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THE EARTH
IN PAST AGES




VESUVIUS IN ERUPTION, 1872.



STORIES of the UNIVERSE

The Earth

In Past Ages

By

H. G. SEELEY, F.R.S.

PROFESSOR OF GEOGRAPHY AND LECTURER ON GEOLOGY AND
MINERALOGY IN KING'S COLLEGE, LONDON



I

WITH FORTY ILLUSTRATIONS



NEW YORK

REVIEW of REVIEWS COMPANY
1910



COPYRIGHT, 1895, 1902,
BY D. APPLETON AND COMPANY.



PREFACE.



I HAVE endeavoured to tell the story of the
Earth so that its past history helps to explain its
present condition.

Explanations are given of the nature of the
common materials which form rocks, of the ways
in which they rest upon each other, and of the
means by which they may be distinguished.

The story of the Earth is divided into epochs
by layers of rock which rest on each other and
rise to the surface of the visible land, and to the
floor of the ocean.

Geological time cannot be defined in years.
The time occupied by an existing river like the
Rhine or the Niagara river, in excavating the
gorge through which it flows, dates back beyond
the antiquity imagined for man by historians.
Yet this incident in sculpture of the Earth's sur-
face is subsequent to the newest of the regular
layers of rock. It is convenient to forget the
human standard of time, and think of a period of
geological time as the age when some rock, such
as coal, accumulated, or when an extinct plant or
animal was dominant on the, Earth.

Fossils are the remains of plants and animals
by which each period of by-gone time is distin-
guished.

5



6 PREFACE.

I. Many kinds of animals, which still live>
date back to the beginning of the Earth's story>
or to an early period.

II. Many groups of animals, such as Trilo-
bites or Ichthyosaurs, endured on the Earth for
long geological ages, varied in form and struc-
ture, and became extinct successively, leaving no
survivor.

The life which now exists on the Earth is a
survival of ancient types of life known from
fossils, which have undergone substantially no
change since first they became known in the
rocks. They are associated now with groups,
like the Mammalia, which are changing rapidly.
The diversity of mammal orders in structure of
the skeleton, is not unlike that which the ancient
Saurians put on before they became extinct.
Animals' orders which vary rapidly last for a
relatively short time.

I have used some scientific names of these
fossils in the story of the Earth, since names
give the easiest identification for fossils as for
our fellow-men. The characteristics or lives of
fossil animals and of living men give interest
to their names. Practical knowledge of fossils
ensures this enduring interest, and is gained by
collecting them in the sea-cliff, quarry, or pit,
and by comparing such specimens with named
examples in museums.

H. G. SEELEY.
KENSINGTON, W., 1895.



CONTENTS.



I. INTRODUCTION . . . .- ,'-.' .-. . 9
II. THE EARTH'S INTERNAL HEAT ... 12

III. MATERIALS OF MOUNTAIN CHAINS v ,,; ; . 18

IV. VOLCANIC ROCKS . . ); Y . . . 26
V. THE MATERIALS OF STRATA . '.", . 31

VI. THE SUCCESSION OF STRATA . . ,- . 51
VII. ORIGIN OF STRATIGRAPHICAL GEOLOGY . 58
VIII. FOSSILS . T. ; .< . ' ..' . . 62
IX. THE CLASSIFICATION OF WATER-FORMED

ROCKS . . . ..... . . 74

X. THE ARCH^AN ROCKS . . . . .78

XI. CAMBRIAN AND ORDOVICIAN ROCKS . . 81

XII. OLD RED SANDSTONE AND DEVONIAN . . 92

XIII. CARBONIFEROUS STRATA . . . . .97

XIV. PERMIAN AND TRIAS 115

XV. LIAS . . . ' ;V . . . / . 125
XVI. OOLITES . . . . . . . .130

XVII. THE NEOCOMIAN PERIOD . . . . 142

XVIII. LOWER CRETACEOUS ROCKS .... 149

XIX. THE CHALK 156

XX. THE LOWER TERTIARY STRATA . . . 162
XXI. THE MIDDLE TERTIARY PERIOD . . . 173

XXII. THE CRAG ; v / 178

XXIII. GLACIAL PERIOD AND GRAVELS . . .182



LIST OF ILLUSTRATIONS.



