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A number of systems are connected together to form each of the
great divisions of the Geological Eecord. This classification will-
be best understood if placed in tabular form, as in the subjoined
subdivisions, which occur in the Cretaceous System. 1



Stratigraphical com-
ponents.

A stratum, layer, seam,
or bed, or a number
of such minor subdi-
visions, characterised
by some distinctive
fossil.

Two or more zones

Two or more sets of
connected beds or
assises



Descriptive Names.



= Zone or horizon,



Beds or an assise. .. .

I Group or stage, which
I may be subdivided
I into sub-groups or
sub-stages

Series, section, or for-
mation



Two or more groups or
stages

Several related forma-
tions
1 For an account of the Cretaceous System, see Chapter XXIV.



System



Examples from the Cre-
taceous System.



Zone of Pecten asper.



Warminster beds.

Cenomanian stage, com-
prising the Rotho-
magian and Caren-
tonian sub-stages.



Neocomian formation.
Cretaceous System.



232 GEOLOGY

The names by which the larger subdivisions of the Geological
Record are known have been adopted at various times and on no
regular system. Some of them are purely lithological ; that is,
they refer to the mere mineral nature of the strata, apart alto-
gether from their fossils, such as Coal-measures, Chalk, Green-
sand, Oolite. These names belong to the early years of the prog-
ress of geology, before the nature and value of organic remains
had been definitely realised. Other epithets have been suggested
by localities where the strata occur, as London Clay, Oxford Clay,
Mountain Limestone. The more recent names for the larger
divisions have, in general, been chosen from districts where the
strata are typically developed, or where they were first critically
studied, e.g. Silurian, Devonian, Permian, Jurassic. In some
cases, the larger subdivisions have received names from some dis-
tinguishing feature in their fossil contents, as Eocene, Miocene,
Pliocene. 1 But it is mainly to the minor sections that the char-
acters of the fossil contents have supplied names.

The designation of any particular group of strata has gradually
come to acquire a chronological meaning. Thus we speak of the
Oolites or Oolitic formations of England, and include under these
terms a thick series of limestones, clays, sandstones, and other
strata, replete with organic remains, and containing the records
of a long interval of geological time. But we also speak of the
Oolitic period a phrase which, in the strict grammatical use
of the word, is of course incorrect, but which conveniently desig-
nates the period of geological time during which the great series
of Oolites was deposited, and when the abundant life of which
they contain the remains flourished on the surface of the earth.
This chronological meaning has indeed come to be the more
usual sense in which the names of the major subdivisions of the
Geological Record are generally employed. Such adjectives as
Devonian and Jurassic do not so much suggest to the mind of
the geologist Devonshire and the Jura Mountains, from which
they were taken, nor even the rocks to which they are applied,
as the great sections of the earth's history of which these rocks
contain the memorials. He compares the Jurassic or Devonian
rocks of one country with those of another, studies the organic
remains contained in them, and then obtains materials for form-

1 For the meanings of these names see Chapter XXV.



FOSSIL REMAINS 233

ing some conception of what were the conditions of geography
and climate, and what was the general character of the vegetable
and animal life of the globe, during the periods which he classes
as Jurassic and Devonian.

SUMMARY. Fossils are the remains or traces of plants and
animals which have been imbedded in the rocks of the earth's
crust. From the exceptional nature of the circumstances in
which these remains have been entombed and preserved, only a
comparatively small proportion of the various tribes of plants
and animals living at any time upon the earth is likely to be
fossilised. Those organisms which contain hard parts are best
fitted for becoming fossils. The original substance of the organ-
ism may, in rare cases, be preserved; more usually the organic
matter is partially or wholly removed. Sometimes a mere cast
of the plant or animal in amorphous mineral matter retains the
outward form without any trace of the internal structure. In
other instances, true petrifaction has taken place, the organic
structure being reproduced in calcite, silica, or other mineral by
molecular replacement.

