T. Davidson — Continental Geology. 253 



Begion of the Central Jura. Subalpine Region. 



(Type situate^^Ween Besaii9on and ^r^^^ neighbourhood of Grenoble.) 



Gault. Gault. 



i Limestone of the Porte-de-France, large 

 mass (No. 1 of M Vi,i,i) Amm. flex- 

 uosus, A. tortisulcatus, A. plicatihs, 

 tatricus, iphicerus, etc. ; Belem. hastatus. 

 i Argillaceous Limestone and Marls, with 

 geodes, Belem. hastatus^ B. latisulcatus, 

 Amm. plicatilis, catialiculatus, tortisul- 

 catus, oculatus, etc. 



M. Lory writes to me, on the 29tli of April, " In all the papers 

 where I have had occasion to refer to the beds with Tereb. diphya 

 (or janitor, Pictet), of the Porte-de-France, of the lithographic 

 limestone and of tlie hydraulic cement beds which lie above, either 

 at the Porte-de-France, at Aizy, and other places in the neigh- 

 bourhood of Grenoble, or at Lemenc, and other spots in the vicinity 

 of Chambery, I had always, until within the last few years, con- 

 sidered these beds as Jurassic. In 1851 I made known the fauna 

 of the breccia of Aizy, as a last remnant of the extension of the 

 coral limestone which terminated at the border of the Alpine mass 

 (Bull. Soc. Geol. France, 2nd ser., vol. ix., p. 54), also the diiSculty 

 of tracing a limit between the limestone of the Porte-de-France and 

 the base of the true Neocomien of the Alps (ibid. p. 52). I concluded 

 that the Alpine mass had emerged above the sea after the deposition 

 of the upper Oxford clay, before (or during) the deposition of 

 the Coral rag, and subsequently replaced by a subsidence under 

 the sea at the commencement only of the Neocomien period [ibid. 

 p. 236, 238) an opinion adopted and maintained by M. Hebert, and 

 upon which we were always agreed. I have since shown that the 

 Neocomien deposits must have commenced in the Alpine region 

 before they spread in the Jurassic one : Since in the latter 

 the first deposits were the Valanginian limestone represented at 

 (jrrenoble, by the limestone of Fontanil, above which we find again 

 the large deposit of the marls with small Ammonites, and with 

 Belemnites latiis of the Mediterranean region (Bull. Soc. Geol., 

 vol. XV., p. 30), so that the Neocomien sea came from the South, 

 after having formed this deposit, and extended itself over the Jura 

 at the period of the deposition of the Valanginian limestone and 

 later again in the Haute Savona where we find only the marls 

 with Spatangus (Desc. Geol. of the Dauphine, p. 159 and 189). 



" In my opinion, and I believe in that of M. Hebert, this deposition 

 of Lower Neocomien strata took place during the long epoch of 

 intermission of the marine deposits in the Anglo-Parisian basin, 

 represented by the fresh water deposits of the Wealden. 



"In the last paper which I published on these beds, I still 

 maintained the upper beds of the Porte-de-France to be of Jurassic 

 age on account of the Ammonites which D'Orbigny had identified 

 as A. anceps, viator, Hommairei, Adelce, etc., occurring in beds, some 

 immediately below, and others above the principal horizon of the 

 Ter. diphya (Bull. 2nd ser., vol. xxiii., p. 516), and above all, 



