520 
THE CRETACEOUS ROCKS OF BRITAIN. 
Composition of the Fine Eesidue. 
As in the case of the Lower Chalk the finest material of the 
chalk composing the zones above described may be classed under 
the following different heads : — 
1. Definite calcitic crystals which have probably been formed 
since the deposition of the rock. 2. Minute particles probably 
derived from shell. 3. Thin flakes often perforated and doubtless 
derived from the ’ tests of Foraminifera. 4. Crystalloids and 
kindred forms. 5. Very minute particles too small as a whole 
to give any clue to their derivation, though some may be recognised 
as broken crystalloids. 
The difference between the finest material of the Rhynchonella 
Cuvieri zone and that of the Terebratulina chalk may almost be 
anticipated. In the shelly lower zone the matrix of the rock 
contains a larger proportion of calcitic particles and crystals, 
the former probably derived from the numberless Inocera- 
mus prisms with which the rock abounds ; in fact, it is possible 
to find particles which come away in the finest washings of whose 
identity as minute pieces of a prism there can be little doubt. Not 
only are large particles present in greater numbers, but the mass 
of the rock seems to consist largely of calcitic crystals, minute, it 
is true, but large enough to possess a definite outline, and giving 
a very finely-granular effect when mounted in balsam and viewed 
under the microscope with an objective of one-tenth inch focus. 
We were, moreover, struck by the fact that much of the matter 
held in suspension in the water after washing a specimen of the 
zone of Rhynh. Cuvieri subsided more rapidly than that of 
the overlying zone. 
Crystalloids are abundant in both zones; in the zone of 
Rhynchonella Cuvieri circular forms seem to predominate, but 
those of elliptical shape are also very common. None were 
like the true coccolith, the shirt-stud form was not seen. 
The constitution of the matrix of the Terebratulina chalk is 
very similar to that of the underlying zone, except that particles 
which may be referred to shell and the larger calcitic crystals are 
less numerous, the minute calcitic particles forming the greater 
part of the material. 
The amorphous inorganic matter, the presence of which is 
quite appreciable in the lower beds of the Chalk, cannot be recog¬ 
nised at this horizon. 
The fine clay of the residues presents no feature of interest. 
Mounted in Canada balsam it appears structureless matter, con¬ 
taining here and there a minute particle of some mineral. 
