138
THE SUBTERRANEAN GEOLOGY OF SOUTH-
EASTERN ENGLAND.
By T. VINCENT HOLMES, F.G.S., M.A.I.
(Being the Presidential Address, delivered January 28th, 1888.)
CONTENTS :—Geological maps show rocks on the surface at a given spot.—
Knowledge of subterranean geology depends partly on the evidence afforded by
the disposition of the surface rocks, partly on that of deep borings.—The geology
of South-Eastern England as shown on a geological map.—The evidence of
districts outside; of Belgium, the Boulonais, and Western England; bearing
upon the geology of S.E. England.—Of deep borings.—Conclusions.
By the subterranean geology of South-eastern England, I mean those
details of its geological structure which do not exhibit themselves on
the surface of that district, and are consequently not marked on any
geological map of it. For the function of a geological map is to
show, in such detail as the scale allows, the nature and age of the
formations occupying the surface of a country. It is sometimes
difficult to say whether a map giving or omitting the drifts or super-
ficial beds, which in many districts overspread the older rocks of
various ages, is the more desirable. All depends on the economical
importance of the superficial as compared with the older rocks, and
on the point of view of the purchaser of the map. In a district like
the Yorkshire Coalfield where the superficial beds are thin, do not
cover much of the surface, and are of trifling economical importance
compared with the coal-measures immediately below, a map omitting
patches of drift may be slightly preferable, as allowing a clearer view
of the range of the various coal seams and the faults, or dislocations
of the strata affecting them. In a district like that of Cumberland
north of the Lake Country, where Glacial Drift and other superficial
beds cover the greater part of the surface to a depth of many feet,
the older rocks giving no signs of their existence over many contin-
uous square miles of country, there is obviously need for maps of
both kinds. And in Norfolk, Suffolk and Essex, geological maps
omitting the drift are comparatively worthless.
For my object this evening, however, the most useful map is that
which most clearly shows the older rocks and the various folds into
which they have been thrown, as on the nature and range of these
folds, as shown on the surface, deductions as to the probable nature
of the wholly subterranean geological arrangements must necessarily
be based.
Examination of a good geological map of England and Wales, such