VIGURB

Vesuvius in Eruption, 1872 Frontispiec*

1. Gneiss 89

2, 3. Hertfordshire Pudding Stone 35

4. Laminated Sand 39

5. Ripple-marked Sandstone . . 41

6. Lithographic Stone ....... 46

7. Carboniferous Limestone 47

8. The Chalk in Yorkshire 56

9. Snowdon to Flintshire 83

10. Inlier at Usk .... . ' . .87

11. Feet of the Trilobite. ...... 91

12. To the Forest of Dean ...... 93

13. East of the Pennine Chain loo

14. Productus . . . . . . . . IOI

15. Pleu rot om aria IO2

16. Sigillaria . \ Iio

17. Teniseopteris ..... Hi

18. Pareiasaurus 119

19. Lias Outlier 126

20. Gryphsea Incurva . ,127

21. Cardinia Listen 128

22. Ichthyosaurus .....*.. 131

23. Belemnites Oweni 136

24. Shotover Hill 137

25. Skeleton of Archseopteryx . . . . .139

26. Skull of Archseopteryx 140

27. Hythe to Folkestone 148

28. Ammonites Deshayesii . . . . . .150

29. Section at Hunstanton . . I 5 I

30. Ammonites Planulatus . .152

31. Poterocrinus ........ 153

32. Micraster Coranguinum 159

33. Galerites Subrotundus 160

34. East of Herne Bay 164

35. Ostrsea Bellovacina 165

36. Cyprina Morrisi 166

37. Strata in Alum Bay ....... 169

38. Planorbis Euomphalus . . '*'' 175

39. Cardita Senilis ........ 179

40. Fusus Antiquus reversed ...... 181



THE STORY OF THE EARTH.



CHAPTER I.

INTRODUCTION.
I.

THE building of the surface layers of the Earth
is recorded in rock materials, which are accumu-
lated upon each other. But there is no trace of a
beginning to their story of the Earth's history.
In the remotest period of past geological time of
which evidence has been found, the earth was in-
habited by types of animals, some of which still
survive. There is no evidence that the most
ancient animals which have, been discovered were
the first that existed, or that the oldest rocks at
present known mark the beginning of geological
records. It is as unprofitable to enquire for evi-
dences of the origin of the earth, as it is to ask
for proofs of the mode of origin of the life which
has flourished upon it.

Because the earth is a planet we may assume
that it had a similar history in its origin to some
of the heavenly bodies. The light which comes
to the earth from the most distant stars in the
universe, proves, when analysed, to result from
the incandescence of elements which are mostly
identical with those found in the earth. The
small masses of matter, termed meteorites, which
fall from time to time to the earth's surface, con-



10 THE STORY OF THE EARTH.

sist of iron and other metals, and of minerals like
those which combine to form crystalline rocks.
The forces which act on the earth are like those
manifested in other heavenly bodies. If the
Earth's surface is not incandescent, as in the
luminous stars, its interior demonstrates in many
ways an internal heat, which has played an im-
portant part in its history. So that, with the mat-
ter and force substantially the same, there is some
justification for the old definition of geology as
that department of astronomy which tells the
story of the Earth.

The geological story differs from that told by
the astronomer in giving results of unceasing ac-
tion of the forces of nature upon the rock materi-
als of the globe. They have worked during a time
which is immeasurably long, when estimated by
such changes on the earth as have happened dur-
ing human history. This time cannot be expressed
in centuries. The work of rivers in carving chan-
nels upon the existing surface of the earth has
been computed at from 15,000 to 30,000 years, in
the case of Niagara river, without reaching the
age when the newer layers of the globe were de-
posited from the sea. This stupendous duration
of time has brought about revolutions in the po-
sitions of oceans and continents ; in the types of
life which were predominant on the earth, as well
as in the distribution of life over the globe, and
in the succession of different kinds of life in the
same region in successive ages, which would be
incredible but for the evidence of fossil animals
and existing animals which are everywhere around
us. These changes have come about, not as result
of catastrophes which have destroyed the fair sur-
face of the land and its life, but as parts of the



INTRODUCTION. 1 1

order of nature, and as conditions of that stability
of government of the world by which the cre-
ations of earlier times have been preserved, and
passed on from one geological age to another to
survive at the present day.



ii.

On various parts of the globe, meteorites have
been found which vary in weight from a few ounces
to a few tons. Examples of 400 of them are pre-
served in the British Museum. Some have been
seen to fall. It may therefore be inferred that
ever since the earth has been in existence it has
probably received such additions of material.
Meteorites however do not demonstrate that the
earth has been built up of meteoric matter; but
they are the only clue of a practical kind to the
origin of the globe, which the geologist encoun-
ters.

