326 M. F. J. Pictet on the present state of the question as to 



To these possible solutions, which have already been discussed, 

 two others may be added, which we shall analyze further on, and 

 which tend to acquire a certain amount of probability. 



May it not pass through the middle of No. 4, separating a 

 Jurassic from a Cretaceous Tithonian ? 



And, lastly, is not this line an illusion ? and is it not possible 

 that there has been no interruption between the two periods ? 



Before going into details, I must say that it is here especially 

 that, under some points of view, I have diverged from M. He- 

 bert; but our divergences have been considerably exaggerated, 

 and relate especially to a question of method. Now the more 

 we approach the solution, the more will the facts acquired predo- 

 minate over this question of method; and when the truth shall 

 be rendered incontestable, it will be of less importance to know 

 by what course we have arrived at it. 



M. Hebert now declares* that he has occupied himself only 

 with the question of the Porte-de-France, and that the bed with 

 Terebratula diphya {janitor) is incontestably Neocomianf. He 

 retains some doubts with regard to the Coralline breccia super- 

 posed upon or associated with the limestone of Aizy, of which I 

 have already spoken. 



The opinion which I put forward in my first note, and which 

 I have since constantly maintained, is that the question must be 

 solved by a general comparison of the strata in dispute over a 

 large geographical space. The solution must be capable of ap- 

 plication not only to the south of France, but also to the Car- 

 pathians, to Italy, to the Tyrol, to Spain, to Algeria, &c. ; we 

 may consequently expect that it will be very complex. 



I must remark, indeed, that the Department of the Tsere alone, 

 notwithstanding the excellent works of M. Lory, does not fur- 

 nish sufficient documents. M. Hebert and myself have worked 

 upon the same materials, which are but scanty, and among which 

 there are numerous species of uncertain origin. We have de- 

 duced from them, I believe, all that can be deduced ; we may, 

 perhaps, even be accused of having acted sometimes with a little 

 temerity. For absolute certainty, more well-characterized species 

 found in situ in a very clear section would be necessary. I con- 

 tinue to be profoundly convinced that the Lithographic limestones 

 of the Porte-de- France and Aizy must follow the lot of those of 



* Geological Magazine, July 1869, vol. vi. p. 301. 



t M. Hebert would perhaps have done better had he made use of the 

 word Cretaceous ; for however possible we may consider it that the Titho- 

 nian stage forms the oldest member of the Cretaceous period, we think we 

 should refuse the name of Neocomiati to a formation so profoundly different 

 from the stage which bears this name, and which has not 5 per cent, of 

 species in common with it. But this is a secondary question, and we shall 

 proceed with the discussion as if M. Hebert had employed the broader word. 



