THE GEOLOGICAL RECORD
'All the Epochs of the Past are only a few of the front carriages,
and probably the least wonderful, in the van of an interminable
procession.'
J. B. Bury (_The Science of History_).
The portion of the earth's surface accessible to investigation is made
up in part of accumulations of old sediments, some indistinguishable
from the shingle, sand, and mud now in process of formation by the
ceaseless action of denudation; others have been hardened, gently
folded or violently contorted and so far altered by crust-movements
as to render their sedimentary origin well nigh unrecognisable. It is
these sediments of former ages, the dust of lost continents, in which
are preserved the majority of the fragmentary remains of plants and
animals, the flotsam and jetsam of successive phases of evolution.
The crust of the earth, as Darwin wrote, 'with its imbedded remains
must not be looked at as a well filled museum, but as a poor collection
made at hazard and at rare intervals'(19). It is from this imperfect
record that we seek to discover the relative antiquity of the several
groups or genera of living plants, and in the structure of extinct
types we endeavour to discover connecting links between divisions of
the plant kingdom which in the course of evolution have retained little
or no signs of a common descent.
Sir Joseph Hooker in a letter to Darwin in 1859 speaks of his
'conviction that we have not in a fossilised condition a fraction of
the plants that have existed, and that not a fraction of those we have
are recognisable specifically'(12). Considering the nature of the
palaeontological documents the wonder is how much they have taught us,
and we may look with confidence to the results of future research in
a field of which the importance has only recently been appreciated.
With the strata of sedimentary origin are frequently associated igneous
rocks, and in many continental regions, as in the majority of oceanic
islands, the crust of the earth consists wholly of volcanic material or
of rocks produced by the gradual solidification of molten magmas. Rocks
composed mainly of carbonate of lime, such as limestones and chalk,
bear witness to ocean beds or to sediments deposited on the floors of
inland seas beyond the reach of land detritus where coral reefs were
reared or the shells and other calcareous skeletons of animals supplied
the material for future land. In such rocks the remains of calcareous
seaweeds are frequently recognisable and occasionally, as in the
English chalk, fragments of wood testify to transport from a distant
land.
While there is little difficulty in explaining the nature of much of
the earth's crust, in several parts of the world the strata are totally
unfossiliferous and closely simulate crystalline rocks. In many cases
it is believed that such strata represent ancient sediments which in
the course of ages have been reduced by metamorphic agencies to a
condition which has obscured or entirely obliterated all traces of
their pristine state.
Since the pioneer work of William Smith, who in the early days of the
nineteenth century first realised the importance of fossils as aids to
the determination of relative age, geologists have devoted themselves
to the task of correlating the sedimentary rocks of the world, using
as criteria the order of superposition of the strata and the nature of
their organic remains. The result has been to classify portions of the
earth's crust into periods or chapters, which together constitute a
record of geological evolution as complete as it is possible to obtain
from the available data. The accompanying table shows the order of
sequence of the epochs, which stand for terms of years of a magnitude
beyond our powers to grasp.
The division of geological history into larger and smaller periods does
not imply the recurrence of sudden revolutions; it is in some measure
dictated by considerations of convenience, but more particularly by
our ignorance of certain stages in the history of the world due to the
imperfection of the record.
GEOLOGICAL TABLE.
Showing the position in the Geological Series of the strata referred to
in this volume.
Tertiary (Cainozoic)
{ =Recent= { Superficial accumulations containing human
{ =Pleistocene= { remains (Metal age, Neolithic and
{ { Palaeolithic ages, Glacial deposits)
{
{ =Pliocene= Cromer Forest-bed, etc.
{
{ =Miocene= Absent from Britain.
{
{ =Oligocene= Bovey Tracey beds, etc.
{
{ =Eocene= London Clay, etc.
Secondary (Mesozoic)
{ =Cretaceous= { Chalk
{ { Wealden beds
{
{ { Purbeck and Portland beds (Upper Jurassic)
{ =Jurassic= { Oolites (Middle Jurassic)
{ { Lias (Lower Jurassic)
{
{ { Rhaetic beds
{ =Triassic= { Keuper " (Marls with rock-salt, etc.)
{ { Bunter "
Primary (Palaeozoic)
{ =Permian= { Red Sandstones, etc.
