02 Observations on the Petrifaction of Shells 



stitute, in the iiiiddle of the seas, large and extensive masses, of 

 which we find only the fragments. It cannot be doubted that they 

 are formed in the middle of the salt waters, when we remark, that 

 the part which does not rest on the bottom is often covered, not 

 only by Annelides of the genus Scrpula, but also by different Zoo- 

 phytes. We sometimes also find Barnacles upon them, as on many 

 of the fossil remains of terrestrial mammifera which have been car- 

 ried into the sea in geological times. 



The formation of these shelly sandstones presents a great number 

 of curious cii'cumstances, which we shall rapidly point out. Metallic 

 objects remaining long in the sea, become, as it were, centres of at- 

 traction for the substances in solution in the waters. These sub- 

 stances are pi-ecipitated on their surface, and cover them with a coat- 

 ing often of vei'y considerable thickness and hardness. 



We possess a musket which appears to have lain long in the sea. 

 It is covered with a sandy shelly layer, from 5 to 6 centimeters thick, 

 and of great hardness. In our collections we possess many iron in- 

 struments, shewing the same peculiarities. We have exhibited to 

 the Academy the blade of a knife, still adhering to its handle, which 

 has been surrounded by a layer of shelly sand nearly 4 centimeters 

 in thickness. Besides the fragments of shells collected by us, and 

 hardened by means of a ferruginous cement, which compose this kind 

 of rock, we also observe small pebbles in it, like those generally found 

 in the sea. We can easily perceive, from the examination of the 

 knife thus encased, the influence which the oxide of iron has exerted 

 on the production of the cement which agglutinates the sand, the 

 shells, and small pebbles. The iron composing the blade and the 

 nails of the handle is converted into limonite or hydrated peroxide, 

 and, by spreading through the mass of sandstone, it has communi- 

 cated a considerable solidity and hardness to this new rock. 



The phenomenon which we have seen exemplified on small objects, 

 is likewise produced on a large scale, and in circumstances too curious 

 not to be at least rapidly noticed. In 1827, by the advice of Davy, 

 the English Admiralty caused the copper sheeting of vessels to be 

 covered with a certain number of plates of zinc, in order to oppose, 

 by a galvanic action, the rapid corrosion of the metal in sea-water, 

 particularly in some parts of the coast of Africa. But this expedient 

 had soon to be abandcned, because considerable deposits of shells and 

 agglutinated sand encrusted the vessel so rapidly, that its progress 

 was retarded. Here the galvanic action accelerated the phenomenon. 

 The copper, rendered negatively electrical by the pile formed by the 

 superimposed zinc and copper, attracted the insoluble bases, the mag- 

 nesia and lime held in solution iu the sea-water, and the sides of the 

 vessel began to be covered with carbonate of lime and magnesia ; 

 the shells and sand were then precipitated on these earthy deposites. 

 The electrical action induced and accelerated the phenomenon, but 

 it is evident that it is entirely of the same order as those we have 

 been examinino-. 



