318 



THE GEOLOGIST. 



The second question is purely applicable to what we ob- 

 serve in fossiliferous strata. The shells of Ammonites 

 floating on the surface of the sea, have thus been carried to 

 the shores of the epoch at which they existed^ and, conse- 

 quently, were not deposited in the midst of the seas where 

 the chief depositions took place. 



This supposition explains the reason why the number of 

 Ammonites, found in different basins, is in the inverse ratio 

 to the thickness of the deposits. The thickest contain few 

 Ammonites, whilst those which are thinner, have many 

 more. It also explains why we generally find more Am- 

 monites at the circumference of any geological basin than in 

 its centre, the shells being always thrown on its shores. 

 When we only find isolated adult Ammonites in a bed, we 

 may suppose that this bed was formed, when the fauna of 

 that epoch was at its full development, whilst the accumula- 

 tion of Ammonites of all ages, which we remark in some 

 places, points to general causes which have destroyed all the 

 individuals of a species, and cast them together on the shore ; 

 powerful, but unknown causes, in which \yg must evidently 

 seek the reason: 1st. of the extinction of the species of an 

 entire fauna, 2nd. of their being replaced by the species of 

 another fauna, and 3rd, of the separation of formations, of 

 the groups of these formations, and of the distinction of the 

 strata proper to the groups of the different rocks containing 

 a particular Fauna. 



From the shell floating on the surface of the water, can 

 also be explained the cause of the Ammonites, of the lower 

 strata, being found in strata superior to them, without having 

 been in existence at the same period. I imagine that some Am- 

 monites were deposited on the shores of the sea towards the end 

 of a certain geological period, and that they were gradually en- 

 closed in the foreign matter forming the strata. If, from the time 

 when they were deposited up to the period of the commence- 

 ment of a new fauna, sufficient time elapsed to allow the cham- 



