OF THE ORGANIC FOSSILS. 413 



through the rock, and nothing hut the impression 

 remains, sometimes with a mere film of charcoal. 



What is called petrifaction is another condition of 

 organic fossils, much more rare, if we adhere to a 

 true definition. Where this process is perfect, the 

 organic body has disappeared, hut its form is preserved 

 in the stony matter, often to the very anatomy; espe- 

 cially in wood and coral converted into chalcedony. 

 In other instances, the process is incomplete, or a part 

 of the original remains, easily discoverable by sulphuric 

 acid, when not visible, especially if it be a vegetable; 

 as, in shells, where those become chert, the animal 

 matter is sometimes united to the flint, and can also 

 generally be detected by chemical means. 



Each of these classes of bodies are petrified by sili- 

 ceous and by calcareous matter. The former is either 

 quartz, flint, chert, chalcedony, or opal : and the other 

 may be pure carbonat of lime, as it may also be 

 argillaceous, or cherty. If shale has been considered 

 as a petrifying substance, its produce is more properly 

 ranked with casts. If metallizations, as they are 

 termed, are to be included here, they are nearly li- 

 mited to pyrites in the animals, though sulphuret of 

 mercury has been mentioned. But bog iron ore and 

 phosphat of iron may be also ranked with these in 

 the case of vegetables. I know not that bitiunini- 

 zation should have a place here ; but it is at least 

 an analogous fact, to which some writers have given 

 a place. Lastly, it has been said, that ostreae, gry- 

 phites, terebratuloe, corals, and serpulse, have com- 

 monly preserved their shelly matter, when porcel- 

 laniteS; volutes, and others, have lost it, that echinites 

 and encrinites are, particularly, converted into cal- 

 careous spar, that ammonites and others vanish, and 

 that belemnites outlast those which they accompany, 



