IV] FOSSIL WOOD. 61 



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 



Pig. 7. Aegoceras planicosta Sow. on a piece of coniferous wood, Lower Lias, 

 Lyme Begis, Prom a specimen in the British Museum. Slightly reduced. 



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 en- 

 closing 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 



