Chapter VI: Part 6

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NOTABLE EVENTS B. C.
8000

The earliest civilisation known is that of Egypt, traces of
which have been found dating back to 7,000 or 8,000 B.C.
Equally early civilisations were probably established in the
Euphrates Valley. 4000

In the fifth millennium Khufu built the Great Pyramids; in
the fourth a Semitic migration, spreading westward from Asia,
peopled Babylonia, Assyria, Canaan, and Phœnicia afresh,
establishing new nations and kingdoms. 3000

The third millennium saw the Aryan invasion of India; the
beginning of Chinese history; and Aryan and Semitic waves of
migration towards Europe. 2000
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Egypt was conquered by the Hyksos, a Semitic nomadic race. 1500

Hittite Empire established in Syria.

During the next three hundred years, of which the history
is obscure, the dynasty of the Ramesides was established in
Egypt, which waged wars with the Hittite Empire. Rameses 1400
II. is popularly identified with the Pharaoh of the Exodus,
an event which is also identified with the expulsion of the
Hyksos. The supremacy in the Mesopotamian regions alternates 1300
between Assyrian and Babylonian dynasties.

---------------------------------------------------------------- 1200

Rise of a Hebrew nation.

Age of Phœnician prosperity; commercial importance of Sidon
and Tyre.

---------------------------------------------------------------- 1100

Ionic and Doric migrations.

Predominance of Phrygia among kingdoms of Asia Minor.

1048 B.C. David captures Jerusalem and becomes King over all
Israel.

---------------------------------------------------------------- 1000
]

JAPAN

CHINESE
EMPIRE

INDIA

PARTHIAN
EMPIRE

ARABIA

ROMAN EMPIRE

BRITAIN
]

---------------------------------------------------------------- 1000

975 B.C. Division of the Hebrew kingdom into Judah and Israel
after the death of Solomon.

Growth of the Hellenic States.

The age of Homer.

---------------------------------------------------------------- 900

850 B.C. Foundation of Carthage.

Beginnings of the Latin and Etruscan peoples.

---------------------------------------------------------------- 800

Assyrian conquest of Babylon, Syria, and Israel.

753 B.C. The foundation of Rome.

Rapid spread of the Greek Colonies.

---------------------------------------------------------------- 700

Beginnings of the Macedonian kingdom.

Rise of Media.

Beginnings of Japanese history.

Decline of Assyria, fall of Nineveh, and establishment of new
Babylonian Empire.

---------------------------------------------------------------- 600

Cyrus, King of Persia, conquers Media, establishes his empire
over Lydia, Assyria, and Babylonia (538 B.C.). His son
Cambyses conquers Egypt, 525 B.C.

---------------------------------------------------------------- 500

The Greek States revolt against Persia and are triumphant.

Egypt regains independence.

Steady growth of Roman ascendancy in Italy.

Struggle between Athens and Sparta.

---------------------------------------------------------------- 400

Conquests of Alexander the Great (334-322 B.C.). He conquers
Persia, masters Egypt, and invades India. At his death his
empire is divided: Egypt falls under the Ptolemies, Syria
under the Seleucidæ.

---------------------------------------------------------------- 300

Babylon absorbed by Parthian Empire.

Carthage dominates Spain.

Wars between Rome and Carthage. Overthrow of Carthage (202
B.C.).

---------------------------------------------------------------- 200

Judea attains independence under the Maccabees.

Growing power of Rome. Macedon a Roman province; Egypt and
Syria made Roman protectorates. The Greek States are absorbed
into province of Macedon.

---------------------------------------------------------------- 100

Cæsar conquers Gaul and lands in Britain.

Egypt becomes a Roman province.

Augustus Cæsar. Establishment of the Roman Empire. B.C.

JAPAN

CHINESE EMPIRE

INDIA

PARTHIAN EMPIRE

ARABIA

ROMAN EMPIRE

BRITAIN]

----------------------------------------------------------------

NOTABLE EVENTS

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For the first four centuries of the Christian era the Roman
Empire absorbed the “known” world, bounded in Europe by
the ocean, the Rhine, and the Danube, and in Asia by the
Euphrates, and including the Mediterranean districts of
Africa. Germanic tribes bore with ever-increasing pressure
upon her European borders, and the Parthians defied her in
the East. At the close of the third century the centre of
political gravity was passing from Rome itself to Byzantium,
preparing for the scission of the Empire, into Eastern and
Western, which was practically at the close of the fourth
century, when it was becoming increasingly clear that Rome
could not stand against the Barbarian invaders, notably the
Goths under Alaric.

----------------------------------------------------------------

In the fifth century the Empire, long weakened by corruption
and the tyranny of the army, was overwhelmed by the
Barbarians. Vandals, Western Goths, and Suevi poured into
Spain; Franks and Alemanni spread over Gaul; Ostro-Goths and
Lombards settled in North Italy; Huns and Avars attacked
Thrace.

Britain was invaded by Saxons, Jutes, and Angles.