{ { Magnesian limestone
{
{ { Coal Measures
{ =Carboniferous= { Millstone Grit
{ { Carboniferous limestone
{
{ =Devonian= { Devonian limestones, etc.
{ { Old Red Sandstones
{
{ =Silurian= { Sandstones, shales,
{ { some limestone
{
{ =Ordovician= { Slates, sandstones,
{ { Volcanic rocks, etc.
{
{ =Cambrian= { Slates, Sandstones, etc.
{
{ =Pre-Cambrian= { Slates, Volcanic rocks, etc.
{ =or Archean=
In certain parts of the world, as for example the north-west Highlands
of Scotland, the Malvern Hills, Scandinavia, and in many other regions
in Europe and North America, geologists have recognised what they
believe to be the foundation stones of the world. These Archaean
rocks, which underlie the oldest fossiliferous strata, belong to a
period of geological evolution from which it appears to be hopeless to
obtain any light as to the nature of the contemporary organic world.
The earliest vestiges of life so far discovered exhibit a high degree
of organisation, which unmistakably points to their being links in a
chain extending far beyond the limits of the oldest Cambrian strata
in which recognisable fossils first occur. The rocks of the Cambrian
and Ordovician epochs, as represented by the grits, shales, slates and
other sedimentary strata in Wales, Shropshire, the Lake district and
elsewhere, though in places rich in the remains of animals, afford
no information in regard to the land vegetation. From the succeeding
Silurian epoch very little evidence has been gleaned as to the nature
of the flora, and it is not until we come to the sedimentary rocks of
the Devonian era that records of plant-life occur in any abundance. The
almost complete lack of botanical data in the pre-Devonian formations
is in part due to the fact that these older rocks consist to a large
extent of marine deposits formed under conditions unfavourable to the
preservation of plants. That the land-surfaces of the older Palaeozoic
eras supported an abundant vegetation there can be little doubt.
The relics of plant-life furnished by the Devonian and succeeding
formations represent the upper branching-systems of a deeply rooted and
spreading tree, the lowest portions of which have been destroyed or
have left no sign of their existence.
In descending the Geological series, we begin with superficial
deposits, such as peat and river-gravels found subsequently to the
underlying boulder-clay of the Glacial period. The remains of forest
trees preserved in the peat and in submerged forests round the
coast connect the vegetation of the historic period with that of the
Neolithic age. At the base of the Pleistocene series, the name given
to the latest chapter of geological history, we find evidence of the
prevalence of arctic conditions in the widely spread boulder-clays and
other deposits of the Glacial period.
From deposits of post-Glacial date abundant plant remains have
been obtained, but we cannot say with any degree of certainty what
proportion of these plants remained in Britain during the Ice age,
and whether the greater part of the vegetation, the relics of which
have been discovered in pre-Glacial beds, was destroyed or driven
south by the advancing ice. We may briefly consider some of the more
interesting facts brought to light by the investigation of the fossil
plants in the Lower Pleistocene and Upper Tertiary beds. It is mainly
to the researches of Mr Clement Reid into the vegetation of Britain
immediately preceding the Glacial period, that our knowledge of this
phase of the history of the British flora is due.
[Illustration: Fig. 3. Pre-Glacial plants from Mundesley (A), Norfolk
and Pakefield (B, C), Suffolk. (Photographs by Mr Clement Reid and Mrs
E. M. Reid.) A. _Bidens tripartita_ Linn. (× 6); B. _Picea excelsa_
Linn. (nat. size). C. _Stellaria holostea_ Linn. (× 12).]
On the coast of Norfolk in the neighbourhood of Cromer the sections
of the cliffs reveal the existence of a succession of sands, clays,
and gravels underlying Glacial deposits; this material was probably
laid down near the mouth of the ancient Rhine, which in the latter
part of the Tertiary period flowed across a low area, which is now
occupied by the shallow southern half of the North Sea(27). The
plant-fragments found in these river-sediments indicate a temperate
climate. Among the plants of this pre-Glacial flora are many familiar
British species, such as _Caltha palustris_ (marsh marigold), species
of butter-cup, _Stellaria holostea_ (greater stichwort) (Fig. 3, C),
_Bidens tripartita_ (bur-marigold) (Fig. 3, A), maple, hawthorn, the
alder, hazel, the yew, Scots pine and numerous others. If, as is not
improbable, these pre-Glacial plants were swept away by the subsequent
arctic conditions, the great majority of them returned to their old
home when a warmer climate ensued. There are however some pre-Glacial
plants, such as the spruce fir (_Picea excelsa_), a cone of which
is shown in Fig. 3, B, the water chestnut, _Trapa natans_ (Fig. 4),
and a few other species no longer represented in the British flora.
