PAL/EON TOLOGY. 129 



II. Mode of Occurrence of Fossil Remains : Petri- 

 faction. When organisms have once become buried 

 in deposits formed at the bottoms of seas, lakes, or 

 rivers, and their soft parts have decayed away, it might 

 be thought that the hard portions would remain then 

 unchanged, until disentombed by the hands of the geolo- 

 gical excavator. This, however, is not the case ; on the 

 contrary, they have in most instances been subjected 

 either to contemporaneous or subsequent mineralizing 

 processes, by means of which they are more or less 

 hardened and petrified. 



It often happens that the animal matter in decom- 

 posing has given rise to various chemical reactions, 

 which have resulted in the formation of fresh mineral 

 matter and its concentration round the body of the de- 

 caying organism ; in such a case the animal becomes 

 included within the mass, and its harder parts, being 

 usually preserved from further change, are disclosed on 

 the splitting open of the nodule or concretion which is 

 thus formed. 



Where, however, this does not take place, and the 

 fossil is simply enclosed in the ordinary matrix of the 

 rock, it is always liable to more or less subsequent 

 .alteration ; in pure clays this change is generally very 

 slight, simply consisting in the complete abstraction of 

 the organic constituents, and if the test or skeleton is 

 siliceous little alteration is effected ; but in other rocks, 

 and where the shell or test is of a calcareous nature, it 

 frequently happens that the whole of it has disappeared, 

 and has either been replaced by other mineral matter, 

 or else the space which the shell once occupied is left 

 open, and nothing remains but the external impression 



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