----------------------------------------------------------------

The seventh and eighth centuries were marked by the rapid
rise of Mohammedanism in Arabia; the conquests of the
Saracens in Egypt, Africa, and West Asia; the establishment
of the Caliphate at Bagdad; and their invasion of Spain. Here
they were checked by the Franks.

Charlemagne, son of Pippin, King of the Franks in Germany and
Gaul, was crowned in 768, conquered Lombardy in 774, calling
himself “King of the Franks and Lombards and Patrician of
the Romans.” His empire was divided after his death; from
it emerged modern France and Germany. His coronation by the
Pope at Rome (A.D. 800) originated the idea of the Holy Roman
Empire.

----------------------------------------------------------------

JAPAN

CHINESE
EMPIRE

{BRITISH
{INDIAN
{EMPIRE

AFGHANISTAN

PERSIA

ARABIA

EGYPT

TURKEY
{BALKAN
{STATES

GREECE

RUSSIAN
EMPIRE

ITALY

AUSTRO-HUNGARY

GERMAN
EMPIRE

BELGIUM

HOLLAND

SWITZERLAND

FRANCE

PORTUGAL

{SOUTH
{AMERICAN
{STATES

SPAIN

MEXICO

MOROCCO

GREAT
BRITAIN
&
IRELAND

UNITED
STATES

DENMARK

NORWAY

SWEDEN

----------------------------------------------------------------

Disintegration of the Empire of the Caliphs, and rise in Asia
Minor of the Seljuk Turks, making war against the Byzantine
Empire and the Crusaders, and conquering Egypt.

India is invaded by Mohammedan Afghan rulers, who eventually
establish a dynasty at Delhi.

----------------------------------------------------------------

The Kingdoms of Hungary, Bohemia, and Poland, converted to
Christianity in the tenth century, come into increasing
prominence.

The Kings of Castile, Navarre, Aragon and Portugal war
against the Moors, who (A.D. 1248) are restricted to Granada.

The Mamelukes (Slave kings) conquer Egypt (1252).

Switzerland attains independence.

----------------------------------------------------------------

Failure of England to absorb Scotland, or to conquer France.
The Hundred Years’ War.

----------------------------------------------------------------

The Turks capture Constantinople (1453).

The Netherlands (Burgundy) united to the House of Hapsburg.
(1477).

Spain united; overthrow of the Moorish dominion.

----------------------------------------------------------------

Bohemia and Hungary united to Austria. Spain and Portugal
take possession of the New World. Mogul Empire established in
Hindostan. The Reformation leads to revolt of the Netherlands
from Spain; Spain absorbs Portugal.

----------------------------------------------------------------

Union of English and Scottish crowns (1603); followed by
legislative union (1707). Disruption of Germany in the Thirty
Years’ War. Establishment of English Colonies in America.
Portugal recovers independence.

----------------------------------------------------------------

Spain becomes a Bourbon Power. Rise of Russia and Prussia.
Partition of Poland between Russia, Prussia and Austria.
Further disintegration of German Empire. British dominion in
India and North America. Independence of United States.

----------------------------------------------------------------

France predominant under Napoleon. Rise of South American
States. Establishment of British India. Italy independent.
Egypt, Greece, and Balkan States freed from Turkey.
Foundation of German Empire.

----------------------------------------------------------------

Independence of Norway (1905).

----------------------------------------------------------------

THE BEGINNING OF THE EARTH

BY PROFESSOR SOLLAS

The origin of our planet is a problem which has appealed to the intellect of thoughtful men from the most remote times, and the earliest recorded speculations concerning it--those of the Mosaic cosmogony--possess a peculiar interest, since they embody the views of the ancient Chaldeans, who were not only systematic observers of the heavens, but made practical use of their results.

[Sidenote: Beginning of a Famous Theory]

The Mosaic cosmogony is not unworthy of the great people among whom it took its rise; it recognises the fact that the earth had a history antecedent to the advent of man, and its account of the order of events in this history is not only remarkable as a feat of _a priori_ reasoning, but accords in some respects with the results achieved after much labour by modern science.

It was not until the middle of the eighteenth century that the reign of evolution began, and attempts were made to trace the history of a planetary system from its source in a primeval nebula on purely mechanical grounds. Swedenborg (1735) was the pioneer in this direction, then came Thomas Wright (1750) of Durham, whose work furnished inspiration to Emanuel Kant (1755), and led him to construct a consistent scheme of the Universe. The last of this group of cosmic philosophers is Laplace (1796), whose admirable description of the evolution of the solar system was arrived at independently, and without knowledge of the previous work of Kant.

Laplace assumed as his starting-point the existence of a nebula formed of incandescent gas, and extending beyond the limits of the outermost planet of our system. It was in rotation about a central axis, and possessed in consequence a disc-like or lenticular form. Radiating its heat away in all directions through surrounding space, it grew continually colder, and in cooling diminished in bulk. As a consequence of this contraction its rate of rotation increased, till at length the centrifugal force of the outermost part became so great that this could no longer continue to follow the contracting mass within, and thus remained behind as a great rotating ring. The continued contraction of the internal mass, and the resulting increase in the velocity of rotation, again brought about the same condition of things, and a fresh ring was left behind.

