THE CHILDHOOD OF THE EARTH
Let us now sum up the various stages in the early growth of the
earth, as most geologists believe them to have occurred.
The first stage was that when the earth, shining like a star, existed
as a fluid globe surrounded by heavy vapours of great thickness,
which contained the future waters of the globe.
Then began the second era of the earth's existence, when it was a hot
solid globe--solid at any rate at the surface, and with a temperature
of about 2500° F. The globe's atmosphere still contained all the
waters of the earth. It contained all the carbonic acid gas which now
exists in the limestones and in coal and other minerals containing
carbon. It contained also all the oxygen since shut up in the rocks
and in vegetation and in animal substances. Such an atmosphere was
probably at least two hundred times as great as the atmosphere which
now surrounds the earth.
Then followed an epoch when great volcanic action set in. We have
partially described it already. Just a thin solid crust was all that
covered the molten interior of the globe. It was too thin always
to contain the boiling liquid, and Titanic explosions, followed by
enormous overflows of lava, continually broke up the crust. The
pressure was relieved by these explosions, and gradually the earth
would settle down again to its process of consolidation. Another
explosion would follow; again a great flow of lava; and again the
effects of the catastrophe would subside. After each explosion and
outflow the earth's crust would grow a little thicker. All this time,
and for long succeeding ages, the earth was attracting to itself,
as a magnet attracts iron filings, all the small bodies which it
encountered on its path round the Sun. These little rocks or masses
of matter, some great and some small, would each add something to the
size of the earth, and, by the shock of collision with it, something
to the earth's heat--just as a bullet flattening itself against a
target melts in the heat of collision. Just also as the bits of
matter which we call "shooting-stars" are set on fire by friction as
they rush into the earth's atmosphere. These meteorites, as they are
called when they are comparatively small bodies, or "planetismals,"
as they are called when they are large, still exist. But the earth,
in the millions on millions of years which it has been coursing round
the Sun, has swept up all the large ones that are likely to fall into
it, and there remain only the small ones which occasionally cross its
path. These are called "Leonids" or "Lyrids" or "Perseids," and these
meteor showers occur at nearly the same time every year when the
earth runs through a swarm of them on its pathway round the Sun. But
they are very small. Some of them are no bigger than a slate pencil.
Few are as big as brickbats, and nearly all are burnt up by the
air-friction before they reach the earth's surface. Larger ones still
fall on the earth, however, and it is calculated that many hundreds
reach us in fragments every year. But when the earth was young many
thousands fell every day.
To this era, or immediately before it, belongs the birth of the Moon.
It is a subject of interest to geologists, because it is admitted
that the materials of which the Moon is constituted are similar
to those of the earth; and it is believed that its history up to
a certain point was very like that of the earth. It had its great
volcanic era such as we have described; but its development closed
shortly afterwards. We are considering, however, at this moment
its origin. It was once part of the earth. All of us have read of
those little animals, of which one form is the amœba and another
form the white corpuscles of the blood. If we watch them under the
microscope we may see one of them slowly lengthen out, then break in
two, and each part go swimming away by itself, a perfect animal. It
was Sir G. H. Darwin, F.R.S., who proved mathematically in 1879 that
the origin of the Moon was such that we may properly compare it to
the splitting up of the little animals just described. The date of
this event cannot be fixed even approximately--beyond saying that
as astronomical events go it must have been rather recent, though
not less than fifty million years ago. The Moon is therefore one of
the younger members of the Solar System. But no other catastrophe,
either before or since, has occurred on the earth to compare with
its prodigious birth. Five thousand million cubic miles of material
left the earth's surface never again to return to it. Whether it all
left at once or whether the action was prolonged we do not know, but
we may try in vain to imagine the awful uproar and fearful volcanic
phenomena exhibited when a planet was cleft in twain and a new moon
was born into the Solar System.
