500 the cretaceous rocks oE rritaiK. 
to those whose form blends with and is hardly distinguishable 
from the surrounding chalk. In size they vary from a minute 
fragment not larger than a pin’s head, but whose outline is clearly 
defined in the surrounding material, to a rounded lump, as large as 
a fair-sized apple. 
Though the Melbourn Rock forms the basement-bed over the 
area occupied by Middle Chalk, it ceases to hold this position in 
South and West Dorset, where more or less of it is replaced by a 
bed or beds of hard smooth chalk destitute of the nodules which 
are so characteristic of the Melbourn Rock, In Devon, the basel- 
bed is a hard, rough yellowish limestone, which differs both from 
the Melbourn Rock and the smooth chalk of Dorset, 
Microscopic Aspect. 
Examined under the microscope in thin sections the appearance 
presented by the Melbourn Rock is alike nearly everywhere, the 
chief differences being in the definition of the outline of the nodules 
and the varying proportions and size of the shell-fragments, 
Foraminifera, and “ Spheres.” 
Before going further, a fuller reference must be made to the 
nature of the calcareous bodies which have been so often alluded 
to, and which are so important a constituent of many parts of the 
Lower and Middle Chalk. These Spheres are small bodies with 
either a perfectly spherical or slightly ovoid outline. They appear 
to be simple monothalamous cells, with comparatively stout walls, 
the material of which is now finely granular crystalline calcite. 
In both varieties the centre of the cell, though often hollow, is 
frequently filled with minute calcitic crystals. In size the 
Spheres are usually from ‘06 to ‘08 mm. in diameter, a few 
of the largest reaching *1 mm., rarely more. The ovoid forms 
are rather smaller than the average-sized Sphere. 
A great deal of doubt still exists as to the actual nature of these 
bodies. Early geologists referred to them generally as “ the young 
of Globigerina or other Foraminifera,” but if this was really the 
case in the many thousands present in the chalk, it would surely 
be possible to follow the Sphere to its adrdt stage. No such 
progress can be seen towards the fully developed Globigerina or 
any other Foraminifera, and, whatever their real nature may be, 
they certainly appear to have reached maturity. 
Professor Blake, in his paper on the Yorkshire Chalk,* mentions 
the occurrence of minute hollow Spheres. He was “ not able 
definitely to find any apertures in them ; otherwise they look like 
Orbulina universa , abounding to the exclusion of all other Fora¬ 
minifera.” 
# Proc. Geol. Assoc., Vol. v., p. 266 (1S78). 