[Sidenote: Cooling of the Nebula]

This process was repeated time after time, till as many rings were formed as there are planets in the solar system; the central mass which survived within the innermost ring condensed to form the sun. The rings were highly unstable--that is to say, a slight disturbing force was sufficient to destroy their continuity; they broke across and rolled up into great nebulous globes, which revolved round the sun in the same direction as the original nebula, and rotated on their axes in the same direction as that in which they revolved. Most of them repeated the behaviour of the original nebulæ, leaving behind rings as they contracted, and these rings either rolled up to form moons or satellites, or, in the solitary instance of Saturn’s rings, retained their annular form. The rings are now known to consist of a multitude of solid bodies, as proved by Clerk-Maxwell.

[Sidenote: The Temperature of the Earth]

By this hypothesis, so beautiful in its simplicity, an explanation was afforded embracing all the more important facts of our system; the revolution of all the planets in nearly circular orbits and in the same direction as that in which the sun rotates, and the revolution of their satellites, also in circular orbits and in the same direction as their primaries; the comparatively high temperature and consequent low density of the larger planets and the sun, as well as a variety of other phenomena, all seem to follow naturally from it. The fundamental assumption seems to be in harmony with a number of known facts. Thus in the case of our own planet the volcanoes distributed around the margins of the oceans, and the hot springs scattered irregularly over the whole terrestrial surface, suggest that great stores of heat exist beneath our feet, a presumption which finds confirmation in the fact that whenever we descend towards the interior of the earth, as in deep mines or wells, the temperature continues steadily to rise after we have passed a depth below which seasonal and diurnal changes of temperature cease to be felt, the rise being in some cases as much as 3 deg. for 100 ft., in others only 1 deg. for the same distance, but on the average 1 deg. for 60 ft. or 70 ft. If this increase of temperature continues down to great depths, and there seems to be no reason why it should not, then a point will be reached, say, at thirty or forty miles down, where the interior will attain a white heat.

[Sidenote: The Earth as a Star]

Thus the earth might be regarded as a white hot body surrounded with a film of rock growing continually cooler towards the surface. But such a hot body suspended in space must be cooling, just as all bodies which are hotter than their surroundings. It is cooler to-day than it was yesterday, or--what is the same thing--it was hotter yesterday than it is to-day, and so of all previous yesterdays. And thus as we travel backwards in time we perceive that the earth will be growing hotter, the level of white heat will be mounting upwards towards the surface, and will at last reach it, so that the earth, instead of being, as it now is, a dark body shining only with the reflected light of the sun, will be self-luminous, a tiny star of a magnitude so diminutive as to have awakened resentment on the part of some terrestrial inhabitants, who have regarded it as disproportionate to their dignity. But we cannot arrest imagination at this stage; our thought still extends its retrospective glance into the abyss of past time, and we perceive the earth still growing hotter, till its temperature transcends those limits at which it can exist in the solid state. It becomes molten--nay, more, it becomes gaseous, and thus resumes the nebular state from which it sprang. Precisely the same argument applies to the sun; our mighty luminary is also a cooling body, and if we could restore to it the heat which it has lost in the course of past æons it would resume a completely gaseous state. Modified in one way or another, this chain of reasoning seemed irrefragable in those happy days which preceded the discovery of radium.

[Sidenote: Universe still in Evolution]

The question may be considered from another point of view. On searching the heavens we find that many of the stages which are assumed in Laplace’s hypothesis are still represented by actual existences. There are, to begin with, those immense diffused nebulæ, almost incapable of definition, which are proved, on spectroscopic examination, to emit that kind of light which is characteristic of glowing gas; from these we pass to others which are resolvable by the telescope into a central and more condensed nucleus, with two mighty nebulous arms whirled round in a spiral, and bearing more condensed masses in their midst; even ring nebulæ are known to exist; and, finally, there are nebulous halos which surround some of the stars. Then we come to the stars themselves, which are suns of various degrees of magnitude, some immensely larger than our own luminary, and these are evidently in various stages of existence. Some are blue, and afford evidence of a higher temperature than that of our sun; others are yellow, and make a nearer approach to the solar temperature; while, again, others are red, and certainly colder.

These, in conjunction with other considerations, lead to the conviction that the universe is in a state of evolution, and that the solar system at one time existed in a nebular state. But whether Laplace’s description of the series of events through which the original nebula passed is the true one or not is a very different matter; it presents so many difficulties that scarcely any student now supports it.

In the beginning, it is supposed that the earth was part of a vast
nebula of gaseous matter and meteorites, resembling the nebula of
Argo, illustrated above.

Later, as the cooling process advanced, the nebula assumed a
rotatory movement in the form of a spiral. The nebula of Andromeda
affords an excellent illustration of this.

Another stage would be as in the annular nebula of Aquaris, the
mass forming into a ball with the outer ring attached.

