392 JUKES-BROWNE I MICROSCOPIC STRUCTURE OF ZONES OF CHALK. 
The zone of Belemnitella mucronata has the same characteristics, 
generally under one per cent, of clay, no glauconite, and detrital 
minerals very rare. The material is a fine calcareous dust or paste, 
in which small calcareous spheres and a few small Foraminifera and 
shell fragments are distributed. 
4. Inferences. 
There is one fact which seems clearly proved by the preceding 
details, and this is that the inorganic constituents of Chalk are most 
abundant in the South of England. There seem to be several zones 
in which the amount of quartz and glauconite increases as they are 
traced southward. From this we may infer that the main mass of 
land lay somewhere in a southerly direction. 
I may here mention that the evidence of the French Chalk shows 
that this land lay to the east of France, and consequently to the 
south-east of England. At the same time there are indications of 
land to the south-west and good reasons for believing that this was 
a large island which presented a rocky shore to the east, its rivers 
draining toward its western coasts. 
With respect to the depth of the Chalk sea we know that the 
period was one of great and extensive subsidence, but there is evi- 
dence that this subsidence was at any rate in one area interrupted 
by upheaval. It is obvious that if we can form an opinion as to the 
depth in which the Chalk Marl was formed, we shall have a basis 
from which to trace the subsequent changes. 
We must remember that the Chalk Marl is not the base of the 
Upper Cretaceous series. The beds beneath it are not shore sands 
but marly clays and greensands, which seem to have been formed in 
water that was not less than 100 fathoms in depth, and may have 
been much more in the central part of our area. There is every 
reason to suppose that the Chalk Marl was formed in deeper water 
than the Gault, but the large amount of sand and glauconite which 
it contains, especially in the southern counties, makes it probable that 
no part of this southern area was more than 200 miles from land. 
Two lines of evidence lead to the conclusion that the Chalk 
Marl was formed in water of from 300 to 500 fathoms. One is the 
