114 NOTES ON A MAP INCLUDING GREATER PART OF S.E. ENGLAND.
beds than the rest of the district included in Index Sheet No. 12,
makes the omission of the various drifts a special gain as regards
that part of that map. For its use is not to give sound notions
respecting the soil of Essex, but clear information on the general
structure of the London Basin. And it would need a very keen and
practised eye to trace the outcrop of the London Clay beneath the
Boulder Clay from Bishop's Stortford to Sudbury and thence to,
Ipswich but for the omission of the Glacial Drift.
In Index Sheet No. 12, the newest beds shown, with the
exception of the alluvium of the rivers, are those known as the
Coralline, Red and Norwich "Crags," with the Chillesford Beds ;
and, south of the Thames, the deposits discovered on the Chalk
Downs near Lenham in Kent by Mr. Clement Reid. These all
belong to the Pliocene system, and cover a considerable area of
ground around Ipswich. Then, next in date are the considerable
patch of (Eocene) Bagshot Beds, between Guildford and Windsor,
with the smaller outliers to be seen here and there, especially in
South Essex, around Brentwood and at Rayleigh. Then comes the
London Clay, which occupies a very much larger proportion of the
surface than any other Eocene formation, while the lowest Tertiary
deposits, the Thanet Sand and the Woolwich and Reading Beds, with
those of the Oldhaven and Blackheath series, appear as a narrow band
on the outer edge of the London Clay, along a line ranging from
Reading W. and N. of Windsor to Ware, Bishop's Stortford, Sudbury
and Ipswich on the northern edge of the Tertiary Basin, and on the
southern from Farnham and Guildford to Epsom and Croydon, and
thence to Rochester, Canterbury and Sandwich. The Tertiary Beds
of the London Basin thus widen eastward and narrow westward.
Then we have a Chalk district beyond, that is, south of the Tertiary
Beds in Kent and Surrey, and also north of them in north-western
Essex, Bedfordshire, and Buckinghamshire : while beds older than
the Chalk may be seen in the Weald of Kent, Surrey, and Sussex,
and again at the north-west corner of the map beyond Prince's
Risborough, Dunstable, Hitchin and Shelford. The oldest beds
shown in Index Sheet No., 12 are some belonging to the Upper Lias,
a little beyond Wolverton, on the London, and North-Western
Railway.
But my object this evening is not to discourse on the Geology of
the London Basin, but to bring under your notice Index Sheet
No. 12, to point out its merits, and the novelty attending it as the