60 M. Von Buch on the SUicificntion <>/' Organic Bodiea. 



destroyed. This last presses forwards between the lamellae of 

 the shell ; and where the calcareous shell cannot be burst 

 through, by means of the greater expansion of the siliceous 

 jelly, the shell becomes enveloped in that substance. 



The siliceous mass presses through between the fibres. The 

 whole becomes siliceous, and now possesses much more the ap- 

 pearance of wood-opal than of calcedony. Likewise, in this far 

 advanced stage of the silicification, we can always distinguish 

 the different lamellae of the shell. Those which belong to the 

 organic substance, are of a much darker colour ; and by means 

 of the greater extension of the siliceous hydrate, they are much 

 thicker than they were originally. The brighter calcareous 

 lamellae, on the contrary, still retained somewhat of their 

 former fibrous structure ; and frequently we can act so far upon 

 them with acids, as to cause them to effervesce. This is a clear 

 proof that here likewise the calcareous part is not that which is 

 changed, but is only enveloped where it cannot be forced off. 



The inside of the shell, when it is possessed of any thickness, 

 continues very much in its natural condition. The silicicating 

 process goes on only from the outside inwards ; hence the pro- 

 cess is a decided change of the organic substance, which by 

 no means takes place without the operation of external causes. 

 Does this organic substance decompose in some way or other a 

 siliceous combination, by which means the siliceous earth be- 

 comes free, absorbs water, and then appears in the condition of 

 calcedony, opal, or hyalite ? 



We might also believe the animal of the oyster to undergo 

 silicification, although such an assumption is opposed to the 

 views adopted by naturalists, who niaintain that so soft an or- 

 ganic mass is incapable of petrifaction. There is represented in 

 Fig. 3, Plate II, a mass of flint, which fills up the inside of an 

 oyster shell, and which appears to be the animal of the shell.. 

 The animal would have lain in the oyster-shell in this posi- 

 tion had it been alive. The greater mass lies towards the 

 right side, where the muscle fixes it to the shell; insomuch, 

 that one could believe we could distinguish the muscle as it 

 passed upwards from the under to the upper shell. The small- 

 er mass of the petrifaction extends itself out as far as the point 