The genus _Trapa_ is a striking example of a flowering plant which
has disappeared since the Tertiary period from many parts of Europe,
including England, most of Sweden, and from several regions in northern
Europe. It still grows in a few localities in Switzerland and in some
of the Italian lakes. In pre-Glacial times the water chestnut was
widely spread from Portugal and England in the west to Siberia in the
east, and its hard four-pronged nuts have been recorded from many
post-Glacial peat-moors in the north of Europe.
[Illustration: Fig. 4. _Trapa natans_ Linn. (nat. size). From
Mundesley. (Photographs by Mr and Mrs Reid.)]
From the so-called Cromer forest-bed and associated deposits on the
Norfolk coast several pre-Glacial plants have been obtained, indicating
a temperate climate during this phase of the Pleistocene period. A few
arctic species, such as the dwarf birch and arctic willow obtained from
the deposits next above the Cromer forest-bed, herald the near approach
of glacial conditions.
It may be remarked in passing that no satisfactory evidence has been
discovered in Britain of the existence of man in this part of Europe in
pre-Glacial days: it is, however, believed that flints from Tertiary
strata on the continent show traces of human workmanship. As Sir
Edwin Ray Lankester said in 1905, 'It is not improbable that it was in
the remote period known as the Lower Miocene--remote as compared with
the gravels in which Eoliths [primitive stone implements] occur--that
Natural Selection began to favour that increase in the size of the
brain of a large and not very powerful semi-erect ape'(28).
Though comparatively recent in terms of geological chronology, the
remoteness, according to ordinary conceptions of time, of the Tertiary
period is brought home to us when we endeavour to grasp the fact
that it was during this chapter in the earth's history that some of
our highest mountain-ranges, such as the Alps, the Carpathians, and
Himalayas were formed by the uplifting of piles of marine sediments.
From Tertiary strata in the Isle of Wight, on the Hampshire coast, and
in the London basin numerous fossil plants have been obtained, which
afford convincing evidence of climatic conditions much more genial than
those of the present day. The presence of palm leaves and of a wealth
of other sub-tropical plants in Lower Tertiary beds in England reveals
the existence of a flora differing considerably both from that in the
uppermost Tertiary beds of Norfolk and from the modern British flora,
but closely allied to the present Mediterranean flora.
The basaltic columns of the Giants' Causeway and of the Staffa Cave,
and the terraced rocks which form so characteristic a feature in the
contours of the Inner Hebrides, are portions of lava-flows, which in
the early days of the Tertiary period were poured out over a wide area
of land stretching from the north-east of Ireland, through the Western
isles of Scotland, the Faroë islands, to Iceland and Greenland. While
in this northern region volcanic activity was being manifested on a
stupendous scale, a shallow sea extended over part of what is now the
south-east of England in which was deposited a considerable thickness
of sedimentary material derived from the neighbouring land. In this
upraised sea-floor, known as the London clay, which is exposed in the
Isle of Sheppey and in many other localities, numerous fossil fruits
and fragments of wood occur in association with marine shells. The
fact that many of the fruits were ripe at the time of their entombment
led some eighteenth century writers to assign an autumn date to the
universal deluge. One of the Sheppey fruits may be mentioned as an
especially interesting sample of the early Tertiary flora, namely
the genus Nipadites, so named from the very close resemblance of the
fossils to the fruits of the existing tropical plant Nipa. _Nipa
fruticans_, sometimes described as a stemless palm because of the
absence of the erect stem which is usually a characteristic feature of
palms, grows in brackish estuaries of many tropical countries (Fig.
5, A): it has long leaves not unlike those of the date-palm and bears
clusters of fruits as large as a man's head; a single fruit is two or
three inches long and its hard fibrous shell is characterised by four
or five longitudinal ribs (Fig. 5, B). The fruits of Nipa, which may be
carried a considerable distance by ocean-currents without losing the
power of germination, are constantly found with other vegetable drift
on the beaches of tropical islands. The discovery of fruits of Nipa (or
Nipadites), hardly distinguishable from those of the existing species,
in Tertiary beds in England, Belgium, in the Paris basin, and in Egypt
affords a striking instance of changes in the geographical distribution
of an ancient plant now restricted to warmer regions.
