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.