GEOLOGICAL HISTORY.
“But how can we question dumb rocks whose speech is not clear[44]?”
In attempting to sketch in briefest outline the geological history
of the Earth, the most important object to keep in view is that of
reproducing as far as possible the broad features of the successive
stages in the building of the Earth’s crust. It is obviously
impossible to go into any details of description, or to closely
follow the evolution of the present continents; at most, we can only
refer to such facts as may serve as an introduction of the elements
of stratigraphical geology to non-geological readers. For a fuller
treatment of the subject reference must be made to special treatises on
geology.
For the sake of convenience, it is customary in stratigraphical geology
as also in biology, to make use of our imperfect knowledge as an aid
to classification. If we possessed complete records of the Earth’s
history, we should have an unbroken sequence, not merely of the various
forms of life that ever existed, but of the different kinds of rocks
formed in the successive ages of past time. As gaps exist in the chain
of life, so also do we find considerable breaks in the sequence of
strata which have been formed since the beginning of geologic time. The
danger as well as the convenience of artificial classification must be
kept in view. This has been well expressed by Freeman, in speaking of
architectural styles,—“Our minds,” he says, “are more used to definite
periods; they neglect or forget transitions which do indeed exist[45].”
The idea of definite classification is liable to narrow our view of
uniformity and the natural sequence of events.
[Sidenote: ROCK-BUILDING.]
Composing that part of the earth which is accessible to us,—or as it is
generally called the earth’s crust,—there are rocks of various kinds,
of which some have been formed by igneous agency, either as lavas or
beds of ashes, or in the form of molten magmas which gradually cooled
and became crystalline below a mass of superincumbent strata. With
these rocks we need not concern ourselves.
A large portion of the earth’s crust consists of such materials as
sandstones, limestones, shales, and similar strata which have been
formed in precisely the same manner as deposits are being accumulated
at the present day. The whole surface of the earth is continually
exposed to the action of destructive agencies, and suffers perpetual
decay; it is the products of this ceaseless wear and tear that form the
building materials of new deposits.
The operation of water in its various forms, of wind, changes of
temperature, and other agents of destruction cannot be fully dealt with
in this short summary.
A river flowing to the sea or emptying itself into an inland lake,
carries its burden of gravel, sand, and mud, and sooner or later, as
the rate of flow slackens, it deposits the materials in the river-bed
or on the floor of the sea or lake.
Fragments of rock, chipped off by wedges of ice, or detached in other
ways from the parent mass, find their way to the mountain streams,
and if not too heavy are conveyed to the main river, where the larger
pieces come to rest as more or less rounded pebbles. Such water-worn
rocks accumulate in the quieter reaches of a swiftly flowing river, or
are thrown down at the head of the river’s delta. If such a deposit
of loose water-worn material became cemented together either by the
consolidating action of some solution percolating through the general
mass, or by the pressure of overlying deposits, there would be formed
a hard rock made up of rounded fragments of various kinds of strata
derived from different sources. Such a rock is known as a CONGLOMERATE.
The same kind of rock may be formed equally well by the action of the
sea; an old sea-beach with the pebbles embedded in a cementing matrix
affords a typical example of a coarse conglomerate. Plant remains are
occasionally met with in conglomerates, but usually in a fragmentary
condition.
From a conglomerate composed of large water-worn pebbles, to a fine
homogeneous sandstone there are numerous intermediate stages. A body
of water, with a velocity too small to carry along pebbles of rock in
suspension or to roll them along the bed of the channel, is still able
to transport the finer fragments or grains of sand, but as a further
decrease in the velocity occurs, these are eventually deposited as beds
of coarse or fine sand. The stretches of sand on a gradually shelving
sea shore, or the deposits of the same material in a river’s delta,
have been formed by the gradual wearing away and disintegration of
various rocks, the detritus of which has been spread out in more or
less regular beds on the floor of a lake or sea. Such accumulations
of fine detrital material, if compacted or cemented together, become
typical SANDSTONES.
In tracing beds of sandstone across a tract of country, it is
frequently found that the character of the strata gradually alters; mud
or clay becomes associated with the sandy deposit, until finally the
sandstone is replaced by beds of dark coloured shale. Similarly the
sandy detritus on the ocean floor, or in an inland lake, when followed
further and further from the source from which the materials were
derived, passes by degrees into argillaceous sand, and finally into
sheets of dark clay or mud. The hardened beds of clay or fine grained
mud become transformed into SHALES. As a general rule, then, shales are
rocks which have been laid down in places further from the land, or at
a greater distance from the source of origin of the detrital material,
than sandstones or conglomerates. The conglomerates, or old shingle
beaches, usually occur in somewhat irregular patches, marking old
shore-lines or the head of a river delta. Coarse sandstones, or grits,
may occur in the form of regularly bedded strata stretching over a
wide area; and shales or clays may be followed through a considerable
extent of country. The finer material composing the clays and shales
has been held longer in suspension and deposited in deeper water in
widespread and fairly horizontal layers.
In some districts sandstones occur in which the individual grains
show a well marked rounding of the angles, and in which fossils
are extremely rare or entirely absent. The close resemblance of
such deposits to modern desert sands suggests a similar method of
formation; and there can be no doubt that in some instances there have
been preserved the wind-worn desert sands of former ages. Aeolian
or wind-formed accumulations, although by no means common, are of
sufficient importance to be mentioned as illustrating a certain type of
rock.
[Sidenote: CALCAREOUS ROCKS.]
The thick masses of limestone which form so prominent a feature in
parts of England and Ireland, have been formed in a manner different
from that to which sandstones and shales owe their origin. On the
floor of a clear sea, too far from land to receive any water-borne
sediment, there is usually in process of formation a mass of calcareous
material, which in a later age may rise above the surface of the water
as chalk or LIMESTONE. Those organisms living in the sea, which are
enclosed either wholly or in part by calcareous shells, are agents of
limestone-building; their shells constantly accumulating on the floor
of the sea give rise in course of time to a thick mass of sediment,
composed in great part of carbonate of lime. Some of the shells in
such a deposit may retain their original form, the calcareous body
may on the other hand be broken up into minute fragments which are
still recognisable with the help of a microscope, or the shells and
other hard parts may be dissolved or disintegrated beyond recognition,
leaving nothing in the calcareous sediment to indicate its method of
formation.
Not a few limestones consist in part of fossil corals, and owe their
origin to colonies of coral polyps which built up reefs or banks of
coral in the ancient seas.
