cavity a darker coloured spot, which is probably the mineralised
cell nucleus. (Fig. 42, _A_, 1, p. 214.) The contents of secretory
sacs, such as those containing gum or resin, are frequently found as
black rods filling up the cavity of the cell or canal. The contents
of cells in some cases closely simulate starch grains, and such may
have been actually present in the tissues of a piece of a fossil
dicotyledonous stem described by Thiselton-Dyer from the Lower Eocene
Thanet beds[121], and in the rhizome of a fossil _Osmunda_ recorded by
Carruthers[122]. (Fig. 42, _B_, p. 214.)
Schultze in 1855[123] recorded the discovery of cellulose by
microchemical tests applied to macerated tissue from Tertiary lignite
and coal. With reference to the possibility of recognising cell
contents in fossil tissue it is interesting to find that Dr Murray
of Scarborough had attempted, and apparently with success, to apply
chemical tests to the tissues of Jurassic leaves. In a letter written
to Hutton in 1833 Murray speaks of his experiments as follows:—
“Reverting to the Oolitic plants, I have again and with better
success been experimenting upon the thin transparent films of
leaves, chiefly of _Taeniopteris vittata_ and _Cyclopteris_, which
from their tenuity offer fine objects for the microscope.... By
many delicate trials I have ascertained the existence still in
these leaves of resin and of tannin.... I am seeking among the
filmy leaves of the _Fucoides_ of A. Brongniart for iodine, but
hitherto without success, and indeed can hardly expect it, as
probably did iodine exist in them, it must have long ago entered
into new combinations[124].”
Apart from this difficulty, it is not surprising that Dr Murray’s
search for iodine was unsuccessful, considering how little algal nature
most of the so-called Fucoids possess.
Some of the most perfectly preserved tissues as regards the details of
cell contents are those of gymnospermous seeds from Autun. In sections
of one of these seeds which I recently had the opportunity of examining
in Prof. Bertrand’s collection, the parenchymatous cells contained very
distinct nuclei and protoplasmic contents. In one portion of the tissue
in the nucellus of _Sphaerospermum_ the cell walls had disappeared,
but the nuclei remained in a remarkable state of preservation. The
cells shown in fig. 42 are from the ground tissue of a petiole of
_Cycadeoidea gigantea_ Sew.[125], a magnificent Cycadean stem from
Portland recently added to the British Museum collection; in the cell
_A_, 1, the nucleus is fairly distinct and in 2 and 4 the contracted
cell-contents is clearly seen. Other interesting examples of fossil
nuclei are seen in a _Lyginodendron_ leaf figured by Williamson and
Scott in a recent Memoir on that genus[126]. Each mesophyll cell
contains a single dark nucleus. The mineralisation of the most delicate
tissues and the preservation of the various forms of cell-contents
are now generally admitted by those at all conversant with the
possibilities of plant petrifaction. If we consider what these facts
mean—the microscopic investigation of not only the finest framework
but even the very life-substance of Palaeozoic plants—we feel that
the aeons since the days when these plants lived have been well-nigh
obliterated.
Occasionally the plant tissues have assumed a black and somewhat ragged
appearance, giving the impression of charred wood. A section of a
recent burnt piece of wood resembles very closely some of the fossil
twigs from the coal seam nodules. It is possible that in such cases we
have portions of mineralised tissues which were first burnt in a forest
fire or by lightning and then infiltrated with a petrifying solution.
An example of one of these black petrified plants is shown in fig. 74
B. Chap. X. In many of the fossil plants there are distinct traces of
fungus or bacterial ravages, and occasionally the section of a piece
of mineralised wood shows circular spaces or canals which have the
appearance of being the work of some wood-eating animal, and small oval
bodies sometimes occur in such spaces which may be the coprolites of
the xylophagous intruder. (Fig. 24, p. 107.)
[Sidenote: FOSSIL PLANTS IN VOLCANIC ASH.]
It is well known to geologists that during the Permian and
Carboniferous periods the southern portion of Scotland was the scene of
widespread volcanic activity. Forests were overwhelmed by lava-streams
or showers of ash, and in some districts tree stems and broken plant
fragments became sealed up in a volcanic matrix. Laggan Bay in the
north-east corner of the Isle of Arran, and Pettycur a short distance
from Burntisland on the north shore of the Firth of Forth, are two
localities where petrified plants of Carboniferous age occur in such
preservation as allows of a minute investigation of their internal
structure. The occurrence of plants in the former locality was first
discovered by Mr Wünsch of Glasgow; the fossils occur in association
with hardened shales and beds of ash, and are often exceedingly well
preserved[127]. In fig. 18 is reproduced a sketch of a hollow tree
trunk from Arran, probably a _Lepidodendron_ stem, in which only the
outer portion of the bark has been preserved, while the inner cortical
tissues have been removed and the space occupied by volcanic detritus.
[Illustration: FIG. 18. Diagrammatic sketch of a slab cut from a fossil
stem (_Lepidodendron_?) from Laggan Bay. _e_, Imperfectly preserved
bark of a large stem, extending in patches round the periphery of
the specimen; the oval and circular bodies in the interior are the
xylem portions of the central cylinders of _Lepidodendron_ stems,
_x_¹, primary wood, _x_², secondary wood. From a specimen in the
Binney collection, Woodwardian Museum, Cambridge. ⅕ nat. size.]
