INTRODUCTION 7 



has the form of the original skeleton but obviously not its 

 internal structure. 



The interior of the shells of foraminifera may, soon after 

 the death of the animal, become filled with glauconite 

 (silicate of iron and alumina) ; subsequently the shell itself 

 often disappears, leaving only the internal cast. Glauconite 

 occurs in this way in the various greensand strata, and 

 also in some of the deep-sea deposits at the present day. 

 Somewhat similarly the shells of sea-urchins occurring in 

 the Chalk are sometimes filled with flint ; in such cases 

 the shell when buried did not become filled with Chalk, 

 but remained empty until flint was deposited in it from 

 percolating water containing silica in solution. 



5. Petrifaction. In some deposits the fossils show 

 the minute structure as well as the form of the organism, 

 but the original material of the skeleton has been replaced 

 by another mineral. Thus we find fossil wood which shows 

 the cells and vessels just as in existing trees, but in which 

 the walls are formed of silica instead of cellulose. The 

 change has gone on in such a manner that as each particle 

 disappeared its place was taken by a particle of silica. 

 The chief minerals which replace the original substance 

 of organisms in this manner are : — 



(i) Carbonate of lime ; calcite sometimes replaces 

 the silica of sponges. 



(ii) Silica, as in the fossils from the Blackdown 

 Greensand, and the Thanet Sands near Faver- 

 sham ; also in the wood of the Purbeck dirt-bed 

 in the Isle of Portland. 



(iii) Iron pyrites; e.g. Ammonites from the Oxford 

 Clay, Lias, etc., and some graptolites. 



