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
wanting in Hertfordshire. There are, however, some widespread deposits
of loamy sands which may possibly be rearranged material from the Thanet
Beds.

The lowest Eocene deposits in the county are the _Reading Beds_. These
rest directly upon the Chalk and have an average thickness of, say, 25
feet. They may be traced E. to S.W. from the brickfields near Hertford
to Hatfield Park; thence to the kilns on Watford Heath and at Bushey;
they may also be traced from Watford to Harefield Park. These beds
contain flints, usually found close to the Chalk, and consist chiefly of
mottled clays, sands, and pebble-beds. Fossils are but rarely found.
From the Woolwich and Reading Beds come those conglomerate masses of
flint pebbles commonly called Hertfordshire _plum-pudding stone_. These
have usually a silicious matrix and were often used by the Romans and
others for making querns for corn-grinding. It is, perhaps, not
impertinent to mention here the opinion of geologists that during the
_Eocene Period_ a considerable portion of the land usually spoken of as
S.E. England was covered by the ocean.

Resting upon the _Reading B