254: T. Davidson — Continental Geology. 



on account of the small fauna of the breccia of Aizy and Lemenc, 

 which is believed to be incontestably Jurassic. Since then I have 

 attentively re-examined the localities in order to furnish M. Pictet 

 and M. Hebert with the stratigraphical data of which they were in 

 need (neither the one nor the other having visited those localities 

 in the neighbourhood of Grenoble), and I have arrived at the 

 following conclusions: First, that the principal and lower mass of 

 the limestone of the Porte-de-France contains a fauna which is 

 entirely Oxfordian ; it is terminated by a bed with large Aptychus 

 (A. Icevis and A. lamellosus) which form the bottom of the quarry of 

 the Porte-de-France. 



" Secondly : In a bed of compact limestone situated above the last 

 described, the T. janitor (or dipliya) begins to appear along with 

 the Ammonites under discussion, which M. Hebert considers re- 

 ferable to Neocomien types, and which do not in any case occur 

 along with the Jurassic fauna underneath : moreover, no species 

 bearing an evident Jurassic type has been met with in this zone. 

 Above this, beds occur containing scarcely any other fossils save 

 T. janitor with T. gratianopolitana (Pictet). These are followed 

 (always in the ascending order) by others containing Ammonites, 

 the lithographic limestones are more developed and richer in fossils 

 at Aizy, and with them is intercalated, as a local accident, the breccia 

 of Aizy, of Lemenc, and other localities possessing a coralline facies. 



" Thirdly : Finally above all these comes the Argilo-bituminous 

 limestone with hydraulic cement, the fauna of which has been so 

 carefully studied by M. Pictet. We cannot doubt then that this is 

 the Lower Neocomien corresponding to the beds of Berrias. 



" The impending discussion can therefore only bear upon the 

 beds No. 2. Now it appears to me from the palEeontological studies 

 in connection vnth the fauna of this layer, that it has strong affinities 

 with the Neocomien and' contains no Ammonite of a decidedly Jurassic 

 type. As to the Echinoderms and the Brachiopoda found in the 

 breccia of Aizy, upon which my former opinion was chiefly 

 grounded, they are open to discussion. Even supposing some incon- 

 testably Jurassic species should be found therein, as for example 

 Acropeltis cequituherculata, Megerlea pectuncidoides, Terehratulina suh- 

 striata, etc., it is necessary to remark that they occur in a pudding- 

 stone, and in such a conglomerate, it is not impossible that these 

 fossils, detached perhaps from the Jurassic beds, should have inter- 

 mingled with the Neocomien ones, derived from the limestone 

 immediately underneath. To the mind of any one acquainted with 

 the locality, this breccia is evidently a shore-deposit, and this mix- 

 ture of fossils, under such circumstances of deposition, would not be 

 impossible. Therefore I am now entirely of the opinion of M. 

 Hebert, or rather, I believe that the pateontological discussions 

 raised on this subject will require to be solved in accordance with 

 his opinion, which appears the only one that would agree with my 

 stratigraphical studies. 



"It is clear from the comparative table I have sent you, that 

 on both sides, in the Jura and in the Alps there exists a break 