[Illustration: Fig. 5. _Nipa fruticans_, Thunb. A. On the coast of the
Malay Peninsula. (Photograph by Prof. Yapp.) B. Head of fruits (1/5
nat. size). From a specimen in the British Museum.]
While the higher members of the Cretaceous system, as seen in the chalk
cliffs and downs, represent the upraised calcareous accumulations on
the floor of a fairly deep and clear sea, the lower members testify to
shallower water within reach of river-borne sand and mud. 'During the
Chalk period,' as Huxley wrote, 'not one of the present great physical
features of the globe was in existence. Our great mountain ranges,
Pyrenees, Alps, Himalayas, Andes, have all been upheaved since the
chalk was deposited, and the Cretaceous sea flowed over the sites of
Sinai and Ararat'(29).
The Wealden strata, at the base of the Cretaceous system, as seen on
the Sussex coast, in parts of the Isle of Wight, in the Weald district
of Kent and neighbouring counties, point to the existence of a lake
over a portion of the south of England and of the English Channel. The
remains of a rich Wealden flora have been collected from these Wealden
sediments, notably from the plant-beds of Ecclesbourne near Hastings,
in which, so far as we know, flowering plants played no part or at most
occupied a very subordinate position. A few fossil leaves have been
described from rocks assigned to a Wealden age,--and from the older
Stonesfield Slate, of Jurassic age, a single leaf is recorded,--which
seem to be those of Dicotyledons; but it is certain that even in the
early days of the Cretaceous period the present dominant group in
the plant kingdom was in its infancy and in many regions probably
unrepresented. When we glance at the geological table and consider
that in all the floras from the Wealden down to the Devonian period,
flowering plants played no part, we are able to appreciate the fact of
their rapid development, referred to in a previous chapter, when once
this highest type had become established.
The rocks comprised in the Jurassic system extend from East Yorkshire
to the coast of Dorsetshire; they consist of a succession of
limestones, clays, sandstones, and a few thin beds of impure coal.
Sediments of this age also occur, though to a much less extent, on
the north-east coast of Scotland and in a few places in the Inner
Hebrides. Many of the Jurassic strata contain only marine shells,
and corals are occasionally abundant, though in the lower members of
the system in the cliffs near Lyme Regis and at Whitby fossil plants
are fairly common. It is, however, from the middle Jurassic beds,
in the cliffs between Whitby and Scarborough, and in some inland
quarries in East Yorkshire, that we have obtained the richest Jurassic
flora. Rivers from a northern land laden with sediment and carrying
drift-wood, leaves and other plant fragments, deposited their burden in
an estuary which occupied the eastern edge of Yorkshire. Sedimentary
rocks laid down towards the close of the Jurassic period in the island
of Portland in the south and on the Sutherland coast in the north have
furnished valuable records of plant-life.
The passage from the Jurassic to the underlying Triassic system is
formed by some shales and limestones in South Wales containing remains
of fish and other marine organisms. These so-called Rhaetic beds are
poorly represented in the British area, but on the continent of Europe
and in other regions the sediments of this age bulk much more largely
and have yielded a rich collection of plants. The rocks of the upper
division of the Triassic system, as seen in the Midlands, point to
the prevalence of desert conditions; and in the grooved sand-polished
surfaces of granite in Charnwood forest we have a glimpse of a
Triassic landscape. The salt-bearing strata of this period in Cheshire
and Worcestershire suggest conditions paralleled at the present day
in the Caspian and Dead-Sea regions. The vegetation of Britain, and
indeed of the world as a whole, seems to have undergone but little
change during the enormous lapse of time represented by the sediments
comprised between the Wealden and Triassic periods. The Lower Triassic
flora affords evidence of a change in the facies of the vegetation and
prepares us for the still greater differences revealed by a study of
the Permian and Carboniferous floras. To the student of evolution these
Palaeozoic floras are of special interest on account of the facts they
have contributed in regard to the descent and inter-relationship of
different branches of the vegetable kingdom.
It is by a patient study of the waifs and strays of the vegetation
of successive phases of the world's history preserved in sedimentary
strata, that it has been possible to follow the history of many
existing plants and to establish links between the present and the
past.