PROCESSES OF PETRIFACTION. 119 



own composition, and possessing its own power of resisting 

 decay. Not only so, but as the percolation of mineral 

 solutions through the earth's crust is incessant, what is 

 deposited at one time may be dissolved at another and a 

 new substance substituted in its place, or no new substance 

 may be substituted, and merely the hollow mould of the 

 organism left to prove that it once was there. For instance, 

 a shell or coral, which consists of animal-formed carbonate 

 of lime, may be converted into sparry mineral carbonate ; 

 or this may be dissolved and carried away, leaving merely 

 a hollow mould with every ridge and line and pore im- 

 pressed on the containing matrix; or this mould may be 

 refilled with siliceous matter, and the shell or coral then 

 present itself as a flint, with every pore and ridge and wrinkle 

 as delicately perfect as on the original organism the day it 

 was imbedded. This perfection of preservation is often, 

 indeed, truly marvellous. We have seen the faceted eyes 

 of trilobites as perfect in form as when they received the 

 rays of light through Silurian waters ; carboniferous uni- 

 valves with their colour-bands still unobliterated ; internal 

 casts of products with their muscular apparatus displayed in 

 a style of legibility to which no anatomical preparation could 

 approach ; and ink-bags of cuttle-fishes so little changed as 

 to furnish the pigments for their own portraiture. 



Of course, the consideration of these percolations, dissolu- 

 tions and substitutions, involves many intricate questions in 

 chemistry; but enough has been stated to inform the gene- 

 ral reader that fossils may occur in many different conditions 

 as stony conversions, as moulds, as casts, or as mere im- 

 pressions on the matrix in which they are entombed. But 

 in whatever state they may occur, there is usually sufficient 

 left either in general form, in external character, or in inter- 

 nal texture, to enable the palaeontologist to determine their 



