44 INVERTEBRATE PALAEONTOLOGY 



temporaneous moulds of larger shells. The Cambridge 

 Greensand, " Chloritic Marl," and rock-bands of the 

 Chalk include good examples of this phenomenon ; 

 while the pyritous moulds of Liassic Ammonites and 

 flint moulds of Cretaceous Echinoids are somewhat 

 analogous. 



(Ill) PETRIFACTION 



(a) By infilling 



Fissures and other open spaces in rocks tend to be- 

 come clogged by precipitation of "vein-minerals" brought 

 in solution by water. The cavities separating casts 

 from moulds are thus liable to be filled by new mineral 

 matter. This will usually be different from that 

 originally composing the fossils, and normally less 

 soluble. Since the secondary mineral is likely to have 

 crystalline properties differing from those of the missing 

 shell-substance, it may fail to restrict its growth abso- 

 lutely to the cavity, and will then obscure some of the 

 details of the fossil. Thus the petrifactions (in Beekite 

 chalcedony) of Mollusca in the Upper Greensand show 

 peculiar orbicular surfaces that were certainly not 

 present in the originals (PI. iv. fig. 2). It is clear that 

 the nature of the mineral deposited in the hollow 

 depends on the quality of the aqueous solution available, 

 and bears no relation to the original substance. 



The great majority of vein-minerals may occur as 

 pseudomorphs after organisms, but Silica, Calcite and 

 Pyrites are those usually employed. Silica, typically 

 as Chalcedony, often replaces calcite or aragonite in 

 calcareous rocks containing opaline silica in the form 

 of Radiolaria or Sponge-spicules. Such petrifaction 

 is often most satisfactory, since the calcareous matrix 

 can be removed with acid, the silica remaining un- 



