6 INTRODUCTION 



almost in its original condition, except that it has lost its 

 organic matter. Thus the shells in the Pliocene beds of 

 England differ from living ones only in being lighter, more 

 porous and generally colourless. In some instances a 

 certain amount of mineral matter, such as carbonate of 

 lime, has been added to the skeleton, making it heavier 

 and more compact. 



3. Carbonisation. In some plants, and in animals 

 with chitinous skeletons, such as graptolites, the original 

 material usually becomes carbonised. The organism under- 

 goes decomposition and loses oxygen and nitrogen, the 

 relative percentage of carbon therefore increasing. The 

 changes are similar to those which occurred during the 

 conversion of vegetable matter into coal. 



4. A mould of the skeleton. Sometimes the skeleton 

 disappears entirely, a mould only remaining : this is 

 especially the case when it consists of aragonite and is 

 embedded in a porous stratum. After the shell of a 

 mollusk has become covered up with sediment, and the 

 soft parts have been decomposed, the interior becomes 

 filled with the same material. Water containing carbonic 

 acid subsequently percolates through the rock and carries 

 away the shell as bicarbonate of lime, so that there is left 

 only a mould of the interior and of the exterior, the space 

 between the two being that which was originally occu- 

 pied by the shell and, if filled with wax, will give an 

 exact model of it. Excellent examples of this mode of 

 fossilisation are seen in some mollusks from the Portland 

 Oolite, e.g. Ceritliium and Trigonia. Sometimes after the 

 shell has been removed the space left becomes filled up 

 with mineral matter carried in by percolating water ; this 



