264 
THE CRETACEOUS ROCKS OF BRITAIN. 
In dealing with the different parts of the Lower Chalk we shall 
describe them in the following order : — 
A. Chloritic Marl (base of the Am. varians zone). 
B. The quartziferous Limestones of Devon. 
C. The Marls and Chalks of the Am. varians zone. 
D. The Totternhoe Stone. 
E. Chalk above the Totternhoe Stone. 
E. The Belemnite Marls. 
Of these subdivisions, the first two will be dealt with in 
this chapter. 
A. The Chloritic Marl and Basement Beds. 
Macroscopic Aspect. 
In this division is included not only the Chloritic Marl (zone of 
Stauronema Carteri ) but also other beds at the base of the Chalk 
which may not lie on exactly the same palaeontological horizon, 
but which from their glauconitic and sandy nature bear a strong 
lithological resemblance to it. 
The specimens are from the following localities from west to 
east: — Bookham Earm, North Dorset (the basement bed, but 
probably not exactly the same horizon as the Chloritic Marl of the 
Isle of Wight); Lulworth and Mupe Bay in South Dorset; 
Maiden Bradley, near Warminster, Wilts; Yentnor, Isle of Wight 
(Ammonite Bed); Folkestone (Greensand); Cambridge Greensand 
from Arlesey; and a bed of micaceous marl lying at the base of 
the Chalk at Sliouldham, Norfolk. 
There is a considerable diversity in the aspect of the specimens. 
In colour they range from pale grey, Bookham and Sliouldham; 
to a darker grey at Maiden Bradley ; grey, sometimes with greenish 
tint, Isle of Wight; and a decided grey-green at Folkestone. 
The Bookham and Lul worth specimens appear to be chalky 
marls plentifully speckled with glauconitic grains; the highest 
part of the glauconitic marl of Mupe Bay is similar, but passes 
down into a green compact sandy rock; that from Maiden Bradley 
a very sandy, rather hard grey, but somewhat friable stone; 
that from the Isle of Wight a compact glauconitic rock. The 
Folkestone sample is a crumbling glauconitic marl, and that 
from Shouldham a compact marly chalk. 
Microscopic Aspect of Thin Sections. 
Viewed in thin sections under the microscope with an objective 
of two-thirds of an inch focus, the aspect of these specimens varies 
as their external appearance would indicate. That from Bookham 
(1)* is seen to consist largely of fine amorphous calcareous material, 
with which are intermingled the glauconitic and mineral grains with 
a few shell fragments; Foraminifera occur, but take up a very small 
* The numbers in the text refer to the analyses of the specimens in the 
tables on pp. 271 to 324. 
