MODES OF FOSSILISATION. 7 



decomposed, the interior also sometimes becomes filled. 

 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 occupied by the shell and which if 

 filled with wax will give an exact model of it. Excellent 

 examples of this mode of fossilisation are seen in the 

 mollusks Cerithium portlandicum and Trigonia gibbosa 

 from the Portland Oolite. Sometimes after the shell has 

 been removed, the space left becomes filled up with a 

 secondary deposit of mineral matter, but this although 

 having the form of the original skeleton will obviously 

 not have its internal structure. 



6. 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, so that it is truly ' petrified.' Thus 

 we find fossil wood which in thin sections shows the cells 

 and vessels just as in living trees, but with the walls 

 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, and the aragonite of mollusks. 

 (ii) Silica ; as in the fossils from the Blackdown Green- 

 sand, from the Thanet Sands near Faversham; 

 and the wood in the Purbeck dirt-bed in the Isle 

 of Portland. 



(iii) Iron pyrites ; e.g. ammonites from the Oxford Clay, 

 Lias etc., some trilobites and graptolites. 



