AMMONITES. — ZOOLOGICAL, ETC. 



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bers of the Ammonites, to become filled by chemical action, 

 either with carbonate of lime or sulphuret of iron, as we 

 frequently observe, or with extraneous matter by means of 

 the fractures in the shell, it would remain in these strata 

 in the fossil state, and would not again float on the surface ; 

 but, on the contrary, if sufficient time had not elapsed nor 

 more alteration taken place than between our own epoch 

 and the tertiary fossils of Grignon and Dax, the Ammonites 

 would still have their chambers filled with air; the strata 

 destroyed by denudation, would again cause the Ammonites 

 to float : they would be deposited simultaneously with those 

 which existed at the second epoch, and then we should have 

 a mixture of the two faunas, without the species having 

 existed at the same time. It is thus that I explain the 

 union of some species, always the upper ones of one fauna, 

 with those which are the lowest of another, posterior to the 

 first. This explanation satisfactorily accounts for the species 

 of the gault, which I have cited, that this mixture only takes 

 place when the lower group no longer exists, and has been 

 destroyed by denudation, as we observe at Cassis, Vergons, 

 and near Hivre, where the gault is entirely wanting, and 

 has been removed by some unknown cause. 



When we find Ammonites which, from the lightness of 

 the shell, are more likely than other Molluscs, to be trans- 

 ported from one strata to another, belong, notwithstanding, 

 to well defined epochs, we ought to attach more importance 

 to their distribution than to that of other genera; as it 

 evidently proves that between each Geological epoch, dur- 

 ing which Ammonites have existed, sufficient time has 

 elapsed to allow the shell to lose its property of floating on 

 the water, or at least, that at this epoch the chemical action 

 was much more intense than it has been from the tertiary 

 period up to the present time, when this action is no longer 

 general. At any rate, Ammonites, belonging to the most 

 perfect Molluscous animals, ofi'er, for this reason and from 



