116 Considerations on determining 



out there, perhaps, as many analogous species as in the faluns; 

 and from this double examination we may conclude, as I had 

 already done, eight years since, upon a more superficial in- 

 vestigation, that these two deposits are contemporaneous, a 

 mean conclusion, between two opposite results. 



If I have particularly called the attention of geological con- 

 chologists to this difference of determinations, it is because 

 it may have more important consequences than we should at 

 first believe ; and may tend to modify the opinions now ad- 

 mitted, in consequence of the important labours of M. De- 

 shayes, and of many other geologists, upon the relative age 

 of other tertiary groups of the middle of Europe^ more impor- 

 tant than those of the faluns and the crag, but of which these 

 have been considered the representatives and contemporaries. 

 But it is not possible to be too scrupulous in our examina- 

 tion, ere we admit facts or opinions, opposed to such as have 

 in their favor, a course of study as long and as conscientious 

 as that of M. Deshayes ; and which are supported by the ex- 

 amination of a collection, so rich as his. To cite but one ex- 

 ample of the indirect but positive consequences which may 

 result from the proportional numbers of analogous species 

 fixed by M. Dujardin, I shall call to mind that M. M. de 

 Beaumont and Dufrenoy divide, into two large formations, 

 the whole of the Parisian strata, the most recent of which 

 comprehends the marine freestone of Fontainebleau, and the 

 upper freshwater deposit. To this second group they conti- 

 nue, notwithstanding very strong objections, to refer, at the 

 same time with many other deposits in the south of France, 

 the faluns of Touraine ; of which, nevertheless, they admit 

 the incontestible superposition to the upper freshwater depo- 

 sit. Now if the faluns does indeed contain 50 per cent, of 

 species, analogous to recent ones, how can it be regarded as 

 nearly contemporaneous with a marine formation, which does 

 not furnish more than 3 per cent, for such is the computation 

 adopted by M. Deshayes for the whole of the Paris basin ? 

 That is to say, how can we unite the two extremes of the 

 tertiary period ? And the lower marine deposit of Paris, is 

 there too closely connected with the upper deposit, to allow 

 of our perceiving between them any greater difference than 

 between the latter and the faluns, which, at twenty leagues 

 distance, contains hardly ten species common to the Paris ba- 

 sin, among more than three hundred. Must we not conclude 

 from this, either that the law of the proportional number of 

 analogous species is entirely inapplicable, or that the calcu- 

 lations obtained by M. Dujardin are inadmissible, or, finally, 

 that we cannot arrange in one group, the faluns, and the up- 



