ill the Mediterranean. 57 



When the petrifaction has reached its last stage, the carbonate of 

 lime which composed the shell in its fresh state, has totally disap- 

 peared. It has been replaced, pretty generally, by a crystalline cal- 

 careous substance, which retains, more or less, the form and struc- 

 ture of the shell in which it was formed. We pretty often see shells 

 metamorphosed in this manner into new limestone, encrusted with a 

 layer of sand, more or less thick, and always indurated. 



All kinds of shells do not appear susceptible of petrifying in the 

 same degree ; we have hitherto observed very ^evi species of the genus 

 Venus in this state. A certain number, however, are to be found 

 in the Mediterranean, among which wo may mention, as the most 

 common, Venus decussata, virginea, 7'ugosa and gcdinea. This cir- 

 cumstance is so much the more remarkable, as this genus is ex- 

 tremely frequent in the tertiary formations, where it is petrified, like 

 the greater number of the species of this formation. The same thing 

 may be observed of the small Tellinfe, so numerous in the Mediter- 

 ranean, and which, notwithstanding, are very rarely petrified.* 



There is, lastly, another kind of alteration presented by shells, and 

 of which we shall only say a iew words, because it is not connected 

 very directly with the question under discussion ; and which, more- 

 over, takes place among fresh as well as petrified shells. When 

 shells, or rocks containing shells, remain a long time in the mud or 

 in puddles of brackish water, such as are very often found along the 

 shores of the Mediterranean, they become of a black or deep blue 

 colour, more or less intense. This change in the colour does not ex- 

 tend much further than the exterior surface. To be convinced of 

 this, we have only to break an oyster, or some other shell thus dark- 

 ened, when we perceive that the shade of colour does not go beyond 

 a few millimetres beneath the surface ; the rest of the shell presents 

 the white colour of carbonate of lime. This alteration is owing to 

 the sulphur of iron, which is formed at the expense of the oxide of 

 iron, forming part of the shell, and the sulphuretted hydrogen spon- 



* We IjcliGve that the facts contained in this chapter, afford a sufBcicnt an- 

 swer to an objection which has been made to us by forae geologists, and which we 

 shall now state. It is alleged that the petrified shells t'uiind on the shores of 

 the Mediterranean may come from geological formations; the waves may have 

 detached these shells from the tertiary formations, in places where thee forma- 

 tions formed the bottom of the sea, and they may have been thrown up on the 

 bank. The two following remarks are sufficient to destroy this argument: — 



1. We find, in the Mediterranean, shells in every stage of petrifaction, from 

 simple discoloration to their complete transformation into carbonate of crys- 

 talline lime. 



2. The ir.olecular structure of the pctiificd shells of tlic present day is often 

 very different from the structure of fossil shells. Tlie former most ircqucnt!y 

 have a crystalline texture ; the latter are always in a compact state. 



licsides, tlio facts which we shall afterwards inention, in reference to the mo- 

 dern shelly sandstone now produced in the Mediterranean, do not leave us any 

 roon^to doubt the reality of the iiii))ortant phenomenon we are illustrating in 

 this memoir. 



