LOWER CHALK—MICROGRAfTIIC STRUCTURE. 319 
These, as before, show a gradual decrease in the amount of residue 
from west to east, but a point to be noticed here is that the Hun¬ 
stanton specimen contains a slight increase in the percentage of 
fine mud when compared with those from Hitchin, Hertfordshire, 
and Louth, Lincolnshire. The Belemnite Marls are absent in North¬ 
west Norfolk, and it is possible the fine mud which we shall see 
present in the next zone may have reached this district in suffi¬ 
cient quantities to affect the deposit, but not enough to form the 
marked marly bed which exists where the sub-zone of Actinocamax 
plenus is well developed. The specimen was taken about 6 feet 
below well-marked Melbourn Rock. At Cerne there is also a higher 
percentage of fine mud ; in this locality inorganic matter is 
found in the chalk for some distance below the Melbourn Rock, 
and it is possible that this specimen is affected by the incoming of 
fine mud which reached its climax in the Marls above. 
Taken as a whole, specimens of this zone show a gradual decrease 
of fine inorganic material when each locality is compared in vertical 
section. 
F. The Belemnite Marls. 
Macroscopic Aspects. 
The appearance of the Belemnite Marls is everywhere that of a 
marly chalk, breaking into thin laminae under the influence of 
weather. Though usually soft, in a few localities, notably at Louth 
in Lincolnshire, the marl is indurated by infiltrated calcite, and is 
somewhat hard. 
The colour is most frequently grey with a greenish tinge, but 
at Dover (Kent) and at Brockham (Surrey) it is a pale yellowish 
grey; at South Cave (Yorkshire) the colour is grey with inter¬ 
calated layers of almost black clay. 
In the counties of Bedford, Hertford, and Cambridge, especially, 
but in other localities also, the Belemnite marls are separated into 
two beds by a hard, smooth, white rock, the character of which is 
very constant. It consists wholly of “ spheres ” and amorphous 
matter, foraminifera and shell-fragments being rare. This bed of 
hard white chalk appears to be broken up in some localities, and 
a nodular rock occupies its place. The nodules of this rock have 
the same structure of the hard white bed, the surrounding matrix 
being full of shell-fragments. 
Microscopic Aspect. 
The appearance of the Belemnite marls from various localities 
when viewed under the microscope in thin sections was as follows : — 
A specimen from South Cave, Yorkshire (very dark bluish-grey 
in colour).—Fine amorphous matter predominates. It appears to 
be largely inorganic, and is full of opaque particles; minute mineral 
grains occur throughout. No foraminifera! cells or “ spheres ” 
occur, but there are a few small shell-fragments. 