In the white cliffs of Dover, Flamborough Head and other places, we
have a somewhat different form of calcareous rock, which in part
consists of millions of minute shells of Foraminifera, in part of
broken fragments of larger shells of extinct molluscs, and to some
extent of the remains of siliceous sponges. As a general rule,
limestones and chalk rocks are ancient sediments, formed in clear and
comparatively deep water, composed in the main of carbonate of lime,
in some cases with a certain amount of carbonate of magnesium, and
occasionally with a considerable admixture of silica.
In such rocks land-plants must necessarily be rare. There are, however,
limestones which wholly or in part owe their formation to masses of
calcareous algae, which grew in the form of submarine banks or on coral
reefs. Occasionally the remains of these algae are clearly preserved,
but frequently all signs of plant structure have been completely
obliterated. Again, there occur limestone rocks formed by chemical
means, and in a manner similar to that in which beds of travertine are
now being accumulated.
Granites, basalts, volcanic lavas, tuffs, and other igneous rocks
need not claim our attention, except in such cases as permit of plant
remains being found in association with these materials. Showers of
ashes blown from a volcano, may fall on the surface of a lake or sea
and become mixed with sand and mud of subaerial origin. Streams of lava
occasionally flow into water, or they may be poured from submarine
vents, and so spread out on the ocean bed with strata of sand or clay.
Passing from the nature and mode of origin of the sedimentary strata to
the manner of their arrangement in the Earth’s crust, we must endeavour
to sketch in the merest outline the methods of stratigraphical geology.
The surface of the Earth in some places stands out in the form of bare
masses of rock, roughly hewn or finely carved by Nature’s tools of
frost, rain or running water; in other places we have gently undulating
ground with beds of rock exposed to view here and there, but for the
most part covered with loose material such as gravel, sands, boulder
clay and surface soil.
[Sidenote: GEOLOGICAL SECTIONS.]
In the flat lands of the fen districts, the peat beds and low-lying
salt marshes form the surface features, and are the connecting links
between the rock-building now in progress and the deposits of an
earlier age. If we could remove all these surface accumulations of
sand, gravel, peat and surface soil, and take a bird’s eye view of
the bare surface of the rocky skeleton of the earth’s crust, we
should have spread before us the outlines of a geological map. In
some places fairly horizontal beds of rock stretching over a wide
extent of country, in another the upturned edges of almost vertical
strata form the surface features; or, again, irregular bosses of
crystalline igneous rock occur here and there as patches in the
midst of bedded sedimentary or volcanic strata. A map showing the
boundaries and distribution of the rocks as seen at the surface,
tells us comparatively little as to the relative positions of the
different rocks below ground, or of the relative ages of the several
strata. If we supplement this superficial view by an inspection of
the position of the strata as shown on the walls of a deep trench cut
across the country, we at once gain very important information as to
the relative position of the beds below the earth’s surface. The face
of a quarry, the side of a river bed or a railway cutting, afford
HORIZONTAL SECTIONS or PROFILES which show whether certain strata lie
above or below others, whether a series of rocks consists of parallel
and regularly stratified beds, or whether the succession of the strata
is interfered with by a greater or less divergence from a parallel
arrangement. If, for example, a section shows comparatively horizontal
strata lying across the worn down edges of a series of vertical
sedimentary rocks, we may fairly assume that some such changes as the
following have taken place in that particular area.
The underlying beds were originally laid down as more or less
horizontal deposits; these were gradually hardened and compacted,
then elevated above sea-level by a folding of the earth’s crust; the
crests of the folds were afterwards worn down by denudation, and the
eroded surface finally subsided below sea-level and formed the floor
on which newer deposits were built up. Such breaks in the continuity
of stratified deposits are known as UNCONFORMITIES; in the interval of
time which they represent great changes took place of which the records
are either entirely lost, or have to be sought elsewhere.
In certain more exceptional cases, it is possible to obtain what is
technically known as a VERTICAL SECTION; for example if a deep boring
is sunk through a series of rocks, and the core of the boring examined,
we have as it were a sample of the earth’s crust which may often teach
us valuable lessons which cannot be learnt from maps or horizontal
sections.
[Sidenote: INVERSION OF STRATA.]
It is obvious, that in a given series of beds, which are either
horizontal or more or less obliquely inclined, the underlying strata
were the first formed, and the upper beds were laid down afterwards.
If, however, we trusted solely to the order of superposition in
estimating relative age, our conclusions would sometimes be very far
from the truth. Recent geological investigations have brought to
light facts well nigh incredible as to the magnitude and extent of
rock-foldings. In regions of great earth-movements, the crust has been
broken along certain lines, and great masses of strata have been thrust
for miles along the tops of newer rocks. Thus it may be brought about
that the natural sequence of a set of beds has been entirely altered,
and older rocks have come to overlie sediments of a later geological
age. Facts such as these clearly illustrate the difficulties of correct
geological interpretation.
In the horizontal section (Fig. 2), from the summit of Büzistock on
the left to Saasterg on the right, we have a striking case of intense
rock-folding and dislocation[46]. Prof. Heim[47] of Geneva has given
numerous illustrations of the almost incredible positions assumed
in the Swiss Mountains by vast thicknesses of rocks, and in the
accompanying section taken from a recent work by Rothpletz we have a
compact example of the possibilities of earth-movements as an agent of
rock-folding. The section illustrates very clearly an exception to the
rule that the order of superposition of a set of beds indicates the
relative age of the strata. The horizontal line at the base is drawn at
a height of 1650 metres above sea-level, and the summit of Büzistock
reaches a height of 2340 m. The youngest rocks seen in the diagram are
the Eocene beds _e_, at the base and as small isolated patches on the
right-hand end of the section; the main mass of material composing the
higher ground has been bodily thrust over the Eocene rocks, and in this
process some of the beds, _b_ and _c_, have been folded repeatedly on
themselves. Similar instances of the overthrusting of a considerable
thickness of strata have been described in the North-west Highlands
of Scotland[48] and elsewhere in the British Isles. It is important
therefore to draw attention to cases of extreme folding, as such
phenomena are by no means exceptional in many parts of the world.
[Illustration: FIG. 2. Section from Büzistock to Saasterg. [After
Rothpletz, (94) Pl. II. fig. 2.]
_a_ = Sernifit or Verrucano (Permian).
_b_ = Röthidolomit etc. (Permian).
_c_ = Dogger (Jurassic).
_d_ = Malm (Jurassic).
_e_ = Eocene.]
