300 
THE CRETACEOUS ROCKS OF BRITAIN. 
CHAPTER XXIII. 
THE MICROGRAPHIC STRUCTURE OE THE BEDS 
FORMING THE ZONE OF HOLASTER SUBGLOBOSUS. 
by W. Hill. 
D. The Totternhoe Stone. 
Tlie external appearance of this bed is that of a hard grey gritty 
chalk, usually containing small brown-black phosphatic nodules, 
and when examined with a hand lens is seen to be speckled with 
black points which are grains of glauconite. The stone occurs in 
lenticular beds of varying thickness, in which shell-fragments pre¬ 
dominate, and which are separated in Bedfordshire, Hertfordshire, 
and Cambridgeshire, by layers of less shelly chalk. 
Microscopic Aspect of Thin Sections. 
When thin slices of the stone—which is most esteemed for build¬ 
ing purposes, and is quarried for that purpose at Totternhoe 
(Bedfordshire) and Burwell (Cambridgeshire)—are examined under 
the microscope, the shell-fragments are seen to be evenly dis¬ 
tributed and to have a uniform size. The greater part of them can 
be recognised as prisms of Inoceramus-shell, and they constitute at 
least 60 to 70 per cent, of the mass. The remainder consists of glau¬ 
conitic and minute mineral grains, both exceptionally abundant 
compared with the Chalk above and below, and Foraminifera, 
amongst which forms of Glohigerina, Textularia, and Rotalina 
can be recognised. The whole is compacted together in a matrix 
of amorphous material, but in some places, especially where shell- 
fragments are coarse, probably allowing an easy circulation of 
water, the matrix is chiefly granular crystalline calcite (see PI. V.) 
In the more northern counties the Totternhoe Stone, though 
thin, is well marked, and its general characters differ little from 
the stone at Totternhoe, except in the greater irregularity in the 
size of the shell-fragments and the gradual decrease of the sandy 
elements. Through Norfolk, Lincolnshire, and Yorkshire it pre¬ 
serves its character as a thin lied of shelly chalk, and at the final 
exposure in the Speeton Cliffs it can still be recognised, though 
apparently from the increase of amorphous material the bed may 
be again passing laterally to less shelly chalk. 
Examination of the Washings. 
Portions of the Totternhoe Stone from Arlesey in Bedfordshire 
and Heacham in Norfolk were washed in water and much of the 