HOW THE HEAVENS TELL THE STORY OF THE ORIGIN OF THE EARTH]

Or, like the nebula of Cygni, with the central sun well formed and
the gaseous ring far removed, the earth would begin to shape, and
the ring would roll up to form the moon.

Jupiter, which is in a molten state, wreathed in thick vapour, with
the “great red spot” indicating the beginning of the solidifying
process, shows what the earth was like before it assumed its
present solid condition.

This shows the earth and the moon in their relative sizes; while
the diagram below it illustrates the distance apart.
]

[Sidenote: Laplace’s Theory Abandoned]

A fundamental difficulty is the extreme tenuity of the gas which is assumed to have formed the planetary rings. A second difficulty, which has been emphasised by Professors Chamberlin and Moulton, is to be found in the comparatively small amount of rotational energy which the system at present possesses, for this is less than 1/200 of that which, on the most favourable assumption, must have been contained within the original nebula. Less fundamental, but equally fatal, is the fact that one of the satellites of Saturn revolves round its primary in a direction opposed to that of the rotation of the planet itself. [Recently Mr. Stratton, following out a suggestion of Professor W. H. Pickering, has shown that this is quite consistent, and, indeed, is a natural deduction from Laplace’s hypothesis.] Hence for these and other reasons we are reluctantly compelled to abandon an hypothesis which for over a century has exercised an influence on our conception of the cosmos not less profound, penetrating, and far-reaching than that of the famous Darwinian doctrine of natural selection, now on its trial.

[Sidenote: What are the Nebulæ?]

At present, unanimity of opinion, even on questions of the most primary kind, is far to seek. Philosophers are not even agreed as to the constitution of the nebulæ. It is questioned whether even those least resolvable and most diffused forms which give bright line spectra really consist of masses of incandescent gas. Many observers, among them Sir Norman Lockyer, now maintain that they are formed of swarms of meteorites, which, moving with prodigious velocity, meet in frequent collision, and by their impact evolve sufficient heat to become self-luminous. Others, again, like the distinguished investigator Arrhenius, while admitting the gaseous nature of these nebulæ, deny that they are incandescent, and assert that their temperature is not much above that of surrounding space. Their exterior parts consist of the lighter gases in a highly rarefied state, and minute particles of negative electricity, which are always careering through space, on penetrating these gases produce a luminous discharge. A nebula composed of swarms of meteorites would, as Sir George Darwin has shown, behave very much in the same way as one composed of gas, and if in rotation would rotate as a solid mass. The meteorites would stand in the same relation to the nebula as molecules to a gas, and thus the question of the constitution of the nebula, although of great interest in itself, becomes of subsidiary importance in tracing its subsequent history.

[Sidenote: Shaping of the Planets]

One of the latest attempts to frame a nebular hypothesis is that of Professor J. H. Jeans. His reasoning is of a highly mathematical character, and his conclusions are expressed in the most general terms. Starting with a spherical nebula of gas or meteorites endowed with a small amount of rotation, he shows that as it cools or loses energy the temperature of the interior will not fall continuously in precise correspondence with the cooling of the outer parts, and this “lag” of the interior temperature will bring about a tendency to instability. The contraction of the nebula due to cooling will increase the velocity of rotation, and this again will tend to instability. As a result of the instability so produced the nebula will change its form, and become more or less pear-shaped. The narrow end of the pear will then separate from the body and assume an independent existence as a primitive planet. This process will recur again and again till the nebula is resolved into a sun with its attendant planets. The planets, existing at first as gaseous masses or quasi-gaseous masses, will be liable to the same kind of transformation, and may thus bud off moons or satellites.

If the nebula were not in rapid rotation, a slight disturbing cause, acting at the critical moment when a planet was being ejected, might determine the inclination of the planet’s orbit, which might thus be very oblique to the equatorial plane of the nebula. Thus the hypothesis is not open to one of the objections which have been urged against that of Laplace--namely, that the orbits of some of the planets in the solar system are inclined at a large angle with the plane of the sun’s equator.

This illustrates Laplace’s theory, which conceived of a vast nebula
filling the whole space of the solar system and rotating around a
central axis. The outer and thinner part had much greater movement
than the denser central mass, finally being thrown off as a ring,
which in turn rolled up into a ball, still following the same
course as the ring had followed. Thus the earth broke off from the
sun and the moon from the earth. The theory is, however, no longer
credited by scientists.

The pear-shaped nebula is the theory of a young English
mathematician, Professor J. H. Jeans. Starting with a spherical
nebula, he argues that in cooling it will assume the form
illustrated above, and that the smaller part will separate and form
a satellite rotating independently but within a distance influenced
by the parent mass.

The spiral nebula in Canes Venatici, a revolving mass of gas or
meteorites, supplies, according to the nebular hypothesis of
Messrs. Chamberlin and Moulton, an excellent example of how the
earth and moon were formed. We may reasonably imagine the smaller
spiral to represent the moon in the act of being thrown off by the
earth.

