<h2 class="nobreak" id="CHAPTER_IX">CHAPTER IX</h2>
<h3>THE CHILDHOOD OF THE EARTH</h3></div>
<p>Let us now sum up the various stages in the early
growth of the earth, as most geologists believe
them to have occurred.</p>
<p>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.</p>
<p>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.</p>
<p>Then followed an epoch when great volcanic action
set in. We have partially described it already. Just
<span class="pagenum" id="Page_99">-99-</span>
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
<span class="pagenum" id="Page_100">-100-</span>
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.</p>
<p>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, <span class="allsmcap">F.R.S.</span>, 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
<span class="pagenum" id="Page_101">-101-</span>
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.</p>
<p>Then, life still being absent from the earth, the
<i>Oceanic Era</i> 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
<span class="pagenum" id="Page_102">-102-</span>
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.</p>
<p>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.<SPAN name="FNanchor_8" href="#Footnote_8" class="fnanchor">[8]</SPAN> As
soon, therefore, as oceans appeared on the earth the
<span class="pagenum" id="Page_103">-103-</span>
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.</p>
<div class="footnote">
<p><SPAN name="Footnote_8" href="#FNanchor_8" class="label">[8]</SPAN> 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.</p>
</div>
<p>After the lifeless era began the age when the lowest
forms of life came into existence. The initial stage was
perhaps the <i>Era of the First Plants</i>, 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
<span class="pagenum" id="Page_104">-104-</span>
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.</p>
<p>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.</p>
<div class="figcenter"> <ANTIMG class="w100" src="images/fpage104.png" width-obs="419" height-obs="648" alt="" /> <div class="txtlf"><i>Stereo Copyright, Underwood & U.</i></div>
<div class="txtrt"><i>London and New York</i></div>
<div class="figcaption" style="clear: both; padding-top:1em;">
<p class="tdc"><span class="smcap">The Crater of an Extinct Volcano</span></p>
<p>This is the entrance to the long extinct volcano of Red Mountain, Arizona.</p>
</div>
</div>
<p><span class="pagenum" id="Page_105">-105-</span></p>
<p>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."</p>
<p>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.</p>
<p>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
<span class="pagenum" id="Page_106">-106-</span>
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.</p>
<p>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.</p>
<p>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
<span class="pagenum" id="Page_107">-107-</span>
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.</p>
<hr class="chap x-ebookmaker-drop" />
<p><span class="pagenum" id="Page_108">-108-</span></p>
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