826 THE CRETACEOUS ROCKS OF BRITAIK. 
commonly as fossils in these rocks, the interstices between the 
particles being filled with finer calcareous matter derived from 
the same source. 
At Mupe Bay, South Dorset, quartz-sand forms a con¬ 
siderable part of the residue, but it is accompanied by a greater 
proportion of fine mud. At Cerne sand is also present in some quan¬ 
tity, but as usual the finest material is of a calcareous nature, and 
the amount soluble in acid is larger than at Mupe Bay. 
Following the zone to the northward, along the outcrop of the 
Chalk through Wiltshire, Oxfordshire, and Berkshire, the aspect of 
the chalk when viewed in thin sections under the microscope sug¬ 
gests a slight current action, the rock being full of small, evenly- 
sized shell-fragments, and in these counties there is also an increase 
in the size of the mineral particles. Though fine inorganic material 
or fine mud is certainly present, the amount of it is not large. 
Passing through Buckinghamshire into Bedfordshire analysis 
shows that there is a large accession of fine mud in the middle 
part of the zone, both the lower and higher parts containing a 
larger percentage of soluble material. 
From the evidence both of thin sections and analysis it is clear 
that at the top of the zone the proportion of inorganic matter is 
everywhere decreasing and the rock becoming a purer chalk, 
though still containing much terrigenous material. 
The fact seems now well established that to the north-east of Bed¬ 
fordshire, through Norfolk and Suffolk, the rock passes laterally to 
a pure chalk. This continues through Lincolnshire and the greater 
part of Yorkshire, till at Speeton we again touch the fringe of shore¬ 
line deposits. It may be remarked that not until the purer chalk 
is reached do G-lobigerinw frequently appear in the thin sections. 
Returning to the south coast we find in the Isle of Wight an in¬ 
crease in the amount of fine mud in the middle part of this zone, 
as is the case in Bedfordshire ; but at Folkestone there is a gradual 
decrease in the proportion both of coarse and fine inorganic matter 
upwards from the base of the zone. 
In the Totternhoe Stone at the base of the overlying zone 
we again have evidence of the sweeping action of a cur¬ 
rent which has resulted in the sorting of calcareous and 
mineral particles, but there is no evidence of a mud-bearing 
stream. This sorting of the coarser particles and the sweeping 
away of the fine mud has resulted in the formation of a rock 
which, when viewed in a thin slice under the microscope, is seen 
to be a fine, calcareous sand, consisting chiefly of small even-sized 
fragments of shell with a small proportion of mineral grains. 
In the succeeding beds, as we have already seen, the chief feature 
is the progressive decrease of terrigenous material and the gradual 
approach to comparative uniformity in the composition of the mate¬ 
rial throughout England. Features standing out strongly in the zone 
below are still, however, faintly repeated in this even to its very 
summit. Thus specimens from the south-west of England still 
