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.