346 PALEONTOLOGY . 



chalybite, limonite, calcium phosphate, pyrite or selenite. 

 Aragonite may be replaced by any of the same, or may 

 undergo a paramorphic change into calcite. The extent 

 to which these changes may alter the minute structure of 

 the skeleton varies very much : when any recrystalliza- 

 tion takes place the minute structure is usually destroyed, 

 wholly or in part. Amorphous silica (opal) may preserve 

 the finest details, but if it passes into chalcedony it will 

 often produce a structure due to imperfect crystallization 

 in rings (beekite structure), which will not only destroy 

 minute structure but even obscure the outward form. 

 So, too, with the formation of selenite (gypsum), few 

 fossils but belemnites can survive it in recognizable 

 form ; but this chemical change differs from most others, 

 as it is probably quite a recent result of weathering near 

 the surface, while most others took place soon after the 

 entombment of the fossil. 



Originally siliceous skeletons may be altered to calcite, 

 pyrite or marcasite, the last in turn passing into limonite 

 under conditions of weathering. Sponges in the chalk 

 may sometimes be found, partly silica, partly limonite. 



Hard organic tissues generally undergo some degree of 

 carbonization, but they may be replaced by pyrite (as 

 with graptolites), or by calcite or silica (as in plant 

 petrifactions). 



Phosphatic skeletons are the most stable of all, and 

 beyond having their minute cavities filled with secondary 

 phospate they undergo no change. Very rarely they may 

 be changed to vivianite (a blue iron phosphate). 



4. An organism may be represented by casts of its 

 skeleton external casts or moulds, and (if a hollow 

 skeleton) internal casts. External casts are composed 

 of the matrix of the rock, that is to say the sediment in 

 which the organism was buried : in hard enough rocks 

 they are always produced when a fossil is extracted, but 



