SUBTERRANEAN GEOLOGY OF SOUTH-EASTERN ENGLAND. 139
as that of Sir Andrew Ramsay, shows us that the oldest, or Palaeozoic
rocks, are confined to the north-western half of the country. A line
ranging from Hartlepool through Doncaster, Nottingham, Leicester,
Stratford-on-Avon, Bath and Sherborne, to Sidmouth in Devonshire,
would divide a western country, consisting of districts mainly
Palaeozoic, from an eastern wholly composed of Secondary and
Tertiary formations. Thus the Londoner who wishes to see some-
thing of the nearest Palaeozoic rocks must travel either due west to
the Bristol Coalfield or the Mendip Hills, or in a north-westerly
direction to Nuneaton, or beyond Leicester. For a journey by the
Great Northern Railway will not enable him to see any sections in
Palaeozoic rocks south of the county of Durham. And as Nuneaton,
the nearest to London of the places just mentioned, appears to be
97 miles away, it is evident that the difficulty of ascertaining
anything about the nature and depth of the Palaeozoic rocks
beneath the Secondary and Tertiary formations, which alone are seen
in the neighbourhood of the capital, may be very great. For between
London and the places where Palaeozoic rocks appear, the older form-
ations are covered by the whole of the Jurassic series (Lias and
Oolite) of beds, while they are in their turn, as we approach London,
hidden from view by the Cretaceous rocks, of which the Chalk is the
best known example, while the Chalk is largely covered by Tertiary
beds, the most important of which to residents in Essex is the London
clay. It is obvious that, looking simply to the number and thick-
ness of these various formations, the resident in Kent, Surrey,
Sussex, Middlesex or Essex who wished to bore, if possible, down to
the Palaeozoic rocks beneath, would do well to avoid the Tertiary
beds and begin, if practicable, in the lowest of the Cretaceous strata.
Now while in Middlesex or Essex the utmost that could be done
in this way to secure a good site for a boring would be to begin on
the bare Chalk, the counties of Kent, Surrey, and Sussex offer con-
siderably greater advantages. For within the area enclosed by the North
and South Downs, the Chalk is absent. The North Downs, as the
outcrop of the Chalk bounding the northern side of this area is called,
range from Folkestone in a north-westerly direction, keeping a mile
or two north-eastward of Ashford and Maidstone. West of the
Medway their general course is slightly south of west, by Dorking and
Guildford to Farnham. Then the outcrop bends round to the south
and looks eastward in the neighbourhoods of Alton and Selborne,
while near Petersfield it turns so as to face the north, and is known as