12 INVERTEBRATE PALAEONTOLOGY 



Just as the name " fossil " has come to be used some- 

 what loosely (for example, as a mild term of abuse), 

 so "petrifaction," both as a process and as a result, 

 is often applied in cases where it is inappropriate. 

 The two words are frequently treated as synonyms, 

 but neither facts nor etymology support such identi- 

 fication. Passing over the obviously inaccurate use of 

 the word in connexion with the products of a " petrify- 

 ing spring " (where objects become coated with films 

 of calcium carbonate precipitated from " hard " water), 

 it may be stated that, while all petrifactions are fossils, 

 all fossils are not petrifactions. The conversion into 

 stone (or new mineral matter) that is implied by the 

 latter term involves a considerable change in the sub- 

 stance petrified, whether it be achieved by Medusa's 

 head or by less heroic methods. Petrifaction is believed 

 to be carried out by molecular replacement, and will 

 be discussed in a later section of this chapter ; but a 

 large proportion of fossils have undergone no such 

 radical change. Impregnation of the spaces of origin- 

 ally porous shells with the same or some other mineral 

 may possibly rank as petrifaction, since the process is 

 similar to that whereby sand-grains become united 

 into the " rocky " state of sandstone. But, in precise 

 terminology, the word petrifaction must be restricted 

 to those fossils which have undergone change either by 

 complete rearrangement of the molecules of the mineral 

 of which they were composed (when all minute structure 

 is lost), or by substitution of a different mineral (when, 

 if replacement has been gradual, details of structure 

 may be retained). 