THREE FAMOUS THEORIES OF THE BEGINNING OF THE EARTH]

[Sidenote: Heavenly Bodies in Collision]

Jeans mentions two disturbing causes in particular which might easily arise--one the penetration of the nebula by a wandering meteorite, which might precipitate an event already on the verge of happening, and simultaneously determine both the birth of a planet and the obliquity of its orbit; the second, the presence of some distant mass, such as a star, which, by raising a quasi-tide in the nebula, would give the final touch required to overturn its equilibrium. The influence of a distant body, such as a passing star, has been invoked by Moulton in another version of the nebular hypothesis. In conjunction with Chamberlin, he calls special attention to the spiral nebulæ, which are by far the commonest kind, as presenting the closest approach to the conditions which obtain when planets are actually in course of formation. Chamberlin and Moulton enter on a detailed account of the manner in which they suppose the planets to have grown by the gradual accretion of meteoric masses as these encountered each other while moving in various elliptical orbits.

At present it would seem impossible to speak with certainty as to the precise history of the solar system. Meanwhile, we may console ourselves with the closing words of Professor Jeans’ paper, to the effect that “no difficulty need be experienced in referring existing planetary systems to a nebulous or meteoric origin on the ground that the configurations of these systems are not such as could have originated out of a rotating mass of liquid.”

An investigation by Sir George Darwin, which has furnished inspiration to such hypotheses as that of Jeans, brings us nearer the immediate subject of this essay, since it treats of one of the last acts in the great drama of planetary existence, and attempts to derive the earth and moon from a common origin in a single rotating sphere.

[Sidenote: Why the Day is Growing Longer]

It is well known that, owing to the frictional effects produced by the tides, the earth is being gradually slowed down as it rotates upon its axis. Thus the day is constantly getting longer, so that in a few millions of years it will have increased in length from twenty-four to twenty-five hours. On the other hand, in past time it must have been shorter than at present: a few millions of years ago it was only twenty-three hours in length, and many millions of years earlier it was still less, only some five hours or so. At that time the earth was hotter than it is now, less rigid, more yielding, and, owing to its rapid rotation, less stable. The action on the moon of the tides produced in it by the earth is similar, and the rotation of the moon has been so far diminished by them that its day has become as long as the month--_i.e._, our satellite only turns once round on its axis in the time that it takes to revolve once round the earth; it is for this reason that our satellite keeps always the same face turned towards us.

[Sidenote: The Moon Was Part of Our Sphere]

The retardation of the earth in its rotation has, however, a very remarkable effect on the revolution of the moon; it involves--by the principle of the conservation of moment of momentum--a corresponding acceleration of the moon in its orbit, and, as a consequence of this, an enlargement of this orbit--that is, the moon is pushed away from us, as it were, and thus becomes more remote. But if so, the moon must have been nearer to us in times past. It is possible to trace the approach of the moon to the earth as we go backwards in time till the distance between them was only two and a half terrestrial radii instead of the sixty radii which now separate them. Mathematics do not take us farther back than this. But it is difficult to resist the suggestion that in the immediately preceding stage of development the earth and moon formed together a single sphere.

If we may adopt this view, then we must regard the sphere as subject to the tidal influence of the sun. It was much hotter, and therefore more yielding, than the present earth; it was also rotating much faster, probably once in about four or five hours. It would be contracting as a consequence of cooling, and the contraction would lead to instability (gravitational instability); its rapid rotation would also tend toward instability (rotational instability). It is difficult to say which of these two, gravitational or rotational instability, would be the most effective; but the combined result would be to give a pear-shaped form to the rotating mass, and eventually to deepen the constriction between the narrow and the broad end, till the smaller protuberance became completely dissevered from the larger mass, and so entered on an independent existence as the moon. This final step in the process would probably depend on the tide-producing power of the sun; the larger mass remained behind as the earth, whose individual existence may be said to date from this event.

[Sidenote: How the Moon Broke Away]

The young earth would be subject to very much the same conditions after as before the ejection of the moon, and might very possibly again pass into a pear-shaped form, but without proceeding further through those subsequent changes, which would have led to the formation of another satellite; and while possessing some such form as this, she might very well have consolidated. With advancing years she would lose, as we have seen, the activity of her youth, the drag of the tides would cause her to spin ever more slowly on her axis, till the day would become prolonged to the twenty-four hours of the present. With this diminished rate of spin, the earth, if free to yield, would lose the pear-shaped form and become an oblate spheroid, and the oblateness of this spheroid would continually diminish, so that it would continually approach towards a true sphere. Suppose, however, that the earth as it cooled lost its power of readily yielding--and at present it is more rigid than a globe of steel--then it would pass from form to form, not by a flowing movement, but by a series of ruptures, and its form at any moment might be a little in arrear of that which it would have possessed if it had been in the fluid state.

Thus it might indeed be possible still to discover some trace of an old-fashioned form in the existing planet; and a careful examination of the distribution of land and sea as represented on a terrestrial globe does, in fact, reveal a remarkable symmetry, in which we seem to recognise a surviving vestige of its early state. The great continent of Africa projects like the narrow end of a pear; around it are oceans--the Atlantic, the Indian Ocean, and the Mediterranean Sea, which was once of far greater extent; then comes a great dismembered ring of land, the two Americas, the Antarctic continent, Australia, Asia, and Europe. Within these, on the side opposite to Africa, is the great Pacific Ocean, which covers over the broad end of the pear.