The order of superposition of strata has afforded the key to our
knowledge of the succession of life in geologic time, and the
refinements of the stratigraphical correlation of sedimentary rocks
are based on the comparison of their fossil contents. By a careful
examination of the relics of fossil organisms obtained from rocks
of all ages and countries, it has been found possible to restore in
broken outline the past history of the Earth. By means, then, of
stratigraphical and palaeontological evidence, a classification of the
various rocks has been established, the lines of division being drawn
in such places as represent gaps in the fossil records, or striking and
widespread unconformities between different series of deposits.
It is only in a few regions that we find rocks which can reasonably be
regarded as the foundation stones of the Earth. As the globe gradually
cooled, and its molten mass became skinned over with a solid crust,
crystalline rocks must have been produced before the dawn of life, and
before water could remain in a liquid form on the rocky surface. As
soon as the temperature became sufficiently low, running water and rain
began the work of denudation and rock disintegration which has been
ceaselessly carried on ever since. In this continual breaking down and
building up of the Earth’s surface, it would be no wonder if but few
remnants were left of the first formed sediments of the earliest age.
The action of heat, pressure and chemical change accompanying
rock-foldings and crust-wrinklings, often so far alters sedimentary
deposits, that their original form is entirely lost, and sandstone,
shales and limestones become metamorphosed into crystalline quartzites,
slates and marbles.
The operation of metamorphism is therefore another serious difficulty
in the way of recognising the oldest rocks. The earliest animals and
plants which have been discovered are not such as we should expect
to find as examples of the first products of organic life. Below the
oldest known fossiliferous rocks, there must have been thousands of
feet of sedimentary material, which has either been altered beyond
recognition, or from some cause or other does not form part of our
present geological record.
As a general introduction to geological chronology, a short summary
may be given of the different formations or groups of strata, to which
certain names have been assigned to serve as convenient designations
for succeeding epochs in the world’s evolution. The following table
(Fig. 3, pp. 32, 33) represents the geological series in a convenient
form; the most characteristic rocks of each period are indicated by the
usual conventional shading, and the most important breaks or lacunae in
the records are shown by gaps and uneven lines. The relative thickness
of the rocks of each period is approximately shown; but the vertical
extent of the oldest or Archaean rocks as shown in Fig. 3 represents
what is without doubt but a fraction of their proportional thickness.
This table is taken, with certain alterations, from a paper by Prof.
T. McKenny Hughes in the Cambridge Philosophical Proceedings for 1879.
Speaking of the graphic method of showing the geological series, the
author of the paper says, “It is convenient to have a table of the
known strata, and although we cannot arrange all the rocks of the
world in parallel columns, and say that _ABC_ of one area are exactly
synchronous with _A′B′C′_ of another, still if we take any one country
and establish a grouping for it, we find so many horizons at which
equivalent formations can be identified in distant places, that we
generally make an approximation to HOMOTAXIS as Huxley called it. The
most convenient grouping is obviously to bracket together locally
continuous deposits, _i.e._ all the sediment which was formed from the
time when the land went down and accumulation began, to the time when
the sea bottom was raised and the work of destruction began. In the
accompanying table I have given the rocks of Great Britain classified
on this system, and bearing in mind that waste in one place must be
represented by deposit elsewhere, I have represented the periods of
degradation by intervals estimated where possible by the amount of
denudation known to have taken place between the periods of deposition
in the same district[49].”
[Illustration: TABLE OF STRATA.
RECENT ╮
GLACIAL ├ QUATERNARY
PLIOCENE ╯
MIOCENE ╮
│
OLIGOCENE ├ TERTIARY
│
EOCENE ╯
CRETACEOUS ╮
│
JURASSIC │
├ MESOZOIC
TRIAS │
│
PERMIAN ╯
╭ Coal-Measures ╮
│ │
│ Millstone Grit │
CARBONIFEROUS ┤ │
│ Yoredale Rocks │
│ │
╰ Mountain Limestone │
│
╭ Devonian │
│ │
│ Upper Old Red │
DEVONIAN ┤ Sandstone │
│ │
│ Lower Old Red │
╰ Sandstone │
│
╭ Ludlow │
│ │
SILURIAN ┤ Wenlock ├ PALAEOZOIC
│ │
╰ May Hill │
│
╭ Upper Bala │
│ │
│ Lower Bala │
ORDOVICIAN ┤ │
│ Llandeilo │
│ │
╰ Arenig │
│
╭ Tremadoc │
│ │
│ Lingula Flags │
CAMBRIAN ┤ │
│ Menevian │
│ │
╰ Harlech ╯
LAVAS, ╮
VOLCANIC │
ASHES, │
GRANITOID ├ ARCHAEAN
ROCKS AND │
SCHISTS OF │
ENORMOUS │
THICKNESS ╯
Fig 3.]
I. =Archaean.=
“Men can do nothing without the make-believe of a beginning.”
GEORGE ELIOT.
There is perhaps no problem at once so difficult and so full of
interest to the student of the Earth’s history, as the interpretation
of the fragmentary records of the opening stages in geological and
organic evolution. In tracing the growth and development of the human
race, it becomes increasingly difficult to discover and decipher
written documents as we penetrate farther back towards the beginning
of the historical period; the records are usually incomplete and
fragmentary, or rendered illegible by the superposed writings of a
later date. So in the records of the rocks, as we pass beyond the
oldest strata in which clearly preserved fossils are met with, we come
to older rocks which afford either no data as to the period in which
they were formed, or like the palimpsest, with its original characters
almost obliterated by a late MS., the older portions of the Earth’s
crust have been used and re-used in the rock-building of later ages.
In the first place, it is exceedingly difficult to determine with any
certainty what rocks may be regarded as trustworthy fragments of a
primaeval land. Throughout the geological eras the Earth’s surface
has been subjected to foldings and wrinklings, volcanic activity has
been almost unceasing, and there is abundant evidence to show how
the original characters of both igneous and sedimentary rocks may be
entirely effaced by the operation of chemical and physical forces. It
was formerly held that coarsely crystalline rocks such as granite are
the oldest portions of the crust, but modern geology has conclusively
proved that many of the so-called fundamental masses of rock are merely
piles of ancient sediments which have been subjected to the repeated
operation of powerful physical and chemical forces, and have undergone
a complete rearrangement of their substance. As the result of more
detailed investigations, many regions formerly supposed to consist of
the foundation stones of the Earth’s crust, are now known to have been
centres of volcanic disturbance and widespread metamorphism, and to be
made up of post-archaean rocks.