The iron in meteorites is metallic, usually com-
bined with nickel. In the earth iron is rarely me-
tallic, and rarely crystallized with nickel. Minute
particles of metallic iron are present in the vol-
canic rock named Basalt, which has flowed over
the north of Ireland. Iron is found combined
with nickel in the Van mine in Denbighshire.
The percentage of nickel in the iron varies in
different localities. There is only one or two per
cent, of nickel in the great masses of iron, some-
times weighing 50,000 Ibs., embedded in Basalt at
Ovifak in Disco Island, on the west of Greenland.
An alloy of these metals found in New Zealand,
yields 67 per cent, of nickel. Both are regarded
as of terrestrial origin.

Although the mineral quartz is one of the most



12 THE STORY OF THE EARTH.

abundant constituents of surface rocks, no true
quartz has been recognised in any meteorite. But
a rare mineral asmanite with many of the proper-
ties of quartz occurs, which somewhat resembles
the variety of quartz found in some volcanic
rocks, which has been distinguished under the
name tridymite.

About ten rare minerals are met with in me-
teorites which have never been recognised in the
rock materials of the globe.

On the other hand, earthy meteorites have
yielded many of the constituents of volcanic and
crystalline rocks.

Two kinds of felspar named labradorite and
anorthite have been recorded in meteorites, and
such minerals as Augite, Bronzite, Enstatite, Oli-
vine, which upon the earth are often combined with
the felspars in mineral union to form crystalline
rocks. But the facts are too few and too obscure
to do more than stimulate interest in the relation
of the earth to the bodies among which it moves.



CHAPTER II.
THE EARTH'S INTERNAL HEAT.

THE earth has an internal heat of its own,
which is not derived from the sun. The temper-
ature of the outer surface layer varies with sum-
mer and winter. In Java and India at a depth of
12 feet the thermometer is constant all the year
round. In London and Paris an unvarying tem-
perature occurs at about 100 feet below the earth's
surface. The earth's heat begins to increase be-



THE EARTH'S INTERNAL HEAT. 13

low this variable surface layer, though the rate of
increase differs with the kinds of rock passed
through, and with the locality. It averages one
degree Fahrenheit for every 55 feet of depth.

In the famous Artesian well at Grenelle near
Paris, the water rose from a depth of 1794 English
feet, with a temperature of nearly 82 F. The
deep boring at Sperenberg near Berlin appears to
show an increase of i F. in 42 feet at the depth
of TOCO feet; i F. in 57 feet, at 2000 feet; and i
F. in 95 feet, at 3000 and 4000 feet. From these
facts the inference has been made that tempera-
ture does not augment appreciably below a mod-
erate external thickness of rock.

The difference between the surface tempera-
ture and the interior temperature, results from the
loss of the earth's internal heat by radiation. On
this circumstance attempts have been made to es-
timate the duration of geological time. By meas-
uring the amount of heat which the earth radiates
from its surface in a year, Lord Kelvin has con-
cluded that in a period of 20,000 millions of years,
more than enough heat would have been lost to
melt the entire bulk of the earth, if the* rate of
loss had been always what it is now, and if the
earth had consisted throughout of the same ma-
terials as its surface rocks. This is the time which
the physicist conceives as possible for the earth's
origin and history. Sir John Herschel had doubt-
ed the primitive fluidity of the earth. It is per-
haps possible that the heat which the earth loses
may not be the original heat of an igneous fusion,
but the result of strain due to its rigid state. It
rotates so that its surface experiences the lifting
influence of tidal attraction which reduces the
pressure, although the amount is too small to dis-



14 THE STORY OF THE EARTH.

1 %

turb the stability of its surface. By the conver-
sion of this attraction of gravitation upon its
outer layers into heat, at a depth from the surface
sufficient to ensure that the heat so generated
could not be radiated in a day, a store of heat
might accumulate near to the surface of the globe.
The most ancient rocks give no evidence of greater
internal heat, or of greater refrigeration of the
earth, or of tidal action upon its surface having
been in any way different from what it is now.

The greatest depth at which the fractures and
dislocations, termed earthquakes, are known by
actual measurement to originate, is about 30
miles. It has also been calculated that a heat
sufficient to melt granite might occur at a depth
of 20 or 30 miles. This is the maximum depth to
which geological theory extends its inferences.

Attempts have been made to calculate the
pressure under which masses of granite in moun-
tain chains have consolidated. In some cases the
crystal structure appears to indicate a superin-
cumbent pressure equal to no more than 15 miles'
thickness of rock, though the pressure was proba-
bly lateral.

