in the Medilerraneait. 55 



III. On the proyress and difercnt degrees of the Petrifaction of 



Shells. 



The indispensable condition for shells becoming petrified in our 

 epoch is, that they remain for a very long period submerged in the 

 sea. When merely left on the sliore they exfoliate and become 

 disintegrated, but never petrify. The whole is then reduced by a 

 slow but total destruction, the rapidity of which depends on the ex- 

 ternal circumstances to which they are subjected. But the process 

 is very difFerent with those which remain sunk in the water. 



The shells abandoned by the animals which inhabit them, and 

 principally such as are left near the shore, are for a long time tossed 

 about by the waves. The first modification they undergo is in the 

 alteration of their colours. Thus discoloured, they are often thrown 

 out on the beach, where they are found in great plenty after rough 

 weather and violent winds. The loss of their natural colours is the 

 first effect produced on the shells and the solid and calcareous tubes 

 of the Annelides, then on the stony masses of the Polypi ; the se- 

 cond, and the more considerable, consists of the alteration of their 

 substance. This alteration first appears most evidently on the shells 

 provided with angles and elevated ridges. The grooves and promi- 

 nent parts disappear, and the suiface becomes uniform. This is 

 particularly observable in the largo ribbed Buccini, such as Cardium 

 album, trebherea, tiibercidatum, and aculeatum. The prominent 

 ribs, and the osseous interstices, which are so conspicuous in the 

 fresh shells, scarcely leave sensible traces of their existence when 

 the alteration is somewhat advanced. This first modification is par- 

 ticularly obvious in the Pectunculi, in which it often exposes their 

 singular structure beneath the external covering. The Citherea3 

 likewise lose their external coat, and exhibit the structure of their 

 interior layer. 



In proportion as these modifications advance, sand is precipitated 

 into the interior of the valves of tiie shell-bearing Molluscs, where it 

 becomes agglutinated and hardened, occasionally enclosing more or 

 less considerable remains of small shells in the interior of their 

 masses. The calcareous matter which, by the effect of this mineral 

 substitution, is precipitated into the very substance of the shells, 

 often becomes there a kind of sphere of attraction for salts of every 

 kind existing in a state of solution in the sea-water ; in consequence 

 of this attraction, the carbonate of lime is precipitated on the exterior 

 surface, as well as on the interior of the sliells, and there forms a 

 crystallization often well-marked, and sometimes of considerable re- 

 gularity. The form of these crystals is referable, for the most part, 

 to the inverse variety of Ilaiiy. 



^\'o have collected a considerable number of shells, in which the 

 original snbstanco has almost wholly disappeared, and is replaced by a 

 carbonate of crystalline lime, the aspect, colour, and transparency of 

 which have no relation to that which at first composed the shell. The 