[Sidenote: THE OLDEST ROCKS.]
The first formed rocks no doubt became at once the prey of denudation
and disintegration, and on their surface would be accumulated the
products of their own destruction: newer strata would entirely cover up
portions of the original land, to be in their turn succeeded by still
later deposits. There is reason to believe that in the remotest ages of
the Earth’s history, the forces of denudation and igneous activity were
more potent than in later times, and thus the oldest rocks could hardly
retain their original structure through the long ages of geologic time.
The earliest representatives of organic life were doubtless of such
a perishable nature that their remains could not be preserved in a
fossil state even under the most favourable conditions. Such organisms,
whether plants or animals, as possessed any resistant tissues or hard
skeletons might be preserved in the oldest rocks, but as these strata
became involved in earth-foldings or were penetrated by injections of
igneous eruptions, the relics of life would be entirely destroyed.
It is, in short, practically hopeless to look for any fragments of
the primitive crust except such as have undergone very considerable
metamorphism, and equally futile to search for any recognisable remains
of primitive life.
In many parts of the world vast thicknesses of rock occur below the
oldest known fossiliferous strata; these consist largely of laminated
crystalline masses composed of quartz, felspar, and other minerals,
having in fact the same composition as granite, but differing in the
regular arrangement of the constituent parts. To such rocks the terms
gneiss and schist have been applied. Rocks of this kind are by no means
always of Archaean age, but many of the earliest known rocks consist of
gneisses of various kinds, associated with altered lavas, metamorphosed
ashes, breccias and other products of volcanic activity; with these
there may be limestones, shales, sandstones, and other strata more or
less closely resembling sedimentary deposits. Such a succession of
gneissic rocks has been described as occupying a wide area in the basin
of the St Lawrence river, and to these enormously thick and widespread
masses a late Director of the Canadian Geological Survey applied
the term Laurentian. These Laurentian rocks, with similar strata in
Scandinavia, the north-west Highlands of Scotland, in certain parts of
such mountain ranges as the Alps, Pyrenees, Carpathians, Himalayas,
Andes, Atlas, &c., have been classed together as members of the oldest
geological period, and are usually referred to under the name of
Archaean, or less frequently Azoic rocks. In some of the uppermost
Archaean rocks there have been recently discovered a few undoubted
traces of fossil animals, but with this exception no fossils are known
throughout the great mass of Archaean strata. It is true that some
authorities regard the beds of graphite and other rocks as a proof of
the abundance of plant life, but this supposition is not supported by
any convincing evidence.
The term Azoic[50] applied by some writers to these oldest rocks
suggests the absence of life during the period in which they were
formed. Life there must have been, though we are unable to discover its
records. The period of time represented by the Archaean or Pre-Cambrian
rocks must be enormous, and it was in that earliest era that the first
links in the chain of life were forged.
II. =Cambrian.=
The term Cambrian was adopted by Sedgwick for a series of sedimentary
rocks in North Wales (_Cambria_). In that district, in South Wales,
the Longmynd Hills, the Malverns, in Scotland, and other regions
there occur more or less highly folded and contorted beds of pebbly
conglomerate, sandstones, shales and slates resting on the uneven
surface of an Archaean foundation.
It is in these Cambrian rocks that trustworthy records of organic life
are first met with. Among the most constant and characteristic fossils
of this period are the extinct and aberrant members of the crustacea,
the trilobites; these with some brachiopods, sponges, and other fossils
comprise the oldest fauna, of which the ancestral types have yet to
be discovered. During the last few decades the number of Cambrian
fossils has been considerably increased, and in certain regions of
North America and China there are found many thousand feet of strata
above the typical Archaean rocks and below the newer fossiliferous beds
of Cambrian age. It is reasonable to suppose that future research may
extend the present limits of fossil-bearing rocks below the horizon,
which is marked by the occurrence of the widely distributed and oldest
known trilobite, the genus _Olenellus_.
The vast thickness of Cambrian strata was for the most part laid down
on the floor of a comparatively deep sea; other members of the series
represent the shingle beaches and coast deposits accumulated on the
slopes of Archaean islands. There have been many conjectures as to the
distribution of land and sea during the deposition of these rocks; but
the data are too imperfect to enable us to restore with any degree
of confidence the physical geography of this Palaeozoic epoch, of
which the sediments stood out as islands of Cambrian land during many
succeeding ages.
III. =Ordovician.=
Since the days when Sedgwick and Murchison first worked out the
succession of Palaeozoic strata in North Wales, there has always
existed a considerable difference of opinion as to the best method of
subdividing the Cambrian-Silurian strata. Later research has shown
that the rocks included by Sedgwick in his Cambrian system, fall
naturally into two groups; for the upper of these Prof. Lapworth has
suggested the term Ordovician, from the name of the Ordovices, who
inhabited a part of northern Wales. At the base of the system we have
a series of volcanic and sedimentary rocks to which Sedgwick gave the
name Arenig; above these there occur the Llandeilo Flags, succeeded
by a considerable thickness of rocks known as the Bala series. The
rocks making up these Ordovician sediments consist for the most part
of slates, sandstones and limestones with volcanic ashes and lavas.
Much of the typical Welsh scenery owes its character to the folded and
weathered rocks laid down on the floor of the Ordovician sea, on which
from many centres of volcanic activity lava streams and showers of ash
were spread out between sheets of marine sediment. The Arenig Hills,
Snowdonia, and many other parts of North and South Wales, parts of
Shropshire, Scotland, Sweden, Russia, Bohemia, North America and other
regions consist of great thicknesses of Ordovician strata.
IV. =Silurian.=
Passing up a stage higher in the geologic series, we have a succession
of conglomerates, sandstones, shales, and limestones; in other words,
a series of beds which represent pebbly shore deposits, the sands
and muds of deeper water, and the accumulated débris of calcareous
skeletons of animals which lived in the clear water of the Silurian
sea. The term Silurian (Siluria was the country of Caractacus and the
old Britons known as Silures[51]) was first applied by Murchison in
1835 to a more comprehensive series of rocks than are now included
in the Silurian system. The rocks of this period occur in Wales,
Shropshire, parts of Scotland, Ireland, Scandinavia, Russia, the
United States and other countries. After the accumulation of the
thick Ordovician sediments, the sea-floor was upraised and in places
converted into ridges or islands of land, of which the detritus formed
part of the material of Silurian deposits. The limestones of the
Wenlock ridge have yielded an abundant fauna, consisting of corals,
crinoids, molluscs and other invertebrates. In this period we have the
first representatives of the Vertebrata, discovered in the rocks of
Ludlow. In fact, in the Silurian period, “all the great divisions of
the Animal Kingdom were already represented[52].”
