THE EOCENE ROCKS.
_Classification._ The Eocene Beds of the south of England have been
subdivided according to the variations in their lithological
characters, and the subdivisions have received local names. The
following classification is generally adopted, though the different
subdivisions are by no means of equal value:
Upper Eocene { Upper Bagshot Beds
{ Barton Beds
Middle Eocene Bracklesham Beds
{ Lower Bagshot Beds
{ London Clay[103]
Lower Eocene { Oldhaven Beds } Lower London
{ Woolwich and Reading Beds } Tertiary Strata
{ Thanet Sands }
[Footnote 103: Some writers place the London Clay in the Middle
Eocene.]
The deposits vary greatly when traced abroad, and the exact
equivalents of the minor subdivisions of the British rocks can seldom
be ascertained at any distance from England, though the division into
Upper, Middle, and Lower Eocene can be made over wide areas.
_Description of the strata._ The character of the strata of Europe and
Asia indicates the persistence of the northern gulf and southern
ocean of Cretaceous times in Eocene times also, though the area of
each had shrunk in the meantime, owing to the physiographical changes
which occurred at the end of Cretaceous times, giving rise to more
extended land areas, and producing a shallow water phase over wide
extents of ocean,--the final shallow water phase of the third and last
great marine period of the British area. It is difficult to ascertain
the exact importance of the physical break between Cretaceous and
Eocene rocks in the south-east of England, owing to the subterranean
solution of the upper part of the chalk, subsequently to the
deposition of the Eocene strata, but the contraction of the Cretaceous
gulf is shown in several ways, one of the most significant being the
distribution of Cretaceous and Eocene rocks in the south-west of
England. The existence of an outlier of Cretaceous rock at Buckland
Brewer in North Devon, only three miles from the Atlantic Ocean,
indicates the former extension westward of the Upper Cretaceous beds,
while the occurrence of an outlier of Eocene rocks at Bovey Tracey in
South Devon, resting not on Cretaceous but on Palæozoic rocks, shows
that there was an uplift after the deposition of the Cretaceous rocks
and before the Eocene rocks were deposited there, and that during the
period of uplift the Cretaceous rocks were removed.
Owing to these physical changes, the Eocene rocks of Britain are
mainly mechanical sediments, some, as the Oldhaven beds, being
composed of coarse pebbles over a fairly wide district, while some of
the earlier Eocene rocks are estuarine or fluvio-marine.
The Eocene rocks of Britain occur in four areas, namely, the London
Basin, the Hampshire Basin, the Bovey Tracey outlier, and the
north-east of Ireland and western Isles of Scotland. The deposits of
the three southern areas may be considered together, and give general
indications of an approach to land when passing westward. The Lower
London Tertiary strata are fluvio-marine at the east end of the London
Basin; they become shallower water deposits when traced westward, and
begin to disappear. The London Clay is an estuarine deposit, which is
generally supposed to have been laid down at the mouth of a large
river flowing from the west. It is absent in the Bovey Tracey outlier.
Local disturbances caused the existence of a shallow water region in
the east during the deposition of the Middle and Upper Eocene
deposits, and accordingly the well-marked marine deposits which form
the representatives of these divisions in Hampshire are replaced by
the Bagshot beds of the London Basin, consisting chiefly of coarse
mechanical sediments with a poor marine fauna, but even in the west
shallow water prevailed at times during the accumulation of various
plant-bearing strata. The Middle Eocene beds only are found in the
Bovey Tracey outlier, though the Upper Eocene beds may originally have
been laid down in that area, and subsequently denuded.
The fourth area displays a very different succession of Eocene strata,
and one of extreme interest. Mechanical sediments and plant-bearing
clays and lignites alternate with a vast accumulation of basaltic
lavas, indicating the outbreak of the volcanic forces in the British
area, after a period of quiescence which lasted through the greater
part of Mesozoic times. The region in which these lavas were poured
out was probably a land area during the greater part of the period of
volcanic activity, but the horizontal lie of the lava flows and their
wide extent indicate the existence of a flat tract of country,
gradually raised into a plateau by the accumulation of sheet over
sheet of basalt. How far this plateau extended it is impossible to
say. The distribution of the lavas at the present day is somewhat
limited in our isles, but there is no sign of dying out at the present
margins of the accumulations, and they have probably escaped
denudation in these regions, as maintained by Professor Judd, on
account of the faults which have depressed them, while the portions
which were not depressed have been removed by denudation. Two views as
to the origin of the lavas have been put forward: according to Prof.
