380 SPECIAL GEOLOGY. 



The white chalk is nearly a pure carbonate of lime, and 

 contains, in some localities, an immense profusion of micros- 

 copic shells and the debris of radiated animals ; it is divided 

 into flinty chalk and chalk without flints. These siliceous 

 nodules indicate the plane of stratification of the beds. In 

 some parts of Yorkshire the lower chalk contains flints, 

 and the same fact is observed in the lower chalk at Havre : 

 the chalk marl consists of a greyish, earthy, marly chalk, 

 sometimes indurated. 



The upper greensand, in some localities, as at Black Down, 

 consists of a sharp siliceous sand ; in others, of a marly cal- 

 careous sand, with green grains and mica; the celebrated 

 firestone of Mertsham is of this character, and it presents 

 similar lithological features in the Isle of Wight ; the gault 

 is a dark blue tenacious clay, with indurated concretions; 

 in some localities, as at Folkestone, the nacreous layer of 

 f > its fossil shells is beautifully preserved. The lower green- 

 sand consists of an alternation of ferruginous sands, interca- 

 lated with beds of clay and clayey sand, sometimes containing 

 bands of limestone and regular seams of chert ; it attains, in 

 the Isle of Wight, a thickness of 808 feet, and is divided 

 into three subdivisions by Dr. Fitton : our table shows that 

 the lower greensand is the equivalent of the "Terrain 

 Neocomien" of the French geologists, MM. Cornuel and 

 Leymerie, which is largely developed in various parts of the 

 Continent, especially near Neufchatel.* It was supposed 

 that this system formed the marine equivalent of the 

 wealden, but subsequent researches have shown that the 

 opinion is incorrect. The neocomian fossils belong to the 

 same era, and many of the species are identical with those 

 of the lower greensand. Beds of this character, containing 

 similar shells, occur in Surrey and Kent, and at Atherfield, 

 in the Isle of Wight ; as these unquestionably belong to the 

 lower greensand, it results that the hypothesis which would 

 place these rocks, as they exist on the Continent, as the 

 equivalent of the wealden formation, must be abandoned; in 

 all probability the neocomian strata will be found to be a 

 larger development of a portion of our lower greensand 

 group. 



The fossils of the cretaceous system consist of marine 



* From whence the term Neocomien is derived. 



