THE EARTH AT ITS BEGINNING
If we look up at the sky with the eye of knowledge we can read in the
celestial objects with which it is strewn something of the history
of our earth. We can only read it dimly even with the aid of the
greatest telescopes, and it is quite possible that in some respects
we may read it wrongly. Let us, however, consider what the eye and
the telescope will reveal to us. The eye will see the Sun--a great
ball, into which the earth might sink without greatly altering the
Sun's appearance, and surrounded with flaming gases hotter than the
hottest furnace man has ever been able to contrive. In that heat
every solid thing on the earth would melt and be turned into vapour.
The eye will also perceive the Moon--another ball, much smaller than
the earth, surrounded by no gases at all, having as far as can be
seen no water; and being so cold during its long nights that all
gases and liquids of which we know would be frozen solid there.
The eye can also see a myriad of stars of varying brightness, but
for the most part only thus distinguishable. If the telescope be
now called in to aid, the eye will, however, be able to discern
differences and distinctions in the stars. It will see that some
are balls like the Earth, the Sun, and the Moon. If these balls are
studied attentively we shall discover that one of them, Jupiter, is
a great deal hotter than the earth, though a great deal cooler than
the Sun; and that another of them, Mars, is a great deal colder than
the earth, but a great deal warmer than the Moon. Perhaps we might
now begin to surmise that the Sun coming first, Jupiter next, the
Earth next, Mars next, and the Moon last, were all like stages in the
history of one of these balls; and that, for example, any one ball
began by being as hot as the Sun, and ended, after passing through
stages like Jupiter, the Earth, and Mars, in being as cold and
lifeless as the Moon.
But if one had a very good telescope, and could examine those more
distant specks of light which we call stars, we perhaps could spy a
little earlier into the history of these great balls. For example,
among the blazing lights of the heavens--the stars which we know to
be suns--there are others which are not balls at all. There are the
Pleiades, for instance, of whom the Prophet Amos wrote, "Seek Him
that maketh the seven stars and Orion" (Amos v. 8). The seven stars
(in the Authorised Version rendered Pleiades),[4] when seen through a
great telescope, are caught in a mesh or a veil of something that may
be starry matter, but of the exact nature of which we are uncertain.
In other parts of the sky there are great masses of this starry
mist; and to these bright patches astronomers have given the name
of "nebulæ." The most wonderful of them all is the great nebula in
Orion;[5] and one of the most beautiful is the great spiral nebula of
Andromeda. These objects are not only wonderful and beautiful; they
also give us a hint as to what might have been the earliest state
of our earth, and of the Sun itself in those almost inconceivably
distant ages before order took the place of chaos.
[Footnote 4: It has been cogently suggested that by the "seven stars"
the biblical writer meant the constellation of the Great Bear; but
Mr. E. W. Maunder, F.R.A.S., of Greenwich Observatory, is of opinion
that the Pleiades were signified.]
[Footnote 5: "God maketh Arcturus, Orion, and Pleiades, and the
chambers of the south" (Job IX. 9).]
Let us ask the reader to imagine what would take place if the earth
were to come into collision with another planet. Some of our readers,
at any rate, will know that the Sun and all its planets--the earth
among them--is moving swiftly to some unknown destination among
the stars.[6] Suppose that some great planet, not of the Solar
System, barred our path. We should not be taken wholly unawares, for
astronomers would know of the approach of the star and our earth
to one another months, and perhaps years, beforehand. That would
be because the light of the Sun falling on it would be reflected,
just as the reflection of the Sun's rays light up the Moon for our
eyes. If the strange planet were a very large body, like the sun
for bigness, it would become visible far beyond the confines of
the Solar System. It might first be taken for a new star such as
sometimes blazes up in the sky and then sinks into darkness again.
But its steadiness would make it an object of suspicion. There would
be another brief period in which it might be taken for a comet; but
comets have a light quite different from that of reflected sunlight.
So that anxious expectation would be dissipated, and the world would
begin to recognise the monster for what it really was. If its size
were the same as that of the Sun, then it would first become visible
to us when 15,000,000,000 miles distant,[7] or let us say 1600 times
farther away from us than we are from the Sun. We and it would
approach slowly at first. It would be nearly ten years before the
distance had been reduced to 6,000,000,000 of miles, and the intruder
had begun to be visible to the naked eye. In fourteen years it would
have reached the outer edge of the Solar System, and would be the
brightest star in the heavens. In another year it would be twice as
bright as Venus at her brightest, and would be coming nearer with
appalling swiftness. In less than two months it would be as near the
Sun as we are. In a week more it would have plunged into the Sun at
the rate of 400 miles a second, and in the awful heat born in that
collision, Sun and earth and planets would be molten, and the Solar
System overwhelmed
In unremorseful folds of rolling fire.
[Footnote 6: It is usually supposed that this movement, amounting to
perhaps ten miles a second, is in the direction of the constellation
of Vega.]
[Footnote 7: See an article by Mr. Ellard Gore in _Knowledge_,
November, 1905. The object would not be visible at this distance
except through large telescopes.]
