90 THE PRESERVATION OF PLANTS AS FOSSILS. [CH. 



trees and may have been floated in by waters 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^." 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 favour- 

 able 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. 



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 



1 An erroneous interpretation of the Arran stems is given in Lyell's Elements 

 of Geologij : Lyell (78) p. 547. ^ Guillemard (86) p. 322. 