V. =Devonian.=
By the continued elevation of the Silurian sea-floor, large portions
became dry land, and during the succeeding period most of the British
area formed part of a continental mass. Over the southern part of
England, there still lay an arm of the sea, and in this were laid
down the marine sediments which now form part of Devon, and from
which the name Devonian has been taken as a convenient designation
for the strata of this period. In parts of the northern land, in the
region now occupied by Scotland, there were large inland lakes, on
the floor of which vast thicknesses of shingle beds and coarse sands
(“Old Red Sandstone”) were slowly accumulated; and it has been shown
by Sir Archibald Geikie and others that during this epoch there were
considerable outpourings of volcanic material in the Scotch area.
Farther to the West and South-west there was another large lake in
which the so-called Kiltorkan beds of Ireland were deposited. In these
Irish sediments, and others of the same age in Belgium and elsewhere
a few forms of land plants have been discovered; but it is from the
Devonian rocks of North America that most of our knowledge of the flora
of this period has been obtained.
VI. =Carboniferous.=
From the point of view of palaeobotany, the shales, sandstones, and
seams of coal included in the Carboniferous system are of special
interest. It is from the relics of this Palaeozoic vegetation that the
most important botanical lessons have been learnt.
The following classification of Carboniferous rocks shows the order of
succession of the various beds, and the nature of the rocks which were
formed at this stage in the Earth’s history.
╭ Upper Coal-Measures.
╭ Coal-Measures[53] ┤ Transition Series.
│ │ Middle Coal-Measures.
│ ╰ Lower Coal-Measures.
│ Millstone Grit.
CARBONIFEROUS ┤ ╭ Upper limestone shales
│ │ and Yoredale rocks.
│ Carboniferous limestone ┤ Carboniferous or
│ series │ Mountain limestone.
│ ╰ Lower limestone shales.
╰ Basement conglomerate.
In the classification of Carboniferous rocks adopted in Geikie’s
text-book of Geology the following arrangement is followed for the
Carboniferous limestone series[54]:—
╭ _Yoredale group_ of shales and grits passing
│ down into dark shales and limestones.
│ _Thick (Scaur or Main) limestone_ in the
│ south and centre of England and Ireland,
│ passing northwards into sandstones,
│ shales and coals with limestones.
Carboniferous ┤ _Lower limestone shale_ of the south and
limestone series │ centre of England. The Calciferous
│ sandstone group of Scotland (marine,
│ estuarine, and terrestrial organisms)
│ probably represents the Scaur limestone
│ and lower limestone shale, and graduates
│ downwards insensibly into the Upper
╰ Old Red Sandstone.
The thick beds of mountain limestone, with their characteristic marine
fossil shells and corals play an important part in English scenery.
In Derbyshire, West Yorkshire, and other places, the limestone crags
and hills are made up of the raised floor of a comparatively deep
Carboniferous sea, which covered a considerable portion of the British
Isles at the beginning of this epoch.
[Sidenote: CARBONIFEROUS ROCKS.]
The accumulation of the calcareous skeletons of marine animals,
with masses of coral, veritable shell-banks of extinct oyster-like
lamellibranchs, built up during the lapse of a long period of time,
formed widespread deposits of calcareous sediments. These were
eventually succeeded by less pure calcareous deposits, the sea became
shallower, and land detritus found its way over an area formerly
occupied by the clear waters of an open sea. The shallowing process
was gradually continued, and the sea was by some means converted into
a more confined fresh-water or brackish area, in which were laid down
many hundred feet of coarse sandy sediments derived from the waste
of granitic highlands. Finally the conditions became less constant;
the continuous deposition of sandy detritus being interrupted by the
more or less complete filling up of the area of sedimentation, and the
formation of a land surface which supported a luxuriant vegetation,
of which the débris was subsequently converted into beds of coal. By
further subsidence the land was again submerged, and the forest-covered
area became overspread with sands and muds.
Such are the imperfect outlines of the general physical conditions
which are represented by the series of sedimentary strata included in
the Carboniferous system. At the close of this period, the Earth’s
surface in Western Europe was subjected to crust-foldings on a large
scale, along lines running approximately North and South and East and
West, the two sets of movements resulting in the formation of ridges
of Carboniferous rocks. The uppermost series of grits, sandstones and
coal-seams were in great part removed by denudation from the crests
of the elevated ridges, but remained in the intervening troughs or
basins where they were less exposed to denudation. It is the direct
consequence of this, that we have our Coal-Measures preserved in the
form of detached basins of upper Carboniferous beds.
A closer examination of the comparative thickness and succession of
Carboniferous rocks in different parts of Britain shows very clearly
that in the northern area of Scotland and in the North of England the
conditions were different from those which obtained further South.
Seeing how much palaeobotanical interest attaches to these rocks, it is
important to treat a little more fully of their geology.
In parts of Devon, Cornwall and West Somerset, the Devonian strata
are succeeded by a series of folded and contorted rocks which have
yielded a comparatively small number of Carboniferous fossils. To this
succession of limestones, shales and grits the term _Culm-Measures_ was
applied by Sedgwick and Murchison in 1837. The rocks of this series
occupy a trough between the Devonian rocks of North and South Devon.
While some authorities have correlated the Culm-Measures with the
Millstone Grit, others regard them as representing a portion of the
true Coal-Measures, as well as the Carboniferous and Lower Limestone
Shale[55]. It has recently been shown that among the lower Culm strata
there occur bands of ancient deep-sea sediments, consisting of beds
of chert containing siliceous casts of various species of Radiolaria.
There can be no doubt that the discovery of deep-sea fossils in this
particular development of the British Carboniferous system leads to
the conclusion that “while the massive deposits of the Carboniferous
limestone—formed of the skeletons of calcareous organisms—were in
process of growth in the seas to the North, there existed to the
South-west a deeper ocean in which siliceous organisms predominated and
formed these siliceous radiolarian rocks[56].”
The Upper Culm-Measures consist of conglomerates, grits, sandstones
and shales with some plant remains and other fossils, and constitute a
typical set of shallow water sediments. In Westphalia, the Harz region,
Thuringia, Silesia and Moravia there are rocks corresponding to the
Culm-Measures of Devon, and some of these have also afforded evidence
of deep water conditions.