The smaller cylindrical structures in the interior of the hollow trunk
are the central woody cylinders of Lepidodendroid trees; each consists
of an axial pith surrounded by a band of primary wood and a broader
zone of secondary wood. One of the axes probably belonged to the stem
of which only the shell has been preserved, the others must have come
from other trees and may have been floated in by water[128]. The
microscopic details of the wood and outer cortex have in this instance
been preserved in a calcareous material, which was no doubt derived by
water percolating through the volcanic ash. It is frequently found that
in fossil trees or twigs a separation of the tissues has taken place
along such natural lines of weakness as the cambium or the phellogen,
before the petrifying medium had time to permeate the entire structure.
Tree stems recently killed by lava streams during volcanic eruptions at
the present day supply a parallel with the Palaeozoic forest trees of
Carboniferous times.
Guillemard in describing a volcanic crater in Celebes, speaks of burnt
trees still standing in the lava stream, “so charred at the base of
the trunk that we could easily push them down[129].” An interesting
case is quoted by Hooker in his _Himalayan Journals_, illustrating the
occurrence of a hollow shell of a tree, in which the outer portions of
a stem had been left while the inner portions had disappeared, the wood
being hollow and so favourable to the production of a current of air
which accelerated the destruction of the internal tissues.
On the coast near Burntisland on the Firth of Forth blocks of rock are
met with in which numerous plant fragments of Carboniferous age are
scattered in a confused mass through a calcareous volcanic matrix. The
twigs, leaves, spores, and other portions are in small fragments, and
their delicate cells are often preserved in wonderful perfection.
[Sidenote: CONDITIONS OF PRESERVATION.]
The manner of occurrence of plants in sandstones, shales or other rocks
is often of considerable importance to the botanist and geologist, as
an aid to the correct interpretation of the actual conditions which
obtained at the time when the plant remains were accumulating in beds
of sediment. To attempt to restore the conditions under which any
set of plants became preserved, we have to carefully consider each
special case. A nest of seeds preserved as internal casts in a mass
of sandstone, such as is represented by the block of Carboniferous
sandstone in fig. 19, suggests a quiet spot in an eddy where seeds
were deposited in the sandy sediment. Delicate leaf structures with
sporangia still intact, point to quietly flowing water and a transport
of no great distance. Occasionally the large number of delicate and
light plant fragments, associated it may be with insect wings, may
favour the idea of a wind storm which swept along the lighter pieces
from a forest-clad slope and deposited them in the water of a lake. In
some Tertiary plant-beds the manner of occurrence of leaves and flowers
is such as to suggest a seasonal alternation, and the different layers
of plant débris may be correlated with definite seasons of growth[130].
[Illustration: FIG. 19. Piece of Coal-Measures Sandstone with casts of
_Trigonocarpon_ seeds, from Peel Quarry near Wigan. From a specimen
in the Manchester Museum, Owens College. ½ nat. size.]
The predominance of certain classes of plants in a particular bed may
be due to purely mechanical causes and to differential sorting by
water, or it may be that the district traversed by the stream which
carried down the fragments was occupied almost exclusively by one set
of plants. The trees from higher ground may be deposited in a different
part of a river’s course to those growing in the plains or lowland
marshes. It is obviously impossible to lay down any definite rules
as to the reading of plant records, as aids to the elucidation of
past physical and botanical conditions. Each case must be separately
considered, and the various probabilities taken into account, judging
by reference to the analogy of present day conditions.
Various attempts, more or less successful, have been made to imitate
the natural processes of plant mineralisation[131]. By soaking sections
of wood for some time in different solutions, and then exposing them
to heat, the organic substance of the cell walls has been replaced by
a deposit of oxide of iron and other substances. Fern leaves heated to
redness between pieces of shale have been reduced to a condition very
similar to that of fossil fronds. Pieces of wood left for centuries
in disused mines have been found in a state closely resembling
lignite[132]. Attempts have also been made to reproduce the conditions
under which vegetable tissues were converted into coal, but as yet
these have not yielded results of much scientific value. The Geysers
of Yellowstone Park have thrown some light on the manner in which wood
may be petrified by the percolation of siliceous solutions; and it has
been suggested that the silicification of plants may have been effected
by the waters of hot springs holding silica in solution. Examples of
wood in process of petrifaction in the Geyser district of North America
have been recorded by Kuntze[133], and discussed by Schweinfurth[134],
Solms-Laubach[135] and others[136]. The latter expresses the opinion
that by a long continuance of such action as may now be observed in the
neighbourhood of hot springs, the organic substance of wood might be
replaced by siliceous material. The exact manner of replacement needs
more thorough investigation. Kuntze describes the appearance of forest
trees which have been reached by the waters of neighbouring Geysers.
The siliceous solution rises in the wood by capillarity; the leaves,
branches and bark are gradually lost, and the outer tissues of the
wood become hardened and petrified as the result of evaporation from
the exposed surface of the stem. The products of decay going on in the
plant tissues must be taken into account, and the double decomposition
which might result. There is no apparent reason why experiments
undertaken with pieces of recent wood exposed to permeation by various
calcareous and siliceous solutions under different conditions should
not furnish useful results.