Head Professor of Geology, University of Chicago.
Just how the earth was formed at the outset is not certainly known. The
most common view of men of science is that it was once in the form of a
fiery gas. It is supposed that all the planets and satellites that now
revolve around the sun were once a part of a common mass of gas in the
form of a vast sphere which was very large and very hot. This gradually
lost its heat and shrank as most bodies do when they cool. If it was
not already whirling round at the outset it must have come to do so as
it shrank, and as more and more of its heat was lost it rotated more
and more rapidly. At length it came to whirl so fast that the outer
part, which was moving fastest, could no longer be held down to the
surface, and so it separated in the form of a ring around the equator
of the great sphere.
The main mass kept on cooling and shrinking and whirling faster and
faster and hence other rings separated. Each of these rings also kept
on cooling and shrinking and is supposed to have parted at some point
and gradually gathered together into a globe, but still in the form of
fiery gas, even though it had lost much of its heat. But at last this
globe of gas cooled so much that the main part of it became liquid.
This was that part which afterwards became the solid part of the earth.
It then had the form of lava. It was still too hot for the water to
condense and hence it remained in the form of steam or vapor, forming
a vast envelope all about the earth. There are supposed to have been
many other vapors in the air at that stage, and it must have been very
dense. But at length the globe of lava cooled so that the outer part
crusted over, and this crust grew thicker and thicker as time went on.
After a while it became cool enough to permit the water to condense on
the surface and so the ocean began to be formed. The water grew in
depth until nearly all the steam was condensed and many of the other
vapors that had been in the air while it was so hot were condensed
also. And this left the gases which cannot easily be condensed behind,
and they formed the air much as it is to-day. And that is the way the
atmosphere is commonly supposed to have come about.
But all this is theory. It cannot now be proved. But there are several
great facts that fit in with it and make it seem as though it might be
true. As wells and mines are sunk deep in the ground it is found that
the earth grows warmer and warmer. Volcanoes pour out molten rock and
this shows that it is very hot somewhere beneath them. Many of the
mountains on the earth are really wrinkles in its crust, and it has
been thought that these are caused by the cooling and shrinking of the
globe. It is because these and other things fit in so well with the
theory that most scientific men have come to accept it as probably
true. It is known as the Nebular theory. But there are other ways of
explaining all these things, and perhaps it may be proven that there
are better ways.
[Illustration: TERRACED ROCKS, YELLOWSTONE PARK.
_Photograph by F. J. Haynes, St. Paul._
CHICAGO:
A. W. MUMFORD, PUBLISHER.]
Some scientists have supposed that the earth was formed by small masses
or particles of matter gathered in from the heavens. On a clear night
shooting stars may be seen quite often. These are little bits of stone
or metallic matter shooting through space at high rates of speed, which
strike the atmosphere and become hot. The earth also is moving at great
speed--nearly nineteen miles per second. It is not strange then that
when the little stranger collides with the earth it should "make the
fire fly." Usually the outside is melted and carried away so fast that
the little mass is entirely used up in a few seconds. It merely makes
a little streak of light. But sometimes the mass is large enough to
stand the waste and still reach the ground. In such cases it is found
to be mainly stony matter and iron. No substance has ever been found
in any of them which is not found in the earth. Only a few of these
shooting stars or meteorites will be seen in looking at any one point
in the heavens. But the earth is very large and there are many such
points, and when these are taken all together it is found that the
number of these little bodies which fall in a day is very large. It is
estimated at twenty millions. But still they are small and do not add
very much to the size of the earth. But as they are being constantly
swept up from space and are growing fewer and fewer, and as this has
been going on for a very long time, it is reasonable to suppose they
may once have been much more abundant and that the earth then grew
much faster by reason of them. It is thought by some that the earth
may have grown up entirely by gathering them in, the idea being that
it was itself once only a little meteorite that succeeded in gathering
the others in. It is commonly supposed, however, by those who hold to
this view, that the earth was formed from some special cluster of these
meteorites that gathered together. It has been thought that perhaps
the gas of the rings mentioned before may have cooled down into little
solid particles before they were collected together and that they built
up the earth. This brings the two theories together in a measure. The
planet Saturn, you know, has rings of this kind and they are made up of
small solid bodies, and not of gas or liquid, as was once supposed.
If the earth was built up this way we must account for the heat in the
interior, but this would come naturally enough. As the little bodies
fell upon the surface they would strike hot. But unless they came fast
they would cool off before others struck the same spot and the earth
would not get very hot. But as they gradually built up the surface the
matter below would be pressed together harder and harder because of the
growing weight upon it, and this pressing together would make it hot.
