CHALK FORMATION. 79 



IX. 



§ II. CHALK FORMATION. 



Comprising 5. Blue chalk marl, or Gait. 



6. Grey chalk marl. 



7. Lower chalk. 



8. Upper, or flinty chalk. 



This formation constitutes one of the most striking features in the 

 geology of Sussex. It forms four principal divisions, distinguished from 

 each other by their chemical characters, and mineralogical productions. 



The uppermost consists of chalk, with numerous parallel beds and 

 layers of flint. 



The next is the lower chalk, containing but very few flints, and in 

 most localities being wholly destitute of them. 



The third is the grey chalk marl, composed of chalk, and a considerable 

 proportion of argillaceous earth. 



The lowermost is the blue chalk marl, or gait, that intervenes between 

 the grey marl and the green sand, and in some parts of its course, passes 

 into a compact limestone. 



The flinty chalk forms the summit, and the chalk without flints the 

 central mass of the South Downs ; the base of this range being composed 

 of the grey marl, which is denuded in the deep vaUies of the chalk, and in 

 many places unites the insulated portions of that formation. The blue 

 chalk marl rises from beneath the grey marl, and forms a narrow fillet of 

 stiff" land, on the northern edge of the Downs. 



The relative situation of these deposits is shewn in the section an- 

 nexed to the map ; and in the plan of the stratification of the south- 

 eastern part of Sussex, tab. in. fig. 1. 



According to the plan adopted in this work, we shall commence our 

 investigation with the lowermost deposit, the blue chalk marl. 