The materials ejected from volcanos give no
indication of having ascended from more than
very moderate depths. The molten matter of
lava streams does not appear to be the primitive
substance of the earth's interior. That heated
material might be rendered liquid by fractures
which penetrate downward so as to remove the
pressure which keeps the heated rock solid. It is
thus manifest that some cause generates heat near
to the earth's surface, which is associated with the
crumpling of the earth's outer layers, with the
changed distribution of level of land from age to



THE EARTH'S INTERNAL HEAT. 15

age, and with the phenomena of volcanic ac-
tivity.

This cause is believed to be the cooling of the
earth ; by which the shrinkage of the deeper lay-
ers crushes the upper layers together, crumpling
them into folds which are directed alternately up-
ward and downward. As these folds are crushed
closer together, the mechanical energy of com-
pression, resisted by the rock material, becomes
converted into heat along the lines of most intense
squeezing;.

The directions of these folds change from age
to age in geological time; for every land consists
of masses of rock which extend through it in direc-
tions which were once approximately parallel to
its shores.

The late Mr. Robert Mallet believed that the
energy of volcanic eruptions was developed by
these compressions of the crust. He also urged
that the lateral pressure exerted by the sides of
an arch of continental land upon its supports
would result in crushing along the lines of greatest
weakness; and calculated that the temperature
may be raised locally in this way to a red heat, or
even to the fusing point of the. rocky ^materials
which are crushed. This heat, which is produced
locally, he believed to be consumed locally, and to
be the source of the explosive energy which ejects
the materials of which volcanos are built up.

Active volcanos are commonly met with in
regions undergoing upheaval. This is attributed
to the underground compression of the rocks
which causes upheaval, generating heat. The
water near the shore which penetrates to the
heated region is raised by that heat to an explo-
sive temperature. Volcanos have a linear exten-



1 6 THE STORY OF THE EARTH.

sion ; sometimes in islands rising from the sea,
sometimes in mountain chains formed of islands
united together. The linear arrangement is at-
tributed to the opening of fissures, which pen-
etrate downward along lines, in which the rocks
have been folded and fractured in the process of
upheaval. When rain water, in a region so bent
and strained, is held back upon the land and
hindered from escaping by the pressure of the sea
round its shores, the water descends through the
minor joints and capillary interspaces between the
particles of rock. Then it rises in temperature
with the internal heat of the earth, so as to facil-
itate the melting of rocks, with which it combines.
Some of this water eventually ascends through
the planes of fracture and displacement forming
outlets for explosive energy, discharging steam,
dust, and the rock matter, both solid and molten,
which builds volcanic cones.

The past periods of geological time abound in
evidences of volcanic activity. From the imper-
fect nature of the records which remain upon the
earth their linear arrangement is not always evi-
dent ; but they may be inferred to mark lines of
upheaval which brought islands into existence, or
united them into continental masses of land in
successive epochs of geological time. But be-
sides the volcanos which are marked by beds of
ashes and lava-flows, and the throats up which
the molten matter ascended, there are in many
parts of the world extinct volcanos with their
cones well preserved, as though the craters had
been recently active.

A little south of the Pyrenees, in the basin of
the Ebro, there are fifteen cones about Olot in
Catalonia, built of cinders, from each of which



THE EARTH'S INTERNAL HEAT. 17

lava has flowed in streams still to be traced, yet
so long since that the existing rivers have cut
passages for themselves through the lava.

The Auvergne is a granite platform in which
some ancient rocks of the carboniferous period
occur. This district appears to have been an is-
land traversed by a line of fracture from N.W. to
S.E., which corresponds to the uplifting of the
crystalline rocks. A second fracture runs from
N. to S. In this region are the ruins of the four
grand volcanos known as Mont Dore, Cantal,
Canton d'Aubrac, and Mezen. The lava flowed
from Mont Dore for 20 miles. The minor cones,
of which there are hundreds, range through the
country in a broad band, from N. to S. Many
have the craters burst down by the lava which
ascended in them, and overflowed into the neigh-
bouring valleys.

Beautifully preserved volcanic cones are found
to the north of the Moselle river in the district
known as the lower Eifel. It may have been in
this country that the eruption took place which is
mentioned by Tacitus as having affected the
country near Cologne, in the reign of the Roman
Emperor Nero. For a long way up tho Rhine
the rocks are volcanic ; and evidences of extinct
volcanos are found west of the Rhine, in many
parts of central Germany; and a series ranges
through Hungary S.W. of the Carpathians into
Servia.

