6 MODES OF FOSSILISATION. 



1. The entire organism preserved. Occasionally the 

 soft parts of the organism are preserved as well as the 

 skeleton, the whole having suffered very little change. 

 Instances of this are, (a) the woolly rhinoceros and mam- 

 moth frozen into the mud in Northern Siberia, and 

 (6) insects and plants encased in fossil resin known as 

 amber, found in the Oligocene beds on the Baltic shores of 

 Prussia and in the Pliocene beds of Cromer. 



2. The skeleton preserved almost unchanged. Some- 

 times when the skeleton alone is preserved, it remains 

 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, is 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 passes into a carbonaceous substance. The 

 organism undergoes decomposition where there is but 

 little access of air, it loses oxygen, and consequently the 

 percentage of carbon increases. 



4. Incrustation. The organism becomes coated over 

 with a layer of carbonate of lime deposited from a cal- 

 careous spring, as in the case of those of Matlock : or of 

 silica, as in the thermal springs formerly existing in New 

 Zealand. 



5. A mould of the skeleton. Sometimes the skeleton 

 disappears entirely, a mould only remaining. This is 

 especially the case with aragonite organisms embedded in 

 porous strata. When the shell of a moll ask becomes 

 covered up with sediment, after the soft parts have 



