2 CONDITIONS FOR FOSSILISATION. 



to species which are not found living at the present day, 

 but at the same time a fossil is not necessarily now an 

 extinct form, thus in some of the later Cainozoic formations 

 as many as 90 per cent, of the species of mollusks are still 

 living. On the other hand an extinct animal or plant is 

 not always a fossil, as for instance in the case of Steller's 

 Sea-cow, Rhytina gigas, which formerly lived in the seas 

 off Kamtschatka, Alaska, and the Kurile Islands, but of 

 which no remains are found in the rocks. 



In order that an animal or plant may be preserved as 

 a fossil, two conditions are generally necessary: First, it 

 must possess a skeleton of some kind or other, since the 

 soft parts rapidly decompose, consequently such animals 

 as jelly-fishes can leave no trace of their existence, unless 

 it be the imprint of their exterior. Second, it must be 

 covered up by a deposit, otherwise when exposed to the 

 air it will soon crumble to dust. Now, since there are 

 comparatively few places on land where material is being 

 deposited to any extent, it follows that terrestrial animals 

 will stand but little chance of being preserved, the greater 

 number after death will remain on the surface, and, if not 

 devoured by other animals, will in a short time entirely 

 decompose. A few, however, may become entombed in 

 peat-bogs, in the dust and ashes of volcanoes, in the sand 

 of sand-dunes, or by the falling of a landslip ; some will 

 be sealed up in deposits of carbonate of lime, such as the 

 travertine thrown down by calcareous springs, or the sta- 

 lagmite formed on the floor of caves ; and lastly, others 

 may be transported by running water and ultimately 

 buried in the bed of a river, of a lake, or of the sea. But 

 all these are of comparatively rare occurrence. With 

 aquatic animals the conditions are much more favourable, 

 since deposition is more universal in water than on laud. 



