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PAGE 4 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 (INTRODUCTION TO THE CREATION OF THE UNIVERSE ACCORDING GENESIS)—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.
The Mosaic cosmogony is not unworthy of the great people among whom it
took its rise; it recognizes 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 alter much labor
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.
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 behavior of the original nebula, 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.
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.
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 today than it was yesterday, or—what is
the same thing—it was hotter yesterday than it is today, 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 aeons 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.
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 nebula; 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 nebulae 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.
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 emphasized by Professors Chamberlin and Moulton, is
to be found m 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 favorable 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 out 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.
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 nebulae. 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 nebulae,
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.
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.
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 star, has been invoked
by Moulton in another version of the nebular hypothesis. In conjunction with
Chamberlin, he calls special attention to the spiral nebulas, 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. Chamberlain 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, bungs 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.
It is well known that, owing to the gravitational 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.
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 a time till
the distance between them was only two and a halt 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.
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 consequent
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 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 recognize 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.
A line drawn from somewhere in Central Africa to its antipodes in the
Pacific, through the center 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. 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 lax 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 specialized activities were assisting to the same end.
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.
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 and 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 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 characterize 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.
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 Archaean—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 molded 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
Archaean 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 Archaean 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 Archaean 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.
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 Archaean 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 cannon-ball. 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 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.
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
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