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



It then became necessary to have a better knowledge of the 

 TerebratulcB of the dip%a-group*, inasmuch as arguments, in 

 my opinion erroneous, were drawn from them to justify exagge- 

 rated associations. I distinguished several species, the characters 

 of which are, in my opinion, perfectly clear ; and I showed that 

 these species, although but little apart as regarded their age, may 

 serve to characterize certain distinct beds. 



These two preparatory investigations being made, I also took 

 up the study of the limestones of the Porte-de-France, and I 

 summed up my opinion nearly as followsf, taking as my basis, 

 like M. Hebert, the classical memoirs of M. Lory. 



The section of Grenoble and of Aizy is composed of three 

 principal stages. 



The base is occupied by a Jurassic fauna, analogous by its 

 fossils to that of the beds of Baden in Argovia, or to the fauna 

 of Ammonites tenuilobatus. This fauna, which in Germany has 

 often the same geological relations with the Stramberg limestone, 

 is regarded by M. Hebert as Oxfordian, and by MM. Beuecke, 

 Zittel, &c. as Kimmeridgian J. 



Above this, and from the appearance of Terebratula janitor 

 (formerly confounded with T. diphya), there is a limestone which, 

 by its fossils, perfectly resembles the Stramberg limestone, with 

 which it is incontestably contemporaneous. With this limestone 

 there is associated at Aizy a coralline breccia which seems to 

 contain a mixture of Jurassic and Cretaceous species. 



The top of the section (cement-marls) is formed by beds iden- 

 tical with the limestone of Berrias. 



Now, where is the limit between the two periods ? This is 

 what remains to be fixed. I supposed a line, A, between the first 

 formation and the second, and a line B, of less importance, be- 

 tween the second and the third. I showed that the limit was 

 perhaps at the line A, less probably at the line B, and perhaps, 

 again, between the two. M. Hebert accepts the line A. We 

 shall see hereafter the difficulties which surround the solution of 

 this question. 



It may be added, for the comprehension of the relations of this 

 section with our faunas of the Swiss Jura, that the limestone of 



* Melanges paleontologiques, 3 me livr. vol. i. p. 143. 



t Ibid. 4 me livr. vol. i. p. 205. See also "Nouveaux documents sur 

 les limites de la periode Cretacee " &c. (Bibl. Univ., Arch., June 1867), and 

 " Notice sur les Calcaires de la Porte-de- France " (ibid. October 1867). 



X I have adopted this last opinion, but I must add that the determination 

 of the exact age of the Jurassic bed has been to me but a secondary ques- 

 tion, of which I leave the discussion to others. It must be remarked that 

 the facies of these faunas varies much in different places, and renders ne- 

 cessary a long and complicated investigation, for which I do not possess the 

 requisite materials. 



