312 
THE CRETACEOUS ROCKS OF BRITAIN. 
Belemnite Marls, but in those obtained along the South Coast east 
of Portland, and in the Midlands north-east of Buckinghamshire, 
glauconite grains are of rare occurrence, especially in the higher 
parts of the zone, and are only seen in the residues when a 
quantity of chalk has been treated with acid. In the chalk of 
this zone from Lincolnshire and Yorkshire it is also rare. Dr. Hume 
notes it as infilling the tests of Porammifera in the Isle of Wight, 
and a few rods, probably residuary sponge spicules, were found in 
the residue of a specimen from Cerne Abbas, Dorset. Glau¬ 
conite, however, occurs in a specimen of bluish-grey chalk from 
Eastbourne, and abundantly in the “ Bag ” of Ivinghoe. 
In both the samples last mentioned, and more sparingly in the 
Chloritic Marl and in the Chalk Marl, there are masses of amor¬ 
phous green matter easily pulverised when dry, forming a tough 
cake if damp when subjected to pressure. In the residues it 
is seen to be porous, as though there had been included in its 
mass minute calcitic particles which had been dissolved out by the 
acid. Heated on platinum foil it turns to a red-brown like iron 
oxide- It is found associated with the glauconitic grains in the 
Totternhoe Stone of Arlesey (Bedfordshire) and Heacham, (Norfolk), 
and the amount of it equals, if not exceeds, that of the glauconite, 
while in the “ Bag ” of Ivinghoe it predominates. Like glauconite, 
it fills the tests of Foraminifera, forming casts, but it also occurs 
in irregular lumps or masses of comparatively large size, far larger 
than any glauconitic grain. 
There may be some relation between this green amorphous 
matter and the Green Muds described in the report of Deep Sea 
Deposits of the Challenger Expedition. * “ There is also,” says Sir 
John Murray, “ in these muds mixed with glauconite a greenish 
amorphous matter which in part at least appears, to be of organic 
origin, for it blackens on being heated on platinum foil leaving an 
ash coloured with oxide of iron. These muds and sands, are almost 
always developed along bold and exposed coasts where no very 
large rivers pour their detrital matters into the sea.” The deposits 
in which this material occurs most abundantly are certainly free 
from much coarse sandy material, and do not compare in this 
respect with those sandy beds at the base of the chalk where glau¬ 
conite occurs in the greatest profusion. 
Marcasite and Iron Oxides .—No bright brassy nodules of Mar- 
casite occur in any of the residues of this zone, though it is probable 
that small brown nodules and cylinders found in many residues 
may arise from the oxidisation of this material. Masses of porous 
ferruginous matter sometimes form a large part of the residues. 
In order to see if any differences existed between the material 
of the isolated bluish-grey patches so frequently seen in the Lower 
* Deep Sea Deposits. Beport of the Challenger Expedition, p. 236. 
