2 1 8 Gault. 



it has been considered advisable to draw a marked line 

 between the two groups ; the Atherfield Clay and the 

 Lower Greensand, when the term Neocomian is not 

 applied to them, meaning Lower Cretaceous, and all 

 above them to the topmost beds of the Chalk being 

 considered as Upper Cretaceous strata. 



The GTAULT forms the base of the Upper Cretaceous 

 series or of the Cretaceous series, for those who choose 

 to call the Lower Ghreensand Neocomian. It is a stiff 

 blue clay, about 300 feet thick in its thickest develop- 

 ment, but sometimes it is hard to separate it lithologi- 

 cally from the Upper Greensand. It appears in the 

 Isle of Wight, overlying the Lower Grreensand all across 

 the Island ; and ranges round the escarpment of the 

 Weald in the same position, with occasional signs of a 

 kind of unconformable erosion between them ; and in 

 the centre of England, from the neighbourhood of 

 Devizes to the Wash in Norfolk, the Grault occasionally 

 completely overlaps the Lower Grreensand in an un- 

 conformable manner. In proof of this unconformity, 

 occasional outlying patches of the Lower Grreensand 

 occur north of the Chalk escarpment, without any 

 visible signs of it immediately at the base of the neigh- 

 bouring Upper Cretaceous strata, which there ought to 

 be, if these formations lay everywhere conformably on 

 the Lower Grreensand. 



Many Foraminifera have been found in the Grault, 

 and a few Corals, Cyclocyathus Fittoni, Trochosmilia 

 sulcata, and Caryophyllia Bowerbanldi. Its sea-urchins 

 are of the genera Cidaris (C. Gaultind), Hemiaster 

 (H. Asterias, H. Bailey i\ and Diadema tumida. It 

 contains many Crustaceans, such as Astacus, Etyus 

 Martini, Diaulax Carteriana, Palceocorystes Stokesii, 

 &c. Among the Brachiopoda and Lamellibranchiate 