Fossils are of the utmost value in geology, inasmuch as they
indicate (1) former changes in geography, such as the existence
of ancient land-surfaces, lakes, and rivers, the former extension
of the sea over what is now dry land, and changes in the currents
of the ocean; (2) former conditions of climate, such as an Arctic
state of things as far south as Central France, where bones of
reindeer and other Arctic animals have been found; (3) the
chronological sequence of geological formations, and, conse-
quently, the succession of events in geological history, each great
group of strata being characterised by its distinctive fossils. This
is the most important use of fossils. Having ascertained the
order of superposition of fossiliferous rocks, that is, the order in
which they were successively deposited, and having found what
are the characteristic fossils of each subdivision, we obtain a
guide by which to identify the various rock-groups from district
to district, and from country to country. By means of the evi-
dence of fossils the stratified rocks of the Geological Eecord have
been divided into sections and subsections, to which names are
applied that have now come to designate not merely the rocks
and their fossils, but the period of geological time during which
these rocks were accumulated and these fossils actually lived.



234 GEOLOGY



PART IV.

THE GEOLOGICAL BECOED OF THE HISTOEY OP
THE EAETH.

CHAPTEE XVI.

EARLIEST CONDITIONS OF THE GLOBE.

THE foregoing chapters have dealt chiefly with the materials
of which the crust of the earth consists, with the processes
whereby these materials are produced or modified, and
with the methods pursued by geologists in making their study of
these materials and processes subservient to the elucidation of
the History of the Earth. The soils, rocks, and minerals be-
neath our feet, like the inscriptions and sculptures of a long-lost
race of people, are in themselves full of interest, apart from
the story which they chronicle ; but it is when they are made to
reveal the history of land and sea, and of life upon the earth,
that they are put to their noblest use. The investigation of the
various processes whereby geological changes are carried on at
the present day is undoubtedly full of fascination for the student
of nature ; yet he is conscious that it gains enormously in interest
when he reflects that in watching the geological operations of the
present day he is brought face to face with the same instruments
whereby the very framework of the continents has been piled
up and sculptured into the present outlines of mountain, valley,
and plain.

The highest aim of the geologist is to trace the history of the
earth. All his researches, remote though they may seem from
this aim, are linked together in the one great task of unravelling
the successive mutations through which each area of the earth's
surface has passed, and of discovering what successive races of
plants and animals have appeared upon the globe. The investiga-



EARLIEST CONDITIONS OF THE GLOBE. 235

tion of facts and processes, to which the previous pages have been
devoted, must accordingly be regarded as in one sense introduc-
tory to the highest branch of geological inquiry. We have now to
apply the methods and principles already discussed to the eluci-
dation of the history of our planet and its inhabitants. Within
the limits of this volume only a mere outline of what has been
ascertained regarding this history can be given. I shall arrange
in chronological order the main phases through which the globe
seems to have passed, and present such a general summary of the
more important facts regarding each of them as may, P hope,
convey an adequate outline of what is at present known regard-
ing the successive periods of geological history.

As the primitive stages of mankind upon the earth and the
early progress of every race fade into the obscurities of mythology
and archaeology, so the story of the primeval condition of our
globe is lost in the dim light of remote ages, regarding which
almost all that is known or can be surmised is furnished by the
calculations and speculations of the astronomer. If the earth's
history could only be traced out from evidence supplied by the
planet itself, it could be followed no further back than the
oldest portions of the earth now accessible to us. Yet there can
be no doubt that the planet must have had a long history before
the appearance of any of the solid portions now to be seen. That
such was the case is made almost certain by the traces of a
gradual evolution or development which astronomers have been
led to recognise among the heavenly bodies. Our earth being
only one of a number of planets revolving round the sun, the
earliest stages of its separate existence must be studied in refer-
ence to the whole planetary system of which it forms a part.
Thus, in compiling the earliest chapter of the history of the
earth, the geologist turns for evidence to the researches of the
astronomer among stars and nebulas.

