270 THE CRETACEOUS ROCKS OF BRITAIN. 
1-40 to 1-60 in. diameter and less, are marked into projecting lobes 
by deep sideations, generally lined at the bottom with white 
calcareous matter, which serves to mark out the limits of each 
lobe. These forms are derived from Textularia, Lituola and 
Bulimina; in size they correspond very closely with such a 
derivation; just passing through the meshes of a sieve which 
retain almost all the numerous Bulimince mingled with them. 
Each lobation of the cast corresponds to the interior of a chamber 
of the Foraminifer, and each white calcareous line between the 
lobes is the edge of the septum which once separated adjoining 
chambers, and now in some cases extends inwards between their 
casts. This is shown by sections made in various directions through 
the green grains. . .. Sometimes Bulimina are met with having 
part of their shell worn away, revealing a green cast within.” 
Besides the above he noted grains derived from casts of Orthocerina, 
Rotalia, G-lobigerina Nodosaria and Lagena. 
In the basal bed at Shouldham, a finer and more chalk-like 
deposit, the grains are smoother and rounder, a large number of 
them being well-formed casts of Foraminifera. 
In colour the grains vary from dark green, almost black, to a 
pale yellowish-grey, and frequently the grains have a thin, pale- 
green rind of dusty appearance as though they were weathered. 
Professor Sollas remarks that the grains in the Cambridge Green¬ 
sand, when slices are viewed with polarised light, show distinct 
colours, dark green with parallel prisms, and bright lighter-coloured 
green with crossed prisms. 
The largest grains occur in the Chloritic Marl of Folkestone, 
they average '31 mm., and this is closely approached in 
specimens from the West of England. Though the largest grain 
measured occurred in the glauconitic marl of Mupe Bay, and was 
•81 mm. in longest diameter, probably grains of equal size could 
be found in the basement-beds of other localities. 
Marcasite and Ferruginous Matter.—No nodules of bright brassy 
Marcasite occurred in our specimens of the Chloritic Marl, but 
possibly the brown porous masses of iron oxide which were found 
in several specimens have resulted from the alteration of this 
mineral. At Shouldham were many sponge spicules coated with 
Limonite. The amorphous matrix will be dealt with under the 
head of Chalk Marl. 
Tabulation of Results. 
The following table gives the results of the analysis of nine speci¬ 
mens of the basement beds of the chalk in a tabular form. The 
methods by of which these results have been arrived at and 
the explanation of the various columns have already been given 
on page 262 at the commencement of this chapter The speci¬ 
mens in this and all following tables are taken from West to East: — 
