8 MODES OF FOSSILISATION. 



(iv) Oxide of iron ; in the form of limonite in some 

 fossils from the Dogger (Inferior Oolite) of York- 

 shire and the Lower Greensand of Pottou etc., 

 and as haematite in fossils from the Carboniferous 

 Limestone of Cumberland. 



(v) In rare cases there are other replacing minerals, 

 such as sulphate of lime, barytes, blende, galena, 

 malachite, vivianite, and spathic iron. 



7. Infilling. The interior of foraminifera and some 

 other organisms soon after the death of the animal may 

 become filled with mineral matter, such as glauconite (sili- 

 cate of iron and alumina), calcite, or silica, subsequently 

 the shell often disappears leaving only the internal mineral 

 cast. Glauconite occurs in this way in the various green- 

 sand strata, and also in some of the deep-sea deposits at 

 the present day. In the Cambridge Greensand the inte- 

 rior of the organism was filled with phosphate of lime. 



8. Imprints. Some animals leave only their foot- 

 prints, such as labyrinthodonts and reptiles from the 

 Trias. In the Solenhofen Slates*, jelly-fishes have left the 

 imprint of their exterior. 



In Stratigraphical Geology fossils are of immense 

 importance. By their aid alone, any formation may be 

 identified in distant localities and under different litho- 

 logical conditions. This depends on the fact that the 

 genera and species of animals and plants are distributed 

 through the rocks in a definite and regular order. In the 

 geological record each great division (termed a System) 

 is characterised by a particular assemblage of genera and 

 species, some of which pass from one system to another, 

 but a large number are confined to each one, and these 

 * These are really shaly limestones. 