In recent years, more precise methods of inquiry, and, in par-
ticular, the application of the spectroscope to the study of the
stars, have gone far to confirm the speculation known as the
Nebular Hypothesis. According to this view, the orderly related
series of heavenly bodies, which we call the Solar System, existed
at one time, enormously remote from the present, as a Nebula
that is, a cloudy mass of matter, like one of those nebulous,
faintly luminous clouds which can be seen in the heavens. This



236 GEOLOGY

nebula probably extended at least as far as the outermost plane-
tary member of the system is now removed from the sun. It
may have consisted entirely of incandescent gases or vapours,
or of clouds of stones in rapid movement, like the stones that
from time to time fall through our atmosphere as meteorites, and
reach the surface of the earth. The collision of these stones
moving with planetary velocity would dissipate them into vapour,
as is perhaps the case in the faint luminous tails of comets. At
all events, the materials of the nebula began to condense, and in
so doing threw off, or left behind, successive rings (like those
around the planet Saturn), which, in obedience to the rotation
of the parent nebula, began to rotate in one general plane around
the gradually shrinking nucleus. As the process of condensation
proceeded, these rings broke up, and their fragments rushed to-
gether with such force as not improbably to generate heat enough
to dissipate them .again into vapour. They eventually condensed
into planets, sometimes with a further formation of rings, or
with a disruption of these secondary rings, and the consequent
formation of moons or satellites round the planets. The outer
planets would thus be the oldest, and, on the whole, the coolest
and least dense. Towards the centre of the nebula the heaviest
elements might be expected to condense, and there the high tem-
perature would longest continue. The sun is the remaining in-
tensely hot nucleus of the original nebula, from which heat is
still radiated to the furthest part of the system.

When a planetary ring broke up, and by the heat thereby
generated was probably reduced to the state of vapour, its ma-
terials, as they cooled, would tend to arrange themselves in ac-
cordance with their respective densities, the heaviest in the centre,
and the lightest outside. In process of time, as cooling and con-
traction advanced, the outer layers might grow quite cold, while
the inner nucleus of the planet might still be intensely hot.
Such, in brief, is the well-known Nebular Hypothesis.

Now the present condition of our earth is very much what, ac-
cording to this hypothesis or theory, it might be expected to be.
On the outside comes the lightest layer or shell in the form of an
Atmosphere, consisting of gases and vapours. Below this gaseous
envelope which entirely surrounds the globe lies an inner en-
velope of water, the ocean, which covers about two-thirds of the
earth's surface, and is likewise composed of gases. Underneath



EARLIEST CONDITIONS OF THE GLOBE 237

this watery covering, and rising above it in dry land, rests the
solid part of the globe, which, so far as accessible to us, is com-
posed of rocks twice or thrice the weight of pure water. But
observations with the pendulum at various heights above the sea
show that the attraction of the earth as a whole indicates that
the globe probably has a density about five and a half times that
of water. Hence we may infer that its inner nucleus not im-
probably consists of heavy materials, and may be metallic. There
is thus evidence of an arrangement of the planet's materials in
successive spherical shells, the lightest or least dense being on the
outside, and the heaviest or most dense in the centre.

Again, the outside of the earth is now quite cool; but abun-
dant proof exists that at no great distance below the surface the
temperature is high. Volcanoes, hot springs, and artificial bor-
ings all over the world testify to the abundant store of heat within
the earth. Probably at a depth of not more than 20 miles from
the surface the temperature is as high as the melting-point of
any ordinary rock at the surface. By far the largest part of the
planet, therefore, is hotter than molten iron. We need have no
hesitation in admitting it to be highly probable that the earth
was formerly in the state of incandescent vapour, and that it has
ever since that time been cooling and contracting. Its present
shape affords strong presumption in favour of the opinion that
the globe was once in a plastic condition. The flattening at the
poles and bulging at the equator, or what is called the oblately
spheroidal figure of the planet, is just the shape which a plastic
mass would have assumed in obedience to the influence of the
movement of rotation, imparted to it when detached from the
parent nebula.

At present a complete rotation is performed by the earth in
twenty-four hours. But calculations have been made with the
result of showing that originally the rate of rotation was much
greater. Fifty-seven millions of years ago it was about four
times faster, the length of the day- being only six and three-quar-
ter hours. The moon at that time was only about 35,000 miles
distant from the earth, instead of 239,000 miles as at present.
Since these early times the rate of rotation has gradually been
diminishing, and the figure of the earth has been slowly tending
to become more spherical, by sinking in the equatorial and rising
in the polar regions.



