1844.] 405 



deed not improbable that this absence of fossils may be local, or 

 apparent, arising only from imperfect search.* 



It may be observed of all the supposed equivalents of the Neo- 

 comian beds, that while some species are very generally diffused, 

 others (and apparently the greater number) are peculiar to each 

 place. So that although some remarkable continental forms are 

 wanting in this country, we have in return many which do not 

 occur elsewhere. The recent examination of the Isle of Wight 

 has brought to light several species new to the continental faunas 

 as well as our own. f 



§. There can now be no reasonable doubt that the Terrain Neo- 

 comien of Central France, illustrated by M. Leymerie, as well as 

 that of Neufchatel, represent the lower and middle portions of 

 our sections at Hythe and in the Isle of Wight. We want, pro- 

 bably, many species of the Aube ; — but we have long possessed 

 a great number of the characteristic Neocomian forms, and are 

 daily adding to the list. The great difference consists in the 

 absence of calcareous strata below the clay of Atherfield — (the 

 Argiles Ostreennes of M. Leymerie) ; and the general question 

 seems to be reduced to this : — Whether the presence of a certain 

 number of characteristic fossils in two distant places is suificient to 

 establish geological identity — where not only the species, and the 

 numbers of the fossils, and the mineral composition of the strata is 

 varied, but where one of the two objects compared is so very much 

 superior in thickness and extent to the other, as the deposits of the 

 south of Europe are said to be with respect to that of central 

 France, — of England, — of northern G-ermany, — and even, it 

 would seem, of Neufchatel itself I believe that Geologists gene- 

 rally will be in favour of the identification by fossils — even in this 

 extreme case — and will be disposed to regard the greatest diversity 

 of mass and composition as nothing more than accidents — so called 

 perhaps because we are not able to account for them. 



If, on the other hand, it be found that, in the south or elsewhere, 

 the lower members of the so-called Neocomian (for the true Neo- 

 comian and the Lower Green-sand are unquestionably the same) 

 become distinguished notably by fossils new in form and in great 

 numbers, it would seem that the name of that portion of the deposit 

 so distinguished ought to be changed ; the upper strata remaining 

 with the cretaceous groups, and the lower being regarded as some- 

 thing new and different from the Cretaceous system. Whether 



* The place in the series of the Blackdown beds of Devonshire is still a sub- 

 ject of doubt. But if (which is not wholly improbable) they belong to the 

 upper part of the lower green-sand, the fossils in that division will then be nearly 

 as abundant as they are in the lower members at Atherfield, &c. The siliceous 

 casts are so remarkable, that if a similar deposit exists in many other parts of 

 this country, it is unlikely that they should have remained unknown. In the 

 upper part of the sands near Folkestone and Sandgate there are spongy siliceous 

 concretions very like the Devonshire whetstone, but in which shells are very rare. 



f Ammonites asper is one of the species generally diffused on the Continent, 

 which has never yet been discovered here. 



H H 2 



