514 
The cretaceous rocks of Britain. 
C. The Terebratulina Zone. 
Microscopic Aspect. 
The chalk of the last mentioned zone passes imperceptibly into 
that of the Terebratulina zone. The proportion of amorphous 
calcareous matter becomes gradually larger than in the zone 
below, and Spheres and shell-fragments are less conspicuous in 
size and number. The Spheres, moreover, become gradually 
less defined in outline, their exterior shell wall is thinner or is not 
seen at all, their centres are filled with fine amorphous matter 
similar to the surrounding matrix in which they are embedded, 
and the cells instead of standing out in strong relief and in the 
well-marked circles which are so characteristic of the zone below, 
are often mere shadowy representations of them. (See Plate VIII. 
Fig. 1). Not only are the Spheres thus altered, but the cells 
of the Glohigerina are similarly affected, their walls being thinner 
and their general outline less prominent. 
The character of the chalk of this zone seems fairly constant 
throughout all the area east and north-east of Dorsetshire. The 
features of many specimens from the Isle of Wight, Dover, and 
parts of Kent, the Midland counties, Norfolk, Lincolnshire and 
Yorkshire are much alike. From the base Spheres and shell- 
fragments become scarcer until about the top of the lower third of 
the zone is reached, the proportion of amorphous material increas¬ 
ing. In the middle third of the zone the greater part of the rock 
consists of fine amorphous calcareous paste, with comparatively 
few Spheres, shell-fragments or Foraminifera scattered through 
the mass. But as the summit of the zone is approached all these 
ingredients become gradually more abundant, though Spheres 
seem never so large or robust or numerous as in the zone of 
Rhyne. Cuvieri. 
The upper third of the zone is characterised in some localities 
by the occurrence of numbers of minute Glohigerina and Textu- 
laria. This is particularly the case in a wide area of country 
S.W. and N.E. of Hitchin, also near Dover, and in the valle} r of 
the Medway, and again at Hessle, in Yorkshire. In a specimen 
from this last locality, probably about forty or fifty feet from the 
upper limit of the zone, Foraminifera though hardly so numerous 
are larger, and the number of Glohigerina greater than in more 
southern samples. 
Shell-fragments also increase in numbers in the higher parts of 
this zone, and some examples contain a considerable quantity of 
rather small angular caicitic particles. 
In North Dorset this zone presents some exceptional features, 
and beds of hard chalk occur, which seem to indicate that condi¬ 
tions akin to those of the Chalk Rock had already set in in some 
