_The Lower Chalk_ is devoid of flints, and rests, in somewhat steeply
sloping beds, upon the Totternhoe stone. It forms the western slopes of
the Dunstable Downs, and of the Chiltern Hills. It is fossiliferous, one
of the commonest of its shells being the Terebratula.

_The Middle Chalk_, of resonant hardness, is laminated, and has at its
base the Melbourn Rock and at its summit the Chalk Rock. Nodules of
flint, greenish in appearance, and (rarely) arranged in layers, occur
sparsely in the Middle Chalk, which may be traced in the neighbourhood
of Boxmoor, Berkhampstead and Baldock, and also in a few other
districts.

_The Upper Chalk._--Although, as has been stated, the configuration of
Hertfordshire is very undulating, we are able to discern a general trend
in certain districts. Thus, there is a gradual slope to the S. from the
N.W. and central hills, a slope which comprises the larger part of the
county. This slope is formed of the Upper Chalk, a formation abounding
in layers of black flints. The chalk is whiter than that of the lower
beds, and very much softer. Fossil sponges, sea-urchins, etc., are
abundant in this formation.

TERTIARY.--Many of the chalk hills of Hertfordshire are strewn with
outlying more recent deposits which prove that the lower Tertiary beds
were more extensive in remote ages. The beds of sand and clay, of such
frequent occurrence in the S.E. districts, contain fossils so distinct
from those of the Upper Chalk that an immense interval must have elapsed
before those Tertiary deposits were in turn laid down.

_The Eocene Formation._--The _Thanet Beds_, of light-coloured sands,
present in some other parts of the London Basin, notably in Kent, are