Judd, they were poured forth from gigantic volcanoes, while Sir A.
Geikie maintains that they represent portions of massive or fissure
eruptions, the molten rock having welled out from great cracks in the
earth, which are now filled by once molten rock in the form of dykes.
As these dykes extend far away from the present volcanic plateau, one
actually extending to the Yorkshire coast, we may well believe,
whatever was the origin of the sheets of lava, that they were formerly
spread far away from their present terminations[104]. Without entering
here into a discussion of the exact nature of extrusion of these
igneous sheets, it will suffice to say that all the evidence points to
the formation of extensive plateaux, which must have presented a
fairly uniform surface, similar to that which is still found
characterising the volcanic districts of the western territories of
North America.
[Footnote 104: Prof. Judd's views will be found in a series of papers
by him on the "Secondary Rocks of Scotland," _Quart. Journ. Geol.
Soc._, vol. XXIX. p. 95, XXX. p. 220, XXXIV. p. 660, while Sir A.
Geikie's explanation is advanced in a paper in the _Transactions of
the Royal Society of Edinburgh_, vol. XXXV.; see also the same
author's _Ancient Volcanoes of Great Britain_.]
The Eocene rocks of the north-west of Europe possess characters very
similar to those of the south of England, and there are indications
that the northern gulf had diminished in extent towards the east as
well as towards the west.
Passing to southern Europe, Central Asia and northern Africa, we find
the conditions of Cretaceous times reproduced, and an extensive series
of marine deposits extends very widely over these regions, the most
persistent deposit being a mass of limestone of Middle Eocene age,
which is almost entirely composed of the tests of Nummulites, whence
the development is known as the Nummulitic Limestone facies, and we
may speak of the ocean as the Nummulitic Limestone Sea. The incoming
of shallow water conditions marked by accumulation of coarse
mechanical sediments towards the end of the Eocene period in some
parts of the southern European area indicates the setting in, even
then, of those continental conditions which culminated during the
Miocene period.
In North America we get similar evidence of the contractions of the
oceans which in Mesozoic times occupied large expanses of our present
continents.
The climatic conditions of Eocene times have been noticed in passing
in chapter IX., and evidence was given to prove the prevalence of a
warmer climate over the British area than that which now exists. A
study of the floras of various parts of the northern hemisphere
suggests that climatic zones, whose lines of demarcation ran
practically parallel with the Equator, existed in Eocene times also,
though further information upon this subject is desirable.
_The Eocene flora and fauna._ The flora of prevalent dicotyledonous
angiosperms, which appeared in Upper Cretaceous times, also marks the
Eocene and later deposits, but a study of the floras indicates that
the differentiation which now marks off the floras of different areas
from one another had not occurred to so great an extent as at the
present time. The existence of a rich flora in the Eocene beds of
circumpolar regions in the northern hemisphere should be noted, though
perhaps its importance has been somewhat exaggerated.
The invertebrate fauna shows an approximation to that of the present
day. The remarkable ammonite fauna of Mesozoic times has disappeared,
and gastropods and lamellibranchs predominate, many of the forms
belonging to existing genera, though very rarely to existing species.
The Nummulites are the most characteristic Eocene fossils, and the
period may be spoken of as the Nummulitic Period, though it is now
known that Nummulites are not confined to the Eocene strata.
The vertebrate fauna is very noteworthy. The fishes and reptiles are
closely related to existing forms, and the orders of reptiles which
predominated in Mesozoic times have completely disappeared. But the
mammals are the most interesting vertebrates of the Eocene period.
Instead of the lowly organised forms of Mesozoic times, we find
representatives of many orders, including the highest, the Primates.
The generalised forms which serve as links between groups which are
now separated to a considerable extent are of particular importance.
They have been detected in Eocene rocks of various regions, though the
most complete series have been obtained from the Eocene rocks of North
America and made known to us through the numerous memoirs of
Professors Cope and Marsh[105].
[Footnote 105: The Eocene floras of Britain are described by Mr J.
Starkie Gardner and Baron von Ettingshausen in the _Monographs of the
Palæontographical Society_; other Monographs of the same Society
contain an account of the Eocene Mollusca by Mr F. E. Edwards and Mr
S. V. Wood. An idea of the generalised forms of Mammalia may be
obtained by perusal of that portion of Nicholson and Lydekker's
_Manual of Palæontology_ in which the latter author treats of the
Mammalia, and in this connexion the reader will do well to read Prof.
Huxley's "Lecture on Fossil Horses," reprinted in his _American
Addresses_.]