238 GEOLOGY

Of the first hard crust that formed upon the surface of the earth
no trace has yet been found. Indeed, there is reason to suppose
that this original crust would break up and sink into the molten
mass beneath, and that not until after many such formations and
submergences did a crust establish itself of sufficient strength to
form a permanent solid surface. Even though solid, the surface
may still have been at a glowing red-heat, like so much molten
iron. Over this burning nucleus lay the original atmosphere,
consisting -not merely of the gases in the present atmosphere,
but of the hot vapours which subsequently condensed into the
ocean, or were absorbed into the crust. It was a hot, vaporous
envelope, .under the pressure of which the first layers of water
that condensed from it may have had the temperature of molten
lead. As the steam passed into water, it would carry down with
it the gaseous chlorides of sodium, magnesium, and other vapours
in the original atmosphere, so that the first ocean was probably
not only hot, but intensely saline.

Eegarding these early ages in the earth's history we can only
surmise, for no direct record of them has been preserved. They
are sometimes spoken of as pre-geological ; but geology really
embraces the whole history of the planet, no matter from what
sources the evidence may be obtained. Deposits from this orig-
inal hot saline ocean have been supposed to be recognisable in the
very oldest crystalline schists ; but for this supposition there does
not appear to be any good ground. The early history of our
planet, like that of man himself, is lost in the dimness of an-
tiquity, and we can only speculate about it on more or less
plausible suppositions.

When we come to the solid framework of the earth we stand
on firmer footing in the investigation of geological history. The
terrestrial crust, or that portion of the globe which is accessible
to human observation, has been found to consist of successive
layers of rock, which, though far from constant in their occur-
rence, and though often broken and crumpled by subsequent
disturbance, have been recognised over a large part of the globe.
They contain the earth's own chronicle of its history, which has
already been referred to as the Geological Eecord, and the sub-
division of which into larger and minor sections, according
mainly to the evidence of fossils, was explained in the preceding
chapter.



EARLIEST CONDITIONS OF THE GLOBE 239

Had the successive layers of rock that constitute the Geological
Kecord remained in their original positions, only the uppermost,
and therefore most recent, of them would have been visible, and
nothing more could have been learnt regarding the underlying
layers, except in so far as it might have been possible to explore
them by boring into them. But the deepest mines do not reach
greater depths than between 3,000 and 4,000 feet from the sur-
face. Owing, however, to the way in which the crust of the
earth has been plicated and fractured, portions of the bottom
layers have been pushed up to the surface, and those that lay
above them have been thrown into vertical or inclined positions,
so that we can walk over their upturned edges and examine them,
bed by bed. Instead of being restricted to merely the uppermost
few hundred feet of the crust, we are enabled to examine many
thousand feet of its rocks. The total mean thickness of the ac-
cessible fossiliferous rocks of Europe has been estimated at 75,000
feet, or upwards of 14 miles. This vast depth of rock has been
laid bare to observation by successive disturbances of the crust.

The main divisions of the Geological Eecord and, we may also
say, of geological time, are five: (1) Archaean, embracing the
periods of the earliest rocks, wherein no traces of organic life
occur; (2) Palaeozoic (ancient life) or Primary, including the
long succession of ages during which the earliest types of life
existed; (3) Mesozoic (middle life) or Secondary, comprising a
series of periods when more advanced types of life nourished ; (4)
Cainozoic (recent life) or Tertiary, embracing the ages when the
existing types of life appeared, but excluding man; and (5)
Quaternary or Post-tertiary and Eecent, including the time since
man appeared upon the earth. It must not be supposed that
each of these five divisions was of the same duration. The
Palaeozoic ages were probably vastly more prolonged than those
of any later division; while the Quaternary periods must com-
prise a very much briefer time than any of the other four groups.

Each of these main sections is further subdivided into systems
or periods, and each system into formations as already explained.
Arranged in their order of sequence, the various divisions of the
Geological Eecord may be placed as in the accompanying Table.



240 GEOLOGY



THE GEOLOGICAL RECORD,

or, Order of Succession of the Stratified Formations of the Earth's Crust



Recent and Prehistoric.
Pleistocene or Glacial.



a



Pliocene.
Miocene.
Oligocene.



Eocene.



Cretaceous.
Danian.
Senonian.
Turonian.
Cenomanian.
Gault (Albian).
Neocomian.