Then, life still being absent from the earth, the _Oceanic Era_
began. The waters condensed into an ocean over the earth, or else
collected in some great oceanic depression. Lands presently emerged
from it. It was a hot ocean, steaming no doubt, for its temperature
was perhaps about 500° F. Some one may ask--Why, then, did it not
steam away into clouds? The answer is that the atmosphere was still
very heavy in that past era, probably still exerting a pressure
as much as fifty times as great as to-day. The pressure of the
atmosphere at the earth's surface to-day is usually about fifteen
pounds to the square inch. In such circumstances water boils rapidly
away at the temperature of 212° F. But if the water be taken up to
the top of Mont Blanc, where the air pressure is less than that at
the sea-level (or if, which amounts to the same thing, we reduce the
pressure on the water's surface by placing it under the receiver
of an air-pump and partially exhaust the air), it will boil at a
lower temperature than this. If, on the other hand, we increase
the pressure on the surface of the water by any means, such, for
example, as by placing it in a chamber of compressed air, the water
can be heated to a higher temperature without boiling away. In the
bygone era of which we are speaking the pressure of the atmosphere
on the water's surface was 700 lb. or 800 lb. to the square inch;
and therefore it could be heated up to a high temperature without
evaporating rapidly.
Another thing began to happen in those days. All bodies in space
attract one another; the Sun attracts its planets; the planets
attract the Sun and their satellites; and the satellites in their
turn attract the planets. Ages before the earth had a moon these
forces were at work. But the attractions of solid bodies for one
another do not bring about any very perceptible alterations in their
shapes; though if the bodies are spinning they effect slow changes in
their speed of rotation. It is different when the bodies are liquid,
or if they have liquid surfaces. Then the attractions of a sun or
a moon on a planet begin to draw up the waters of the planet and
produce tides. The attraction of the earth would produce tides on
the Moon if an ocean existed there; and, it is suspected, do produce
something resembling tides on the present surface of the Sun.[8] As
soon, therefore, as oceans appeared on the earth the waters began
to ebb and flow in tides. (Another consequence of this constant ebb
and flow was that the friction of these movements began to diminish
the speed of the earth's rotation--just as a string that was placed
round the circumference of a spinning-top would, if constantly
pulled backwards and forwards, gradually help to slow down the top.)
Then oceanic waves and currents would begin to eat a way into the
land that was on their borders, or which was emerging from their
depths. Rivers would begin to arise, and they would carry on the
work of erosion. Other causes tending to break up the rocks would
be the gases in the air--the excessive quantities of carbonic acid
and oxygen would be active chemical agents in this work. Before the
close of this era the limestones and iron carbonates began to form;
sediments arose in the lifeless oceans, and thus began the first
formation of those sedimentary rocks and strata which have been dealt
with in the earlier chapters.
[Footnote 8: A paper read by Mr. E. W. Maunder before the Royal
Astronomical Society in 1907 gave reasons for believing that the
earth has perceptible effects on the movements of sun-spots.]
After the lifeless era began the age when the lowest forms of life
came into existence. The initial stage was perhaps the _Era of the
First Plants_, Algæ, and later still aquatic fungi or bacteria.
This began when the general temperature of the ocean may have been
as high as 150° F. (some water plants can now live in waters up to
and above 180° F.). Limestones began to form from the secretions of
plants, and deposits of silica from silica secretions. Also where
the conditions were favourable there were large sedimentary deposits
and accumulations. In the second part of this æon the earth, still
continuing to cool, and going down in temperature to 115°, gave
opportunity to animal life. At the end of this era the general
temperature of the earth and its oceans was as low as 90° F. The
first animal life had begun to appear; its activity greatly increased
under what were favourable conditions for it. This increase of animal
life had its effect on the earth's crust. We have already spoken of
the formation of limestones from the bodies of sea animals. This was
going on in those ages millions of years ago before any of the higher
forms of life had appeared on the earth, and though it was not going
on so rapidly, still it must be remembered that at some point of
the world's history the oceans were of greater extent than now, and
consequently the deposits of lime and the accumulations of sediment
were more widespread. The sedimentary rocks grew faster and faster,
especially on the floors of the oceans.