[Sidenote: Earth’s Unknown Changes]

Soon after the earth had cooled down, so that the oceans were
formed, the shaping of the great continents began. The action of
moving water in the making of new land is well illustrated by the
vast delta of the Mississippi, where an area larger than Wales has
been formed by debris deposited by the river.
]

A line drawn from somewhere in Central Africa to its antipodes in the Pacific, through the centre of the earth, would correspond to the long axis of the pear; a second, at right angles to this, would correspond to its breadth; and a third, at right angles to both, would correspond to the axis on which it rotates. A diameter of the earth taken through the equator is almost 8,000 miles in length, the Polar diameter is about sixteen miles shorter, and this slight difference measures the oblateness of the spheroid, or the departure of the form of the earth from a true sphere. Further, it would appear that the diameter drawn through Africa is about half a mile longer than the equatorial diameter taken at right angles to it, and this insignificant quantity measures the departure of the form of the earth from that of an oblate spheroid to that of a pear, so nearly complete is the adjustment of its form to existing conditions. Before this nice adjustment was reached, the earth must have suffered many changes, passed through many times of stress and storm, and witnessed many geological revolutions.

[Sidenote: An Age of Red-hot Rain!]

If, at the beginning of her career, the earth was molten, or at a very high temperature, she must have been surrounded by a very deep and dense atmosphere, for all the waters which now rest on her surface--oceans, lakes, and rivers--would have contributed to it in the state of steam; and not till the temperature of the ground had fallen to 380 deg. C. could liquid water have begun to accumulate. Then a steady downpour of almost red-hot rain would have set in, filling up the neck of the pear and extending far and wide over its broad end.

The temperature would now fall somewhat rapidly, and in a short space of time the surface of the earth would have become as cool as it is at the present day. Directly the waters of the firmament had collected into the oceans, leaving behind an atmosphere like that which now exists, geological agencies of the kind we are now familiar with would begin their sway. Air and rain would exert their insidious power upon the rocks, sapping their strength, converting the hardest granite into soft sand and clay, which would be washed away by the rain through brooks and rivulets into the channels of many rivers, all hastening with their burden of sediment, to deposit it finally in the sea. Here it would accumulate, layer after layer, building up those mighty masses of strata which now form the greater part of the visible land. While this general action was everywhere in progress, wearing down continents and islands towards the level of the sea, more specialised activities were assisting to the same end.

This illustrates in striking manner, based on the calculations
of the best authorities, the comparative sizes of the earth,
first as a gaseous mass, and, second, after it had cooled down
and solidified into the planet on which we live. The small dot
represents 8,000 miles, the earth’s diameter.
]

The waves which fall upon our coasts are now constantly undermining the cliffs and extending the margin of the sea at the expense of the land, and rivers not only serve to transport sediment, but cut down their channels deep into the rock, and so carve out the most varied landscapes of hill and valley from monotonous tableland.

[Sidenote: Action of Winds and Tides]

When we enter into calculations we are astonished at the rapidity with which these agents perform their work even at the present day; but as we proceed farther back into the past, when the earth was full of youthful energy, their power must have been greatly enhanced. We might almost take the measure of the day as the measure of their work, for they probably accomplished as much during the eight hours’ day which once existed as they do now in twenty-four hours. A little consideration will make this clear. It is the winds which, blowing over the surface of the ocean, produce the sea waves, and it is these falling on our coasts that perform the work of marine denudation. But the winds are due in the first place to the heat of the sun, and the difference of temperature established at the equator and the poles; and, in the next place, to the rotation of the earth. Thus, with the increased rapidity of rotation which we know to have existed, and with increased radiation from the sun, a very probable contingency, the winds would increase in strength and more powerfully erode our coasts. Again, with the moon in greater proximity, and with a more rapid rotation of the earth, the tides would be much higher and more frequent, and these, raising and lowering the cutting edge of the sea, greatly assist it in its work of destruction. The winds and the tides produce various marine currents, and these help to distribute the sediment which the rivers deliver into the sea, so that when stronger currents flowed as a result of more powerful tides and more violent winds, the sediments would be strewn over wider areas; hence, the more ancient strata of our planet are far more widely distributed than are those of later time.

In the days when the earth’s crust had formed but was still
unstable, the process of cooling not having gone far enough,
there would not be the mountains which now characterise it. These
came when the earth contracted and crumpled up along certain
well defined lines, which are now represented by the three great
mountain chains of the world.
]

[Sidenote: Building Up the Earth]

Finally, a heavier rainfall would result from a more active atmospheric circulation, creating larger rivers, and thus, at the beginning, all those denuding agents which are engaged in wearing the land down into the sea would be working at a more rapid pace. Correspondingly, all the agents which are occupied in building up deposits of sediments would have extended their operations over a wider area, laying down a foundation broad and deep.

