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.]