CHAPTER IV.
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.
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.
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].”
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].
Fig. 7. Aegoceras planicosta Sow. on a piece of coniferous wood, Lower Lias, Lyme Regis. From a specimen in the British Museum. Slightly reduced.
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.
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.
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].”
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]”
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.
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.
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].”
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.
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.
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.
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.
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.
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.)
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.
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].
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].”
Fig. 14.
- Araucarioxylon Withami (L. and H.). Radiating lines of crystallisation in secondary wood, as seen in transverse section.
- 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.
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. x1. Primary xylem. x2. Secondary xylem. c. Innermost cortex.
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 (x1 and x2) 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, x2; 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].
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.
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.
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].
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.)
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.
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, x1, primary wood, x2, 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.
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].
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.