On the other hand, the contraction of the earth, due to the loss of its energy of rotation as well as of its internal heat, would also have proceeded more rapidly, new land would have emerged from the sea, old lands would have been submerged beneath it far less slowly than at the present day; ruptures of the crust, accompanied by earthquakes and volcanic action, would have been more frequent and thus, by the more rapid loss of its intrinsic energy, the renovation of the earth would have kept pace with its accelerated destruction.

One effect of the contraction of the earth which has manifested itself in even late geological times is the crumpling up of the terrestrial crust into the sharp folds of mountain chains; but at the beginning this crumpling must have been far more universal and energetic. In this connection it is interesting to observe that the most ancient rocks known to us--the Archæan--never present themselves under any other form than as intensely plicated masses. They originally consisted of lava flows and volcanic ashes, of ancient sediments and limestones, into which subterranean masses of granite and other molten, deep-seated rocks have been injected; but under the intense pressures to which they were subjected after their formation they and the invading granite have entirely lost their original character, and have been metamorphosed into gneisses, schists, and marble, all sharply and closely folded together. In any given district the direction of their folding is maintained with wonderful constancy over great distances. There is no succeeding system of rocks that has been so completely transformed, so universally plicated, as this ancient Archæan complex.

In later times we can pass from stratum to stratum of the sedimentary series and read their history almost as we turn over the pages of a book; in the Archæan all are kneaded together into a state of such desperate entanglement as to defy the powers of human ingenuity to unravel them. Thus the line of demarcation between the Archæan and subsequent sedimentary systems is the sharpest and most absolute that is known to us in the history of the earth. It marks the close of our planet’s infancy, the several events of which have passed into oblivion as profound as that of our own forgetfulness of our earliest days. Later events, on the other hand, are recorded in the stratified series with a faithfulness which increases as we approach existing times.

[Sidenote: How We Know These Wonders]

[Sidenote: The Ocean 100 million Years old!]

[Sidenote: The Part Radium may play]

A history without dates must seem very unsatisfactory to a historian, and the question will naturally arise whether we can assign any definite time to the various critical events recorded in the evolution of the earth. At present we can only make more or less plausible estimates. Thus, from a consideration of the thickness of the sedimentary crust, and the rate at which sediments are now being deposited, it has been asserted that the interval which separates us from the close of the Archæan era may amount to about twenty-six millions of years. Professor Joly, basing his argument on the undoubted fact that the ocean derives the greater part of its salt from the dissolved material contributed to it by rivers, comes to the conclusion that the ocean first came into existence about one hundred millions of years ago. As regards the birth of the moon, Sir George Darwin has given a minimum limit of fifty-four millions of years, but he adds that it may have taken place many hundreds of millions of years before this. Lord Kelvin has attempted to determine the time which has elapsed since the earth first acquired a solid crust. If we only knew the rate at which the earth is cooling we might calculate back to this time with some assurance of certainty, always, however, on the assumption that the earth is simply a hot body cooling like any other hot body--such, say, as a red-hot cannonball. But a few years ago it began to be seriously suspected that this assumption was a very doubtful one, for a new element--radium--was discovered in 1898, which possesses the remarkable property of spontaneously liberating heat, and this not in small quantities, but at an astonishing rate. One gramme of radium, for example, gives out enough heat in one hour to raise the temperature of one gramme of water to boiling point; hour after hour, year in, year out, this wonderful substance is setting free the energy it contains, and will continue to do so until, some thousands of years hence, it has exhausted its store. If this element should happen to exist in sufficient quantity within the earth, then the earth could not be said to be cooling just like a piece of hot iron, and the increase of temperature we experience as we descend towards the interior of the earth might possibly be due to the heat set free from radium. Indeed, the argument is not confined to the earth; it may apply also to the sun, and much of the heat we derive from that luminary may be provided by bursting atoms of radium. This was pointed out by Sir George Darwin and Professor Joly in 1903.

It became obviously a question of the first importance to discover what proportion of the earth’s crust consists of radium, and an investigation was undertaken for this purpose by the Hon. R. J. Strutt, who finds that the rocks composing the earth’s crust contain a superabundance of radium--sufficient, if this element is uniformly distributed through the whole earth in the same proportion as it occurs at the surface, not only to make good the heat which is radiated away into space, but actually to raise the temperature of our planet, which, on this evidence, should, therefore, be growing not colder, but hotter.

This is a result as disconcerting at first sight as it is astonishing, and its effects are very wide-reaching. Of course, it completely destroys the validity of Lord Kelvin’s argument, but it also deprives the nebular hypothesis of one of its cherished lines of evidence--a loss which the force of the general argument enables us to bear with equanimity.

