in the Mediterranean. 65 



4<7t, The difference which exists in the mode of substitution in 

 the present day and in geological times, consists in this, that the 

 petrifaction formed at these two epochs have a different texture and 

 molecular constitution ; it is essentially crystalline in the former, 

 while it is compact in the petrifactions of the old world. 



bth. Shells petrified in the present day do not arrive at this crys- 

 talline texture till after they have passed through a certain number 

 of stages, easy to be observed. They begin by losing their colour ; 

 then the inequalities, asperities, and expansions of their surface dis- 

 appear, and they become quite smooth. Finally, the penetration of 

 the calcareous fluids causes their transformation into a stony mass, 

 most commonly crystalline, and sometimes having the appearance of 

 alabaster. 



6f/i, Univalve shells petrify less easily than bivalves. The loose 

 and foliated structure, such as is observed in oysters, seems to facili- 

 tate the penetration of the lapidifying liquids. 



"ith. The black tint which bhells often acquire by lying in sea-mud, 

 arises from the reaction of sulphuretted hydrogen, spontaneously dis- 

 engaged from the mud, on the oxide of iron which these shells con- 

 tain. This phenomena has no connection with petrifaction, 



8iA, The phenomenon of petrifaction is very little perceptible on 

 bones in modern times. By lying in the Mediterranean, they merely 

 acquire greater density and solidity. 



9fA, There are formed, in our own day, in the middle of the waters 

 of the Mediterranean, banks of shelly sandstones which represent the 

 analogues of the shelly sandstones proper to geological formations. 



lOi/i, These shelly sandstones are produced with great facility 

 around all metallic objects which remain pretty long in the sea. 



Will, The phenomena which we have proved as existing in the 

 Mediterranean probably take place in the ocean ; and it will be easy 

 to ascertain this when naturalists turn their attention to it. 



The facts contained in this memoir concur, then, with many others, 

 to prove that notliing is changed in the order of Nature, and that the 

 thread of her operations is not broken.* 



* From Annales des Sciences Xaturelles, Jan. 1847, p. 21. 



VOL. XLIV. NO, LXXXVII. — JAN. 1848. 