[Sidenote: COAL-MEASURES.]
_S. W. England, S. Wales, Derbyshire and Yorkshire._ In these districts
the Carboniferous limestone reaches a considerable thickness; in the
Mendips it has a thickness of 3000 feet, and in the Pennine chain of
4000 feet. At the base of this limestone series there occurs in the
southern districts the so-called lower limestone shale, consisting of
clays, shales and sandy beds. Above the limestone we have the Millstone
grit and Coal-Measures; but in the Pennine district there is a series
of rocks consisting of impure limestones and shales, intercalated
between the Millstone grit and Carboniferous limestone; for this group
of rocks the term _Yoredale series_ has been proposed. In the Isle of
Man and Derbyshire sheets of lava are interbedded with the calcareous
sediments, affording clear proof of submarine volcanic eruptions.
_N. England and Scotland._ In the Carboniferous rocks of Northumberland
we have distinct indications of a shallower sea. The regular succession
of limestone strata in West Yorkshire and other districts, gives place
to a series of thinner beds of limestones, interstratified with shales
and impure calcareous rocks. We have come within the range of land
detritus which was spread out on the floor of a shallow sea. The lowest
portion of the Mountain limestone is here represented by about 200 feet
of shales and other rocks grouped together in the _Tuedian series_.
The Upper Carboniferous limestone and Yoredale rocks of Yorkshire are
represented by sandstones, carbonaceous limestones and some seams of
coal, included in the _Bernician series_. Further north, again, another
classification has been proposed for the still more aberrant succession
of rocks; the lowest being spoken of as the _Calciferous sandstone_,
and the upper as the _Carboniferous limestone_. The calciferous
sandstone may be compared with the lower limestone shale and part of
the Carboniferous limestone of England. The Carboniferous limestone of
Scotland probably represents the upper part of the limestone of England
and the Yoredale rocks of the Pennine and other areas.
Turning to the upper members of the Carboniferous system—in the
Coal-Measures, as they were called in 1817 by William Smith,—we have a
series of coal seams, sandstones, shales, and ironstones occurring for
the most part in basin-shaped areas. As a general rule, each seam of
coal, which varies in thickness from one inch to thirty feet, rests on
a characteristic unstratified argillaceous rock known as Underclay.
The accompanying diagram (Fig. 4) illustrates the frequent
intercalation of small bands of argillaceous and sandy rocks associated
with the seams of coal.
The usual classification adopted for the British Coal-Measures is that
of Upper, Middle, and Lower Coal-Measures; between the Upper and Middle
divisions there occur certain transition or passage beds which are
known as the Transition series. Continental writers, and more recently
Mr Kidston of Stirling, have attempted with considerable success to
correlate the Coal-producing strata by means of fossil plants[57].
[Illustration:
10 in. Massive clay-shale with a few
coal films in the lower part.
10½ in. Shale full of thin streaks of
coal.
14 in. Massive shale with a few streaks
of coal and iron pyrites.
5½ in. Bastard coal; more coal than
shale.
6½ in. Good coal, with masses of iron
pyrites.
1½ in. Coal and seat-rock mixed.
5 in. Seat-rock.
Fig. 4.
Vertical section of the Bassey or Salts Coal seam, Rushton
Colliery, Blackburn (Lower Coal-Measures). From a specimen 4 feet
4 inches in height, presented by Mr P. W. Pickup to the Manchester
Museum, Owens College.]
Finally, some reference must be made to the occurrence of Carboniferous
rocks underneath more recent strata. In a geological map, or bird’s-eye
view of a country, we see such rocks as appear at the surface; by
means of deep borings, however, we are occasionally enabled to follow
the course of older beds a considerable distance below the usually
accessible part of the Earth’s crust. In the neighbourhood of London,
Dover, and other places we have Tertiary and Mesozoic strata forming
the surface of the country, but below these comparatively recent
formations, the sinking of deep wells and other borings have proved
the existence of a ridge of Palaeozoic rocks stretching from the South
Wales Coal-field through the South-east of England to northern France,
Belgium and Westphalia. It is from rocks forming part of this old
ridge that characteristic Coal-Measure plants have been obtained from
the Dover boring. In Fig. 5 is shown an almost complete pinnule of
_Neuropteris Scheuchzeri_ Hoffm., a well-known fern, marking a definite
horizon of Upper Carboniferous rocks[58]. The small hairs on the
pinnules, shown in the figure as fine lines lying more or less parallel
to the midrib and across the lateral veins, are a characteristic
feature of this species.
[Illustration: FIG. 5.
Imperfect pinnule of _Neuropteris Scheuchzeri_ Hoffm., showing the
characteristic hairs as fine lines traversing the lateral veins.
From a specimen obtained from the Dover boring and now in the
British Museum. Nat. size.]
VII. =Permian.=
Reference has already been made to the earth-foldings which marked
the close of Carboniferous times; “the open Mediterranean sea of the
Carboniferous period in Europe was converted into a large inland sea,
like the Caspian of the present day, surrounded by a rocky and hilly
continent, on which grew trees and plants of various kinds[59].” In
parts of
Lancashire, Westmoreland, the Eden Valley, and in the East of England
from Sunderland to Nottingham, there occurs a succession of limestones,
sandstones, clays and other rocks with occasional beds of rock-salt
and gypsum, which represent the various forms of sediment and chemical
precipitates formed on the floor of Permian lakes. The poverty of the
fauna and flora of Permian strata points to conditions unfavourable to
life; and there can be little doubt that the characteristic red rocks
of St Bees Head, and the creamy limestones of the Durham coast are the
upraised sediments of an inland salt-water lake. The term Dyas was
proposed by Marcou for this series of strata as represented in Germany,
where the rocks are conveniently grouped in two series, the _Magnesian
limestone_ or _Zechstein_ and the red sandstones or _Rothliegendes_.
The older and better known name of Permian was instituted by Murchison
for the rocks of this age, from their extreme development in the old
kingdom of Permia in Russia. Unfortunately considerable confusion has
arisen from the employment of different names for rocks of the same
geological period; and the grouping of the beds varies in different
parts of the world. It is of interest to note, that in the Tyrol,
Carinthia, and other places there are found patches of old marine beds
which were originally laid down in an open sea, which extended over
the site of the Mediterranean, into Russia and Asia. In Bohemia, the
Harz district, Autun in Burgundy, and other regions, there are seams
of Permian coal interstratified with the marls and sands. From these
last named beds many fossil plants have been obtained, and important
palaeobotanical facts brought to light by the investigations of
continental workers. Volcanic eruptions, accompanied by lava streams
and showers of ash, have been recognised in the Permian rocks of
Scotland, and elsewhere.