[Sidenote: On the Eve of great Events]

In any case, the vast body of facts bearing on the history of the earth suffices to show that its temperature cannot be rising. Mr. Strutt has, therefore, imagined that the radium is not uniformly distributed throughout the mass of the planet, and supposes that it is restricted to an external zone forty-five miles in thickness; this would suffice to maintain the earth at its existing temperature. If, however, we admit a restriction of this kind, we are in no way bound to fix the limit at forty-five miles. All we can say is that we do not know how far downwards the radium reaches--for aught we know five miles, or even less, is as likely a limit as forty-five miles. Professor Joly, indeed, maintains that the radium we meet with is not proper to the earth at all, but comes from the sun.

Radium is a short-lived element, its existence being limited to a few thousand years; but as fast as it decays it is reproduced at the expense of another element--uranium--the lifetime of which is measured by hundreds of millions of years.

The last quarter of a century has proved fertile in great discoveries--more so than any corresponding period in the past. As a result, the whole world of scientific thought has been thrown into commotion; old-established theories, and even the most fundamental notions, seem to be in a state of flux. Under the stimulus of new ideas great questions, such as the constitution of matter, the origin of species, and the birth of worlds are being re-investigated with renewed energy, and we seem to be on the eve of great events.

WILLIAM JOHNSON SOLLAS

FOUR PERIODS OF THE EARTH’S DEVELOPMENT

A Postscript to Professor Sollas’s Chapter on the Wonderful Story
of the World’s Birth, beginning on page 79

The earth was once “a fluid haze of light.” The whole solar system once formed a vast nebula, consisting of glowing gas, or a swarm of meteoroids. Our planet was slowly shaped into a globe out of this primitive nebula.

This globe was at first intensely hot, and probably liquid. A solid crust formed on the surface as heat was lost by radiation, and this crust consisted of the oldest rocks of igneous formation like the granites and gneisses. During this Archæan or Eozoic Period, the earth acquired its atmosphere and its oceans, and it is probable that the mysterious origin of life took place.

The later history of the earth since the stratified rocks began to appear, and life existed, is divided into four main periods, of which the first is known as Primary, or Palæozoic.

The First Period of the Earth

CAMBRIAN SYSTEM. The rocks formed in the Cambrian Age are mainly grits, quartzites, and conglomerates, with shales, schists, and limestones. The earth was then mostly covered by seas, and the first well-defined forms of life were of marine origin.

SILURIAN SYSTEM. The Silurian rocks are mostly sandstones, shales, and slates deposited in the seas. The first vertebrates made their appearance as fishes, whilst insects began to flutter in the air, and occasionally to alight on the emerging land.

DEVONIAN SYSTEM. This was the age of the old red sandstone. Fishes reached a high state of development, whilst the first traces appeared of land vegetation, ferns and lycopods.

CARBONIFEROUS SYSTEM. This system is exceptionally important, because its chief rock is coal, the fossilised remains of the luxuriant vegetation which grew in tropical swamps. The first terrestrial animals, true air breathers, now appeared.

PERMIAN SYSTEM. The last of the primary systems gave us the new red sandstone, distinguished from the old by lying above the coal measures. The Permian Age was apparently unfavourable to life, and is only notable for the first appearance of the land reptiles into which the amphibians developed.

The Second Period of the Earth

The Secondary Period marks the emergence of the dry land into importance greater than that of the sea.

TRIASSIC SYSTEM. The Triassic rocks chiefly consist of sandstones and hardened clays laid down in shallow sea basins. Land vegetation now first began to assume a modern type, with conifers and cycads. The seas were still richly peopled, and the land first gave a home to huge reptiles, or dinosaurs.

JURASSIC SYSTEM. This system is marked by a great variety of limestones, the product of dead sea creatures. It is essentially the age of reptiles. The ichthyosaurus disputed the seas with the plesiosaurus; the pterodactyl ruled the air; whilst on land, huge monsters like the brontosaur and diplodocus browsed on tropical vegetation. From these reptiles the birds were developing, whilst small marsupials, the oldest of the great mammalian race, skipped under the branches.

CRETACEOUS SYSTEM. This was the age of the great chalk deposits. The birds, now emerging from their reptilian ancestry, dominated its life, and the first modern plants appeared on the land.

The Third Period of the Earth

The Tertiary Period marks the true beginning of modern geological history, when the great outlines of geography were laid down, and the first representatives of modern plants and animals made their appearance.

EOCENE SYSTEM. The Eocene rocks are mainly limestones, with sandstone and hardened clays. We owe them to the sea and its organisms. Modern evergreen trees now first appeared. The mammals come to the front, with the tapir-like palæotherium and the first recognisable ancestor of the horse.

MIOCENE SYSTEM. The Miocene Age was a mountain-building period, when the great chain which runs from the Alps into Central Asia received its final uplift. Deciduous trees, like the beech and elm, now made their appearance. The giant mastodon and the formidable sabre-toothed tiger roamed the Miocene forest, and true apes--man’s first forerunners--mopped and mowed in the boughs.

PLIOCENE SYSTEM. The last of the Tertiary ages set the final stamp on the geological moulding of the earth’s crust. Its plants were transitional to the flora of modern Europe. Great herds of herbivora now appeared.

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The Book of History (Vol. 1 of 18)Chapter VI: Part 6

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