112 Considerations on determining 



The particular views according to which different observ- 

 ers form their respective estimates of specific limitations, 

 must then occasion a considerable variation in the numerical 

 results, connected with the identification of fossil with recent 

 shells ; and, consequently, a corresponding difference in the 

 determination of the age of the strata in which the former are 

 contained. It is, however, upon this foundation, that geoge- 

 nic inferences, of the most important character, are based ; 

 such as, the progressive changes of temperature, and the suc- 

 cessive creation of species, or their insensible modification 

 from one epoch to another, and from one, to a neighbouring 

 basin. 



It is then necessary, for greater certainty, that the fossils 

 of different tertiary strata should be compared among them- 

 selves, and with living forms, by the same conchologists ; or 

 that, at least, specific characters should be adopted, which 

 are uniform, and of fixed value. But it will be a long time 

 before this can be the case ; nevertheless, this chronometer, 

 once introduced into the science, will necessarily be made 

 use of by all those who may think themselves sufficiently ac- 

 quainted with comparative zoology to employ it practically, 

 however different may be their principles of specific recogni- 

 tion. 



It is thus that the attentive study of the fossil shells of two 

 tertiary formations, the faluns of the Loire, and the crag of 

 the eastern side of England, has recently led to results so 

 contradictory, that it seems to me important to call attention 

 to them, before we allow them to be definitively established 

 in the science. It is now no longer doubted, that these two 

 important deposits are more recent than the whole of the Pa- 

 risian strata : but if this opinion, (which I have endeavoured* 

 to establish from the direct superposition of the faluns of the 

 Loire, upon the last freshwater stratum of the basin of the 

 Seine, and from a very striking general resemblance between 

 the faluns and the crag,) has been generally adopted, it has 

 not been the same with regard to their mutual agreement in 

 age, which I also pointed out as very probable. Yet it was 

 not merely from external characters, owing to the deposition 

 of these two strata under analogous physical circumstances, 

 that is to say, in shallow gulphs, and upon shores, that I had 

 pointed out this connection, but from the amount of analo- 

 gies among fossils of different classes, common to the two 



* Annales des Sciences naturelles, February and April, 1829, (v. xvi. 

 p. 171 and 402): Observations on a series of marine deposits more recent 

 than the tertiary strata of the basin of the Seine. 