It will be understood by those who have read the foregoing two
chapters closely that the "igneous" or fire-born rocks must lie
underneath the sedimentary ones. But that is only true in general
terms, for a double reason. In the first place, owing to the
inestimable forces which for millions of years were still continually
effective below the earth's crust, the igneous rocks over and over
again were able to burst their way through the slow-forming sediment
of other rocks laid down above them. In the second place, the igneous
rocks, owing to their composition and superior hardness, were much
less worn by wind and weather than the less compact "sedimentary
rocks," and these remained, showing themselves at the surface as
coast-lines of oceans and in mountain ranges, after the sandstones
and shales and limestones had disappeared.
[Illustration:
_Stereo Copyright, Underwood & U._ _London and New York_
The Crater of an Extinct Volcano
This is the entrance to the long extinct volcano of Red Mountain,
Arizona.]
The memorials of volcanic action remain, we had almost said
permanently, among the decay of other rocks, though, of course, even
hard volcanic or igneous rocks will be worn down in time. In many
cases volcanoes themselves are left, though they may have been for
ages extinct. In some volcanic regions where no great central cones
have existed the vast floods of lava that were poured forth extend
to-day as vast black plains of naked rock, mottled with shifting
sand-hills, or as undulating tablelands carved by running water
into valleys and ravines, between which the successive sheets of
lava are exposed in terraced hills. Beyond the limits over which
the lava sheets have spread there are often great veins or parapets
or sunken walls of lava to which are given the names of igneous or
volcanic "dykes." Dykes vary from less than a foot to one hundred
feet in breadth, and often run in nearly straight courses sometimes
for many miles. They consist usually of very hard rock like basalt,
"andesite," or "diabase."
They were fissures in the earth's surface, and, after the manner we
have described, the molten rock welled forth through these fissures,
and spread out sheet after sheet, till like a rising lake it has not
only overflowed the lower grounds but even buried all the minor hills.
Lava eruptions of this kind have taken place in recent years in
Iceland. On a small scale they can be seen to take place in the
island of Hawaii, where the outflows of lava reproduce for us,
like models in miniature, the great outbreaks of the past. On the
largest possible scale similar effects may be seen on the great lava
plains of the Moon, where the giant craters that we can see through
telescopes are not the mouths of extinct volcanoes, but the banked-up
edges and shores of lava outflows.
On a much smaller scale than this, but still on a gigantic scale,
the same thing took place in Western North America, where there are
vast tracts of land which are best to be explained by supposing that
there were once great outbreaks and overflows. The area which has
there been flooded with lava has been estimated to be larger than
that of France and Great Britain put together, and the depth of the
total mass erupted reaches in some places as much as 3700 feet.
Some rivers have cut gorges in this plain of lava, laying bare its
component rocks to a depth of 700 feet or more. Along the walls of
these ravines we see that the lava is arranged in parallel beds or
sheets often not more than ten or twenty feet thick, each of which,
of course, represents a separate outpouring of molten rock.
These are comparatively modern lava plains, although, of course, the
outpourings in North America occurred ages before historic time,
or, indeed, before there are any traces of man's existence on the
earth. Such lava outflows can only occasionally be examined--as
in the instance just quoted when rivers have cut deep into them.
Consequently we have to speculate on the connection between the
dykes and fissures and the lava flood itself. But in various parts
of the world lava plains of much older date have been so eaten into
and worn by the action of the elements that not only the successive
sheets of lava are exposed but the rock floor over which they poured.
Exposed also are the abundant dykes which served as the channels
by which the lava rose to the surface. In Western Europe important
examples of this structure occur from the north of Iceland through
the inner Hebrides and the Faroe Islands to Iceland. This volcanic
belt presents a succession of lava sheets, which even yet, in spite
of enormous waste, are in some places more than 3000 feet thick.
These sheets are nearly flat and rise in terraces over one another
into green grassy hills or into the dark fronts of lofty sea-washed
precipices. Where sheets have been stripped off or worn down by wind
and weather thousands of volcanic dykes are exposed. These dykes are,
as it were, the roots of which the lava sheets were the branches; and
even where the whole of the material that gushed up to the surface
has been worn away the dykes remain as evidence of the vigour and
energy of the volcanic forces.