334 On the Limits of the Jurassic and Cretaceous Periods. 



pendence of the stages. So long as we were acquainted only with 

 the principal stages characterized by abundant faunas, we could 

 believe in their complete independence ; and this was certainly an 

 important general fact ; these successive well-defined faunas will 

 always remain the necessary guide for the teaching and justification 

 of our classification. But since many intermediate beds have been 

 studied and traced to great geographical distances, some transi- 

 tions have been met with. Fossils supposed to be very charac- 

 teristic, have in some places preceded the fauna with which they 

 principally lived, or have survived it a little. We have had to 

 learn that affairs have gone on as would have been the case in 

 the existing seas in consequence of gradual modification in the 

 nature of the bottom, the depth, and the temperature. We have 

 had to discover that, just as in our seas, the most marked faunas 

 are separated by intervals in which the species are more or less 

 mixed. Between the formations, as soon as they are known in 

 all their details, there are analogous passages. Thus, taking for 

 example the Cretaceous series, we know that whilst the principal 

 formations were long regarded as characterized by perfectly special 

 faunas, modern discoveries, while remarkably increasing the num- 

 ber of substages, have rendered most of the boundaries less defi- 

 nite than was supposed. The Valangian stage has many species 

 in common with the Neocomian, and the latter with the Lower 

 Urgonian. The Upper Urgonian has a pjtrt of the palseontologi- 

 cal characters of the Lower Aptian. The Upper Aptian has 

 some of those of the Gault. The Upper Gault, or Uraconian, 

 has come in to diminish the differences which distinguished the 

 Gault from the Cenomanian stage, &c. 



I do not wish to exaggerate these facts, the importance of 

 which is not sufficient to veil the existence of certain faunas re- 

 cognized by every body ; but they teach us that the stages can no 

 longer be characterized by one or two isolated fossils, but only by 

 the totality of the fauna. 



May not what we have just said with regard to the stages 

 into which the great formations are divisible, also apply to these 

 formations themselves? What is the philosophical argument 

 that leads us to regard these limits between the great forma- 

 tions as more strongly marked than others ? Limits are inven- 

 tions of science and useful as landmarks ; but who has told us 

 that there is an insuperable barrier between the last Jurassic and 

 the first Cretaceous stage ? Why should this barrier be so clear 

 in the Anglo-French basin and not throughout ? We should be 

 a good deal embarrassed to furnish a priori a plausible reason for 

 this. 



Here again we are in accord with M. Merian's note. Our 



