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THE GEOLOGIST. 



formation. We see these species gradually disappear, dur- 

 ing the continuance of the Neocomien formation, up to the 

 upper portion of it, where they are replaced by twenty-two 

 species distinct from the first, and which themselves disap- 

 pear with the last deposits of the Neocomien. With the 

 first strata of the lower gault, twenty-two species of Ammo- 

 nites appear, but they bear no resemblance to the species of 

 the upper portion of the Neocomien. They are specifically 

 distinct, and furnished with totally different characters. 

 The same as in the Neocomien, the species of the gault gra- 

 dually disappear, and are, in the upper strata, replaced by 

 twenty-two well- characterized species, none of which sur- 

 vives the epoch of the last portion of the gault. We find, 

 again, with the group of the chloritic chalk, a series of Am- 

 monites essentially different from those of the Neocomien 

 and gault, and distinguished from them by peculiar forms ^ 

 lastly, these disappear entirely with the upper central beds, 

 at the same time that Ammonites are swept for ever from 

 the surface of the globe. The Ammonites of the cretaceous 

 period have thus been created at five successive epochs; 

 three of which, in particular, present, each time, after the 

 complete extinction of the existing species, the appearance of 

 a new series distinct from those preceding them. We may 

 in consequence observe, that the cretaceous formation not 

 only divides itself into three well-defined geological groups, 

 but also that these groups are each subdivided into two 

 series of the strata, the one inferior, the other superior, each 

 possessing their peculiar species. This important result, at 

 which I have arrived after having compared thousands of 

 Ammonites from all parts of France, and verified the super- 

 position of the strata, evidently proves that it is not only a 

 few isolated shells, ivhich are characteristic of formations, as 

 has been hitherto stated, but that all the species of the genus 

 Ammonites, without exception, are characteristic ; that all 

 positively indicate the formation to which they belong, and 



