346 fossils 



(2) Entire loss of substance and retention of form. In this 

 class of fossils all the original material of the organism has been 

 lost, and no trace of its internal structure is retained, but only the 

 external form has been reproduced in some different material. 

 Under this class we may distinguish two principal varieties : (a) 

 Moulds and (J?) Casts. A mould is formed when the fossil is 

 embedded in sediments, which accurately reproduce its external 

 form, and harden so as not to collapse when the fossil is removed. 

 Percolating waters then dissolve away the organism entirely, leav- 

 ing only a cavity, which is the mould. It is often possible to 

 reproduce the form of a vanished fossil by filling the natural mould 

 with plaster of Paris, gutta-percha, or similar substance. Im- 

 pressions of footprints, which may be placed in the same category 

 as moulds, have already been explained (see p. 227). 



Casts are formed when the mould is filled by some solid sub- 

 stance deposited from percolating waters, thus reproducing the 

 form of the fossil, as is done artificially with plaster or gutta-percha. 

 If the fossil were hollow, like a shell, we frequently find a com- 

 bination of internal cast with an external mould in the same 

 specimen. At the time the fossil is embedded its interior is 

 filled with the same sediment, which hardens and forms an inter- 

 nal cast, exactly reproducing the form of the interior. The shell 

 itself is then dissolved away, leaving a space between the outer 

 mould and the inner cast. Moulds and casts are commonest in 

 rocks which permit percolating waters to traverse them freely, 

 such as sandstones and coarse-grained limestones. An interesting 

 form of cast is the brain-cast, which is made by the fine-grained 

 sediment which fills the cranial cavity of an embedded skull, 

 often reproducing the form of the brain with much accuracy. 



(3) Loss of substance with reproduction of form and structure. 

 Fossils of this class are also called petrifactions and pseudomorphs 

 (the latter a term borrowed from mineralogy). Here the original 

 material of the organism has been more or less completely removed, 

 and other material substituted for it ; but the substitution has been 

 so gradual, molecule by molecule, that not only the external form 

 but also the microscopic structure has been perfectly reproduced. 