Jurassic.
Purbeckian.
Portlandian.
Kimmeridgian.
Corallian.
Oxfordian.
Bathonian.
Bajocian.
Liassic.

Triassic.
Rhsetic.

Keuper or Upper Trias.
Muschelkalk.
Bunter or Lower Trias.



EARLIEST CONDITIONS OF THE GLOBE



241



Permian.

Upper Red Sandstones, clays, and gypsum.
Magnesian Limestone (Zechstein).
Marl-Slate (Kupferschiefer).
Lower Red Sandstones, breccias, etc. (Rothliegende),

Carboniferous.
Coal-Measures. ,
Millstone-Grit.
Carboniferous Limestone series.

Devonian and Old Red Sandstone.



Devonian
Type

Old Red

Sandstone
Type



Upper Cypridina and Goniatite beds.
Middle Stringocephalus (Eifel) Limestone.
Lower Spirifer Sandstone, etc.

Upper Yellow and Red Sandstones, with Holop-

tychius, Pterichthys major, etc.
Lower Sandstones, flagstones, and conglomerates,

with Cephalaspis, Coccosteus, Asterolepis, etc.



Silurian.

Ludlow group.

Wenlock group.

Upper Llandovery group.
f Lower Llandovery group.
I Caradoc and Bala group.
1 Llandeilo group.
[Arenig group.



.a &


fc
O rt


Upper Tremadoc Slates.


8 |

O M


a $

oi T3


Lingula Flags.


11


2 1


Lower Menevian group.


Oj P-4

P-l H


11


Harlech group.


O


5





Longmyndian Uriconian.

[Dalradian.]

Torridonian.

Archaean Lewisian, Hebridean.



THE PRE-CAMBRIAN PERIODS.



Owing to the revolutions which the crust of the earth has un
dergone, there have been pushed up to the surface, from under
neath the oldest fossiliferous strata, certain very ancient crystal-



242 GEOLOGY

line rocks which form what is termed the Archaean system. As
already mentioned, these rocks have by some geologists been sup-
posed to be a part of the primeval crust of the planet, which
solidified from fusion. By others they have been thought to
have been formed in the boiling ocean, which first condensed
upon the still hot surface of the globe. In truth, we are still
profoundly ignorant as to the conditions under which they arose.
We have hardly any means of ascertaining in what order they
were formed. We know no method of determining whether those
of one region belong to the same period as those of another. N or
can we always be sure that what have been called Archaean rocks
may not belong to -a, much later part of the Geological Eecord,
their peculiar crystalline structure having been superinduced
upon them by some of those subterranean movements described
in Chapter XIII.

Of Archaean rocks the most abundant is gneiss, passing on the
one hand into granite, and on the other into micaceous and
argillaceous schists, with interstratified bands of various horn-
blendic, pyroxenic, and garnetiferous rocks, limestone, dolomite,
serpentine, quartzite, graphite, hematite, magnetite, etc. These
various materials are more or less distinctly bedded. But the
beds are for the most part inconstant, swelling out into
thick zones, and then rapidly diminishing and dying out. This
bedding somewhat resembles that of sedimentary rocks, and the
manner in which the limestone and graphite occur, recalls the
way in which limestone and coal are found in the fossiliferous
formations. The inference has accordingly been drawn that the
Archaean crystalline bands were really deposited as chemical pre-
cipitates or mechanical sediments on the floor of the primeval
ocean, and have since been more or less crystallised and disturbed.
But from what has been brought forward in Chapter XIII, re-
garding the totally new structures which have been developed in
rocks by subterranean movement, it is evident that a bedded ar-
rangement and a crystalline texture, like those of the Archaean
gneisses and schists, have sometimes been induced in rocks by ex-
cessive crumpling, fracture, and shearing. How far, therefore,
the apparent bedding of Archaean rocks is their original condi-
tion, or is the result of subsequent disturbance, is a question that
cannot yet be answered.

The alternations of gneiss and other crystalline masses form



EARLIEST CONDITIONS OF THE GLOBE 243

bands which are usually placed on end or at high angles, and
are often intensely crumpled and puckered, having evidently un-
dergone enormous crushing (Fig. 114). Attempts have been
made to subdivide them into groups or series, according to their



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