296 THE WONDERS OF GEOLOGY. Lect. IV. 



3. The Cretaceous ok Chalk formation. — The pure 

 white limestone called Chalk, is known to every one ; but* 

 in the nomenclature of geology, the term is applied to a 

 group of deposits very dissimilar in lithological composition, 

 but agreeing in the nature of the organic remains which 

 they contain, and evidently referable to the same geo- 

 logical epoch. The series essentially consists of green and 

 ferruginous sands, clays, marls, and grey and white lime- 

 stones, abounding in marine remains. With this explanation 

 it will be convenient to employ the term in its extended 

 sense. The Chalk Formation comprises the following prin- 

 cipal subdivisions : — 



1. Upper chalk f with flints . . . . ^ Craie blanche of the 



2. Lower chalk, without flints ... J French Geologists. 



( Chalk marl . . . . • Craie tufeau. 



3. Firestone, \ Upper green sand, or \ .^ 



' <ni •* J Glauconie crayeuse. 



4. Gait, or Folkstone marl 

 xreensand, or Shanklin 



green-sand j Terrain Neocomien. 



5. Greensand, or Shanklin, or Lower- ) Glauconie sableuse. 



The Chalk is generally white, but in some countries is of 

 a deep red, and in others of a yellow colour ; nodules and 

 veins of flint occur in the upper division, but seldom in the 

 lower chalk of England. The Marl is an argillaceous lime- 

 stone, which is very constantly found underlying the chalk ; 

 the lower beds contain a large intermixture of green sand, 

 and form a coarse calcareous sandstone, called Firestone. 

 The Gait is a stiff, dark blue clay, abounding in shells, 

 which, as in most argillaceous beds, retain their pearly 

 investment unimpaired in lustre. The Greensand is a 

 triple alternation of sands and sandstone with clays ; with 

 beds of chert and fuller's earth in some localities. 



On the Continent the cretaceous deposits are largely de- 

 veloped ; and taken as a whole, the chalk formation may be 

 described as extending over a great part of the south-east of 



