458 Mr. Weaver 07i M. Alcide d'Orbigny's Fieia 



and may lead to very satisfactory results in geology, if we 

 may judge by the fruits of our observations in this respect. 



We could have desired to establish some general facts of 

 much greater extent, founded on new observations recently 

 made by us on the class of the Foraminifers ; but the pre- 

 sent occasion not admitting such an extension, let us pass to 

 the Foraminifers of the white chalk of the Paris basin. 



The geological position of the white chalk of Paris is so 

 well known that we have not thought it necessary to speak of 

 it; yet, if we seek to determine its position relatively to the 

 other cretaceous beds by means of the Foraminifers it con- 

 tains, compared with living species, the fades of the genera 

 and species proves to us, that the chalk of Maestricht, of Fau- 

 quemont (Belgium), of Tours, of Chavagne, and of Vendome, 

 is above it; while, on the contrary, all the other beds are 

 below it ; thus in the chalk of Maestricht and the upper beds 

 of the basins of the Loire, we recognize only genera still ex- 

 isting, or at least occurring in tertiary tracts, while the white 

 chalk of the Paris basin already exhibits to us different genera, 

 such as Flahellina, Verneuilina, and Gaudrijina, and a great 

 number of species quite distinct. 



It would therefore be easy to establish, by means of the 

 Foraminifers alone, the relative antiquity of the cretaceous 

 beds ; but we must previously make two geographical sections 

 quite independent of each other, founded on the zoological 

 forms; the first comprising the entire basin of the Seine, of 

 the Loire, of Belgium, and of England, in which we find a 

 striking analogy between the species found in all the beds, 

 from the lowest to the highest, with a regular passage from 

 one to the other; the second, comprising the West and South 

 of France, in which the species of Foraminifers have not 

 only no analogy with those of the other section, but in which; 

 moreover, almost all the genera are different. If we seek an 

 example of this fact, we shall find it on comparing the green 

 sand of the environs of Mans with that of the mouth of the 

 Charente. The first, which in fact contains species approxi- 

 mating to those of the white chalk of Paris, contains already 

 several species analogous to those which have lived up to that 

 bed; while the second, with perfectly distinct species, pre- 

 sents to us genera different from all that we know in the cre- 

 taceous beds of the North of France and of Belgium. 



The Foraminifers are sufficient to establish the following 

 descending order of superposition in the cretaceous beds : — 