In North America, Australia, and India the term Permo-Carboniferous is
often made use of in reference to the continuous and regular sequence
of beds which were formed towards the close of the Carboniferous and
into the succeeding Permian epoch. The enormous series of freshwater
Indian rocks, to which geologists have given the name of the GONDWANA
SYSTEM, includes the sediments of more than one geological period,
some of the older members being regarded as Permo-Carboniferous in
age. These Indian beds, with others in Australia, South Africa, and
South America, are of special interest on account of the characteristic
southern hemisphere plants which they have afforded, and from the
association with the fossiliferous strata of extensive boulder beds
pointing to widespread glacial conditions.
VIII. =Trias.=
As we ascend the geologic series, and pass up to the rocks overlying
the Permian deposits, there are found many indications of a marked
change in the records of animal and plant life. Many of the
characteristic Palaeozoic fossils are no longer represented, and
in their place we meet with fresh and in many cases more highly
differentiated organisms. The threefold division of the rocks of this
period which suggested the term Trias to those who first worked out the
succession of the strata, is typically illustrated over a wide area
in Germany, in which the lowest or _Bunter_ series is followed by the
calcareous _Muschelkalk_, and this again by the clays, rock-salt, and
sandstones of the _Keuper_ series. In the Cheshire plain and in the
low ground of the Midlands, we have a succession of red sandstones,
conglomerates, and layers of rock-salt which correspond to the Bunter
and Keuper beds of German geologists. These Triassic rocks were
obviously formed in salt-water lakes, in which from time to time long
continued evaporation gave rise to extensive deposit of rock-salt
and other minerals. From the fact that it is this type of Triassic
sediments which was first made known, it is often forgotten that the
British and German rocks are not the typical representatives of this
geological period. The ‘Alpine’ Trias of the Mediterranean region,
in Asia, North America, and other countries, has a totally different
facies, and includes limestones and dolomites of deep-sea origin. “The
widespread Alpine Trias is the pelagic facies of the formation; the
more restricted German Trias, on the other hand, is a shallow shore,
bay or inland sea formation[60].”
In the Keuper beds of southern Sweden there are found workable
seams of coal, and the beds of this district have yielded numerous
well-preserved examples of the Triassic flora. A more impure coal
occurs in the lower Keuper of Thuringia and S.-W. Germany, and to this
group of rocks the term _Lettenkohle_ is occasionally applied.
In the Rhaetic Alps of Lombardy, in the Tyrol, and in England, from
Yorkshire to Lyme Regis, Devonshire, Somersetshire, and other districts
there are certain strata at the top of the Triassic system known as
the _Rhaetic_ or _Penarth_ beds. The uppermost Rhaetic beds, often
described as the White Lias, afford evidence of a change from the salt
lakes of the Trias to the open sea of the succeeding Jurassic period.
Passing beyond this period of salt lakes and wind-swept barren tracts
of land, we enter on another phase of the earth’s history.
IX. =Jurassic.=
The Jura mountains of western Switzerland consist in great part of
folded and contorted rocks which were originally deposited on the
floor of a Jurassic sea. In England the Jurassic rocks are of special
interest, both for geological and historical reasons, as it is in them
that we find a rich fauna and flora of Mesozoic age, and it was the
classification of these beds by means of their fossil contents that
gained for William Smith the title of the Father of English Geology. A
glance at a geological map of England shows a band of Jurassic rocks
stretching across from the Yorkshire coast to Dorset. These are in a
large measure calcareous, argillaceous, and arenaceous sediments of an
open sea; but towards the upper limit of the series, both freshwater
and terrestrial beds are met with. Numerous fragments of old coral
reefs, sea-urchins, crinoids, and other marine fossils are especially
abundant; in the freshwater beds and old surface-soils, as well as in
the marine sandstones and shales, we have remnants of an exceedingly
rich and apparently tropical vegetation. This was an age of Reptiles
as well as an age of Cycads. An interesting feature of these widely
distributed Jurassic strata is the evidence they afford of distinct
climatal zones; there are clear indications, according to the late Dr
Neumayr, of a Mediterranean, a middle European, and a Boreal or Russian
province[61]. The subdivisions of the English Jurassic rocks are as
follows[62]:—
╭ ╭ Purbeck beds ╮ ╮
│ │ Portland beds ├ _Upper_ │
│ │ Kimeridge clay ╯ │
│ ┤ │
│ │ Corallian beds ╮ │
JURASSIC ┤ │ Oxford clay, with ├ _Middle_ ├ Oolite.
│ ╰ Kellaways rock ╯ │
│ │
│ ╭ Great Oolite series ╮ _Lower_ │
│ ╰ Inferior Oolite series ╯ ╯
│
╰ Lias
In tracing the several groups across England, and into other parts of
Europe, their characters are naturally found to vary considerably; in
one area a series is made up of typical clear water or comparatively
deep sea sediments, and in another we have shallow water and shore
deposits of the same age. The Lias rocks have been further subdivided
into zones by means of the species of Ammonites which form so
characteristic a feature of the Jurassic fauna. In the lower Oolite
strata there are shelly limestones, clays, sandstones, and beds of
lignite and ironstone. Without discussing the other subdivisions of
the Jurassic period, we may note that in the uppermost members there
are preserved patches of old surface-soils exposed in the face of the
cliffs of the Dorset coast and of the Isle of Portland.
X. =Cretaceous.=
In the south of England, and in some other districts, it is difficult
to draw any definite line between the uppermost strata of the Jurassic
and the lowest of the Cretaceous period. The rocks of the so-called
_Wealden_ series of Kent, Surrey, Sussex, and the Isle of Wight, are
usually classed as Lower Cretaceous, but there is strong evidence
in favour of regarding them as sediments of the Jurassic period.
