AMMONITES — ZOOLOGICAL, ETC. 



349 



GEOLOGICO-GEOGRAPHICAL SUMMARY. 



The geologico-geographical conclusions which maybe drawn 

 from the division of the Ammonites amongst the different 

 strata of the cretaceous formation, are : 



] St. That during the Neocomien period, from the first to 

 the last deposits, there existed in France, at least so far as 

 we are at present aware, but two distinct basins, the Pro- 

 venyal basin, bounded on the west by the Cevennes, and on 

 the east by the Alps, and the Parisian basin, whose shore*^' 

 extended from the Department of Aube-et-Yonne by Haute- 

 Marne, as far as the Jura, and on the other side as far as 

 England. These two basins have each their peculiar and 

 distinct fauna, possessing, at the same time, a sufficient 

 number of species common to both, to remove all doubts as 

 to their not being of contemporaneous origin. 



2nd. That at the epoch of the lower Gault, these two seas 

 were in the same condition ; but that during this first 

 period, the great eff'ects of currents, marked by the transport 

 of species, and arising doubtless from partial dislocations, 

 have apparently opened wide communications between the 

 two seas, since in the upper strata of the Gault we find a 

 much larger number of species common to both seas than 

 during the Neocomien period. 



3rd. That the group of the chloritic chalk has entirely 

 changed the aspect of the cretaceous seas. The two first 

 basins remained, relatively to the distribution of species and 

 their proportions, the same as they were at the epoch of the 

 upper Gault; but to the Parisian basin was joined the 

 gulfs of Cotentin and the Loire, up to that period distinct 

 from the cretaceous formation ; and the chalk formation in- 

 vaded, at the same time, the immense basin of the Pyrenees, 

 from the Department of Charente-Inferieure as far as the 

 Eastern Pyrenees ; thus, towards the latter epoch of the Cre- 



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