Suppose that this catastrophe were to take place. Would that be the
end of all things? No. Out of the fiery mist many millions of miles
across--like one of those great nebulæ which the telescopes reveal
to us--order would be evolved. Two things would at once begin to
happen, since in the Universe nothing stands still. The fiery mist
would be giving out heat all about it, sending out heat-waves as a
fire or a red-hot poker will do. The red-hot poker cools; so, too,
would the fiery nebula. Then the nebula would begin to condense; not
quite in the same way that a cloud of steam does, for it would be
whirling all the time, and its fiery particles would be all trying
to fall inwards, just as anything dropped from our hands tends to
fall towards the centre of the earth. As this whirling mass of gas
condensed some great masses of it would become detached, and would
begin to enjoy separate existences of their own.
Let us imagine, for the sake of argument, that a mass of gas vast
enough, when it condensed, to form the earth itself became detached
from the parent nebula. Suppose we follow its history. At first it
may have been a globe hardly distinguishable from a whirling flame.
In brightness it was like the Sun, and like the Sun, it was covered
with elemental gases. It was, in fact, in its earliest days a sphere
of gas continually giving out heat, and continually cooling, till
from a sphere like the Sun it became a ball like Jupiter. It had
an intermediate stage when its gases were condensing into liquids,
as steam condenses into water; for though the nebula as a whole
was hot it was always travelling through cold space. Gradually the
earth became partly liquid and partly gas. For millions of years it
continued to revolve as a ball of liquid--still cooling--still being
pressed very hard at its central parts by the weight of all the gases
and liquids round it, till at last the first crust of solid matter
began to form on the liquid surface. This crust continued to thicken,
but it was subject to many appalling catastrophes and breakages.
We have already used the occurrence of the tides of the earth as an
instance of the Sun's attraction. The Sun (and the Moon) attract the
waters of the earth, pulling them up towards themselves. So would
they also attract the molten materials of which the early earth was
composed. The liquid mass would be continually surging like a tide
against its wall of solid crust, and the liquid would now and again
burst through. There must have been a time when the thin solid crust
covering the molten interior became, owing to the solidification
and contraction of the crust, much too small to contain the liquid
material. The lava would then break through, and would form huge
craters, not unlike some of those which we see on the Moon. We can
faintly imagine these terrible outbreaks in which the molten tide
rose not thirty or forty feet but many miles high!
Later, after some relief had been given by these outbreaks and the
crust thickened, the interior regions of the earth by cooling shrank
away from the solid shell, which was now too large. This solid shell
being insufficiently supported sometimes caved in, and other great
outflows of lava resulted. These lava floods dissolved the original
solid shell whenever they came in contact with it. The earth probably
once had gigantic craters like those which we can see in their
extinct form on the Moon; but they were destroyed by these outflows
of lava of which we have spoken.
Then, still cooling, the earth's crust grew thicker and thicker. The
great outflows and eruptions of molten elements from underneath grew
fewer, and more liquid elements cooled into solids, and more gases
condensed into liquids. There was another thing happening of which,
as conscientious recorders of the history of the earth's geology, we
must take note; and it is that in those early days meteorites were
falling on the earth in vastly greater numbers than they do to-day.
Meteorites are masses of cooled rock flying through space which still
occasionally fall on the earth, and specimens of them are still to be
found, many of them preserved in museums, such as the Natural History
Museum in Cromwell Road, London. But the earth in its path has
swept most of them up, as the housemaid's dusting-pan collects the
fragments of dust. When the earth was young there were incomparably
more fragments to collect, and they fell on the earth like rain.
Meanwhile the cooling water vapour became the oceans; clouds and
rain and cool winds and eventually snow and ice became possible; and
the hardening lavas, or fire-born rocks, became subject to their
influences, till above them were raised the stratified rocks, of
which we have spoken in our earlier chapters, and the lineage and
descent of which is part of the study of geology. On the earth most
of the traces of its earlier history have been removed, but there
are some signs of them perceptible to the comprehending eye. The
earth is probably seamed with great cracks that do not now reach to
the surface, but which are indicated by the presence of chains of
volcanoes. The volcanoes of the great chains of the Andes lie along a
straight crack reaching from Southern Peru to Terra del Fuego, 2500
miles in length. The volcanoes of the Aleutian Islands lie along a
curved track equally long. Other shorter lines of volcanoes are very
numerous, and since countless others existed in former times, the
cracks in the earth's crust must be exceedingly numerous. There is
one crack which comes to the surface in various places in Eastern
Asia and Western Africa, and stretching from the Dead Sea to Lake
Nyassa, reaches the enormous length of 3500 miles.
From this brief sketch of the formation of the earth, its progress
and processes, and from the hints which the volcanoes and earthquakes
of to-day afford us, we may obtain some idea of the underworld that
lies far beneath our feet. Not much, however, for we are ignorant of
the actual conditions which exist towards the centre of the earth.
But there seems a strong case for supposing that there is an outer
solid crust of earth and an inner molten core of very great heat.