The latest volcanic outbursts in the British
Isles were at the beginning of the Tertiary period
in Skye, Rum, Mull and the adjacent mainland of
Scotland, and in the north of Ireland, where
streams of mud due to volcanic dust, washed
down by rains, covered up the vegetation of the



1 8 THE STORY OF THE EARTH.

country before it was deluged with the black lava
named basalt. Branches of the conifer Sequoia,
and of plane trees covered with leaves, are pre-
served in the consolidated mud which underlies
these lava-flows.



CHAPTER III.

THE MATERIALS OF MOUNTAIN CHAINS.

THE same cause which produced the local heat
and fractures which led to volcanic outbursts, has
folded the earth's crust. Rocks many thousands
of feet thick have been bent, folded and crumpled.
This structure, which is shown in the succession
of rocks on the surface of every country, in folds
termed saddles and troughs, is most astounding in
its intensity in mountain chains. The upheaving
of the parallel ridges of limestone rock known as
the Jura chain, forming the frontier between
Switzerland and France, is a beautiful example of
troughs which form valleys, parting the elevated
ridges from each other. In that part of the Alps
known as the Grisons, all the geological deposits,
from the tertiary down to the oldest, have been
turned upside down, in the process of folding by
lateral displacement ; which is the sole cause
which lifts mountain ranges. The curved form of
the earth necessitates that every axis of elevation
must be accompanied by spurs at right angles to
itself, or by parallel ranges. The parallel system
is exemplified in the chains of North America,
which lie between the Rocky Mountains and the
Pacific.



THE MATERIALS OF MOUNTAIN CHAINS. 19

These folds once formed remain for all time.
They may be raised higher, or depressed beneath
the sea, and new rocks laid down upon them ;
but as those ancient folds increase in intensity
with the slow succession of geological ages, the
newer rocks become folded with their folds, and
the folds run in the same direction.

In such puckered and crumpled rocks as moun-
tain chains exhibit on their denuded heights, there
is almost invariably evidence of a crystalline tex-
ture. This may be attributed to the influence of
the heat produced by the mechanical power, trans-
formed by the resistance which the rock mass of-
fered to compression.

The rocks which form mountains are chiefly
slaty rocks, and schists, with here and there some
granite masses or sheets of volcanic rock. They
have only been laid bare by the removal of vast
thicknesses of water-formed rock which once ex-
tended above them. If the crystalline materials
are not the necessary products of the upward
thrust of the mountain chain and adjacent land
which supports it, it may be difficult to account
for the uniform character of the rocks of which
the durable central masses of mountain cnains are
built. There are stages in this process of change.
The flanks of a mountain range commonly show
the fine microscopic crystalline texture of slate,
while the central masses show the coarse crystal-
line texture of schist, or granite.

Slate. The part which slate plays in the forma-
tion of mountain masses is well seen in the struc-
ture of the mountainous regions of North and
Central Wales, in parts of the Lake District in
Westmoreland and Cumberland, and in the south
of Scotland. It is certain that slate was originally



20 THE STORY OF THE EARTH.

a water-formed rock, a mud which consolidated
into clay. It often shows successive parallel beds
marked by differences of colour. Welsh slates
sometimes contain clay pebbles, such as occur at
the present day on shores where the cliffs are of
compact clay. Many slates contain fossil remains
of animals which lived in the sea when the old
mud was accumulating. Those fossils are often
distorted and squeezed into half their original
breadth or length, showing that the whole moun-
tain mass has undergone compression and con-
densation. The compression has bent the rocks
into synclinal troughs and anticlinal saddles.
The slaty texture is most developed in the troughs.
The effect of this lateral pressure has been in
the first place to turn the films of water contained
between the particles of the old mud at right
angles to the direction from which the pressure
came. The resistance offered by the rock trans-
formed a large part of the motion imparted to its
particles into heat. That heat raised the temper-
ature of the water contained in the rock, enabling
each film, under the pressure, to dissolve some
constituents of the mineral matter in which it was
contained. These slaty rocks often give evidence
of having been fractured through their thickness
by minute dislocations, and subsequently re-united.
Such breakage, relieving the pressure, would cause
the temperature to fall, and the substances which
had been dissolved then crystallize in minute
films, parallel to each other, extending throughout
the mountain mass, and having no relation to the
original planes in which the mud was deposited.
These microscopic crystal films resemble such min-
erals as mica or chlorite. They impart to the
rock the property termed slaty cleavage. This


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