The Cretaceous rocks of England are generally speaking parallel to
the Jurassic strata, and occupy a stretch of country from the east
of Yorkshire and the Norfolk coast to Dorset in the south-west. The
Chalk downs and cliffs represent the most familiar type of Cretaceous
strata. In the white chalk with its numerous flints, we have part
of the elevated floor of a comparatively deep sea, which extended
in Cretaceous times over a large portion of the east and south-east
of England and other portions of the European continent. On the bed
of this sea, beyond the reach of any river-borne detritus, there
accumulated through long ages the calcareous and siliceous remains of
marine animals, to be afterwards converted into chalk and flints. At
the beginning of the period, however, other conditions obtained, and
there extended over the south-east of England, and parts of north and
north-west Germany and Belgium, a lake or estuary in which were built
up deposits of clay, sand and other material, forming the delta of
one or more large rivers. For these sediments the name _Wealden_ was
suggested in 1828. Eventually the gradual subsidence of this area led
to an incursion of the sea, and the delta became overflowed by the
waters of a large Cretaceous sea. At first the sea was shallow, and in
it were laid down coarse sands and other sediments known as the _Lower
Greensand_ rocks. By degrees, as the subsidence continued, the shallows
became deep water, and calcareous material slowly accumulated, to be
at last upraised as beds of white chalk. The distribution of fossils
in the Cretaceous rocks of north and south Europe distinctly points to
the existence of two fairly well-marked sets of organisms in the two
regions; no doubt the expression of climatal zones similar to those
recognised in Jurassic times. In North America, Cretaceous rocks are
spread over a wide area, also in North Africa, India, South Africa,
and other parts of the world. Within the Arctic Circle strata of this
age have become famous, chiefly on account of the rich flora described
from them by the Swiss palaeobotanist Heer. The fauna and flora of this
epoch are alike in their advanced state of development and in the great
variety of specific types; the highest class of plants is first met
with at the base of the Cretaceous system.
XI. =Tertiary.=
“At the close of the Chalk age a change took place both in the
distribution of land and water, and also in the development of organic
life, so great and universal, that it has scarcely been equalled at
any other period of the earth’s geological history[63].” The Tertiary
period seems to bring us suddenly to the threshold of our own times. In
England at least, the deposits of this age are of the nature of loose
sands, clays and other materials containing shells, bones, and fossil
plants bearing a close resemblance to organisms of the present era.
The chalk rocks, upheaved from the Cretaceous sea, stood out as dry
land over a large part of Britain; much of their material was in time
removed by the action of denuding agents, and the rest gradually sank
again beneath the waters of Tertiary lakes and estuaries. In the south
of England, and in north Europe generally, the Tertiary rocks have
suffered but little disturbance or folding, but in southern Europe and
other parts of the world, the Tertiary sands have been compacted and
hardened into sandstones, and involved in the gigantic crust-movements
which gave birth to many of our highest mountain chains. The Alps,
Carpathians, Apennines, Himalayas, and other ranges consist to a
large extent of piled up and strangely folded layers of old Tertiary
sediments. The volcanic activity of this age was responsible for the
basaltic lavas of the Giants’ Causeway, the Isle of Staffa, and other
parts of western Scotland.
During the succeeding phases of this period, the distribution of land
and sea was continually changing, climatic conditions varied within
wide limits; and in short wherever Tertiary fossiliferous beds occur,
we find distinct evidence of an age characterised by striking activity
both as regards the action of dynamical as well as of organic forces.
Sir Charles Lyell proposed a subdivision of the strata of this period
into Eocene, Miocene, and Pliocene, founding his classification on the
percentage of recent species of molluscs contained in the various sets
of rocks. His divisions have been generally adopted. In 1854 Prof.
Beyrich proposed to include another subdivision in the Tertiary system,
and to this he gave the name _Oligocene_.
Occupying a basin-shaped area around London and Paris there are beds of
Eocene sands and clays which were originally deposited as continuous
sheets of sediment in water at first salt, afterwards brackish and
to a certain extent fresh. In the Hampshire cliffs and in some parts
of the Isle of Wight, we have other patches of these oldest Tertiary
sediments. Across the south of Europe, North Africa, Arabia, Persia,
the Himalayas, to Java and the Philippine islands, there existed in
early Tertiary times a wide sea connecting the Atlantic and Pacific
oceans; and it may be that in the Mediterranean of to-day we have a
remnant of this large Eocene ocean. Later in the Tertiary period a
similar series of beds was deposited which we now refer to as the
Oligocene strata; such occurs in the cliffs of Headon hill in the Isle
of Wight, containing bones of crocodiles, and turtles, with the relics
of a rich flora preserved in the delta deposits of an Oligocene river.
At a still later stage the British area was probably dry land, and an
open sea existed over the Mediterranean region. In the neighbourhood
of Vienna we have beds of this age represented by a succession of
sediments, at first marine and afterwards freshwater. Miocene beds
occur over a considerable area in Switzerland and the Arctic regions,
and they have yielded a rich harvest to palaeobotanical investigators.
On the coast of Essex, Suffolk, Norfolk, the south of Cornwall, and
other districts there occur beds of shelly sand and gravel long known
under the name of ‘Crag.’ The beds have a very modern aspect; the
sands have not been converted into sandstones, and the shells have
undergone but little change. These materials were for the most part
accumulated on the bed of a shallow sea which swept over a portion of
East Anglia in Pliocene times. In the sediments of this age northern
forms of shells and other organisms make their appearance, and in the
Cromer forest-bed there occur portions of drifted trees with sands,
clays and gravels, representing in all probability the débris thrown
down on the banks of an ancient river. At this time the greater part
of the North Sea was probably a low-lying forest-covered region,
through which flowed the waters of a large river, of which part still
exists in the modern Rhine. The lowering of temperature which became
distinctly pronounced in the Pliocene age, continued until the greater
part of Britain and north Europe experienced a glacial period, and such
conditions obtained as we find to-day in ice-covered Greenland. Finally
the ice-sheet melted, the local glaciers of North Wales, the English
Lake district and other hilly regions, retreated, and after repeated
alterations in level, the land of Great Britain assumed its modern
form. The submerged forests and peat beds familiar in many parts of the
coast, the diatomaceous deposits of dried up lakes, “remain as the very
finger touches of the last geological change.”
[Sidenote: GEOLOGICAL EVOLUTION.]
The agents of change and geological evolution, which we have passed in
brief review, are still constantly at work carrying one step further
the history of the earth. A superficial review of geological history
gives us an impression of recurring and widespread convulsions,
and rapidly effected revolutions in organic life and geographical
conditions; on the other hand a closer comparison of the past and
present, with due allowance for the enormous period of time represented
by the records of the rocks, helps us to realise the continuity of
geological evolution. “So that within the whole of the immense period
indicated by the fossiliferous stratified rocks, there is assuredly
not the slightest proof of any break in the uniformity of Nature’s
operations, no indication that events have followed other than a clear
and orderly sequence[64].”