226 



of each stage, more than on its lithological character and physical 

 development, that we must depend for a true determination of 

 its age and correlation. 



KiMMERIDIEN = KiMMEEIDGE ClAT. 



The upper Jurassic rocks present a greater diversity of character 

 in different regions than the members of the middle and lower 

 divisions, and the difficulty of establishing the synchronism of 

 their several stages increases in our ascent from the Coral Ea^ 

 to the summit of the Purbeck series ; several reasons may be 

 given for this, but our chief obstacle is the want of a better 

 classification, and more detailed study of the Kimmeridge clay 

 and Portland formations of England, and this is the reason for 

 my reticence regarding the correlation of some of these deposits. 

 M. Maetin defi-nes I'etage Kimmeridien thus : — 



KiMMERiDiEN ^ ^I'lrnes et calcaires marno-compactes a Ostrea virgula, 

 { Calcaires a Pteroceren avec quelques Ostrea virgula. 



M. G. de Nerville * includes the calcaire a Pteroceren in 

 his groupe corallien of which it forms the uppermost terrain, 

 and thus defines it "Calcaire a Pteroceres. Calcaire jaune, a 

 points verts, sableux, renfermant quelques minces bancs de 

 marnes sableuses, caracterises par le Pterocera Oceani." The 

 marnes a Ostrea virgula he makes the base of his groupe Kim- 

 meridien et Portlandien, and describes as "marnes blanchatres 

 renfermant une grande quantite de Gryphees virgules, Terre a 

 fours." In the Haute-Saone M. Thirria unites the two in 

 one group, as "calcaires et marnes a Grj'phees virgules," and 

 describes as raarne grisatre, divisee en plusieurs assises par de 

 minces bancs de calc. marn. avec Ammonites gigas, Nautilus 

 giganteus, Pterocera Oceani, Pholadomya protei, Ceromya excen~ 

 trica, Ceromya inflata, Exogyra virgula, Exogyra Bruntrutana, 

 Ostrea solitaria, Nerincea grandis, Nerincea cylindrica, &c. These 

 beds form the groupe de Porrentruy of M. Marcotj in his Jura 

 sulinois, and they are les argiles a Ostrea virgula de Loxeville, 

 of M. E. PiETTE, in the department of the Meuse, and are 



* Legendo explicative de la carte geolog. du Depart, de la C6te-d'0r : 1853, 



