of the Isle of Wight. 285 



signate, with MM. Thirria and Thurmann, by the epithet of 

 Portlandian. The other formation, that which occupies the 

 upper part, is the lower greensand, which admits as its pecu- 

 liar andevidently characteristic fossil the Exogyra sinnatay so 

 constant on the continent above the Neocomian formation. 



Thus here are two formations, one marine (Neocomian 

 formation), and the freshwater (Wealden), which appear to be 

 comprised between the same limits, and to which it is in the 

 first place very natural to assign the same level. The follow- 

 ing considerations support this view. 



When the Wealden, the extended prevalence of which be- 

 speaks a lengthened time of deposition, was formed in an es- 

 tuary into which fresh water flowed abundantly, the ordinary 

 sedimentation was probably not suspended in the seas at that 

 epoch ; there must therefore have been sediments deposited 

 on the margin of these seas, and these sediments appear to us 

 to be the Neocomian formation. 



Some would now regard the freshwater strata of the Wealds 

 as the representative of the last deposits of the Jurassic sea; 

 truly, we do not see any reason to think so. In fact, the upper 

 strata of the limestone of the Jura, which are intimately con- 

 nected, in the east of France, with the Kimmeridge group, 

 which forms the base of the deposit, cannot, it seems to us, 

 rise above the level of the Portland stone of England. We 

 should be rather led, by the consideration of the fossils, to 

 place them lower, on the contrary, towards the upper part of 

 the Kimmeridge clay. With respect to the analogy remarked 

 between the forms of the fishes and reptiles of the Weald for- 

 mation and of the Jura, we may be permitted to say that it is 

 too vague and too uncertain to attract our serious attention. 



Moreover, purely geological considerations are also opposed 

 to the adoption of the new theory. In fact, the commencement 

 of the freshwater deposit, which is called the Wealden^ must 

 have been occasioned by a removal of the waters of the seas, 

 and by a corresponding change in the mineralogical and zoo- 

 logical conditions which it is reasonable to attribute to the 

 general cause which produced the annihilation of the Jurassic 

 fauna, by preparing a new order of things which is designated 

 by the epithet of cretaceous^ The end of the freshwater period 

 here in question would then find its cause very naturally in 

 this new transport of the waters, which denuded the Neoco- 

 mian formation before the deposit of the Exogyra sinuata, and 

 which gave rise to the unconformity of stratification which we 

 have remarked between the Neocomian formation and the 

 greensand of Champagne. 



It would then be at the end of the Neocomian period that 



