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



tion the limits of the Jurassic and Cretaceous periods is a me- 

 moir by Oppel*, the purpose of which was to show that, between 

 those Jurassic stages which have been regarded as the uppermost, 

 and the lowest Cretaceous stage, there exist thick strata charac- 

 terized by a very rich fauna. These deposits had, indeed, been 

 noticed by several geologists, and especially in Switzerland by 

 MM. Studer, Brunner, and Fischer-Ooster, and in Germany by 

 MM. Suess, von Hauer, Peters, &c. ; but Oppel, by a detailed 

 comparison of the facts, gave them quite a new significance. He 

 grouped the whole of this series of deposits under the name of 

 the Tithonic stage, which he limited below by the Kimmeridgian 

 stage on which it often rests, aud above by the Neocomian stage. 

 He was inclined to regard it as the upper term of the Jurassic se- 

 ries. We have not here to discuss the value of this new name, 

 which seems to us to represent rather too large a totality j\ The 

 author himself moreover announced his Tithonic stage as constitu- 

 ting a provisional association, destined to be subdivided hereafter. 

 Considered generally and rather vaguely, it represents a right 

 idea by which science has made a positive advance. Applying 

 these data to the special case of the limestones of the Porte-de- 

 France, to which we shall refer by-and-by, Oppel was the first who 

 conceived the idea of reducing the age of these beds, by raising 

 them in the chronological series of formations. 



Whilst Oppel sought in this way to raise the superior term of 

 the Jurassic series, M. Hebert, taking up another phase of the 

 question, was inclined to lower the inferior term of the Cretaceous 

 series. For some years the learned Professor of the Sorbonne 

 had remarked that, below the bed designated by M. Lory the 

 Lower Neocomian marls, which passed as the oldest Neocomian 

 bed, there were some limestones possessing the lithological ap- 

 pearance of the subjacent Jurassic rocks, and nevertheless con- 

 taining Neocomian fossils [Ammonites macilentus &c.) . In 1861 

 our .colleague, Professor Studer, accompanied M. Hebert in a 

 journey the principal result of which was to show the existence 

 of this new term in the Cretaceous series. At first this discovery 

 attained but little publicity beyond that which was given to it 

 by general and personal statements made in the laboratory of the 

 Sorbonne to all the geologists and palaeontologists who visited 

 those interesting collections. 



I myself knew of it by conversations with M. Hebert; and 

 after having often pressed him to give a real publicity to these 



* " Die Titonische Etage," in the Zeitschrift der Deutschen Geologischen 

 Gesellschaft for 1865. Translated and abridged by M. de Loriol, Bibl. 

 Univ. {Archives) for January 1866. 



t MM. Benecke and Zittel have already proposed to exclude from it a 

 certain number of local beds, such as those of Solenhofen &c. 