It is figured out that it would become very, very hot indeed, though
this might not seem so at first thought, and that the volcanoes and
mountains may all be explained in this way quite as well, and perhaps
better, than in the other way. This is called the Accretion theory.
It may be that neither of these theories is right, and we will do well
to hold them only as possible ways in which the earth may have been
formed at the beginning. But, at any rate, the earth has been shaped
over on the surface. In a certain sense its outer part has been remade.
And this concerns us more than the question of its far-off origin,
because our soils, ores, marbles, and precious stones, as well as our
lands and seas, are all due to this reshaping. In the deepest parts of
the earth which we can get at for study, we find that it is made up of
rocks of the granite class; not always granite proper, but rocks like
it. What is below this in the great heart of the earth we do not know,
except that it is very dense and heavy. Rocks of the granite class are
formed under great heat and pressure, or by the cooling of molten rock
material. They may be called the basement rock or great floor, on which
all the other rocks near the surface are laid. They underlie all the
surface, but at different depths. In some places they have been crowded
up by the pressure that came from the shrinking of the earth, of which
we spoke before, and so have come to be actually at the surface, except
that soil, clay, sand, or gravel may cover them. Under about one-fifth
of the land these rocks lie just below the clays, gravels, sands, and
soils that occupy the immediate surface. Sometimes they come out to the
actual surface, and may be seen in ledges or bluffs. But usually the
soils, sands, gravels, and clays cover them up more or less deeply, but
even then they are often struck in sinking wells.
Under the other four-fifths of the land they lie much deeper, often
several thousands of feet, and there are spread over them sandstones,
shales, and limestones. These are the rocks we usually see in the
quarries and cliffs of the interior states. The materials to form these
were taken from the older rocks of the granite class by a process
which is now going on--so we know how it is done. This is the way
in which it takes place: The air and the rains and the water in the
ground act upon the rocks, and cause them to soften and fall to pieces,
forming soils, or sand, or little rock fragments. This material is
gradually washed away by rains and floods. This does not usually quite
keep pace with the softening; so the surface is covered with soil and
other loose material. But it is little by little washed away, and
carried down to sea, where it settles on the bottom, and forms layers
of mud or of sand. The mud afterwards hardens, and becomes a kind of
rock known as shale. The sands become cemented by lime or iron, or
some other substance, and form a sandstone. The lime in the rocks that
softened and decayed is chiefly dissolved out by the carbonic acid in
the waters of the ground, and is carried away to the sea in solution.
This lime is then taken up by sea animals to form their shells,
skeletons, teeth, and other hard parts. Afterwards the animals die, and
these hard, limy parts usually crumble more or less and form a bed of
lime material, and later this hardens into limestone.
Some of the lime is also separated from the waters by evaporation or
by other changes. You have noticed that on the inside of a tea kettle
there gathers a stony crust. This is made of the same material as
limestone--indeed, it is limestone. It was dissolved in the water put
in the tea kettle, but as the water was heated and partly changed into
steam it could no longer hold all the lime, and some or all of it had
to be deposited. So, in a similar way, sea-water is dried up by the sun
and air, and deposits lime, and so beds of limestone are formed. You
will readily see from what has been said why shales, sandstones, and
limestones take the form of beds lying upon each other.
Now, away back towards the beginning, when the ocean was first formed,
and some part of the earth was pushed up so as to form land, this
process began, and has been at work ever since. The surface of the land
has been moistened by the air and moisture, and then has been washed
away to the ocean and laid down in beds. When these grew thick, and
were pressed by the weight of the newer beds that were laid down on
them, they hardened into rock again. And this has gone on for a very,
very long time, and the beds of sandstone, shale, and limestone so
formed have come to be many thousand feet thick in some places. The
land would all have been worn away down to the level of the sea if the
earth had not kept shrinking and wrinkling, or pushing up in places.
At different times portions of what was once the ocean bottom have
been lifted and have become land. If these beds are examined they will
be found to contain shells and corals and other sea animals which
were buried in them when they were forming, and thus it is known that
they were laid down under the sea. It is found also that the lower
beds contain kinds of life different from those above, and the lower
beds were, of course, formed first. So, by studying the sea-shells
and other relics in the beds, from the lowest ones up to the highest
ones in the order in which they were formed, the various kinds of life
that have lived in the sea from the beginning are found out. The life
at the beginning was simpler than it is now, and quite different in
many respects. There were gradual changes from time to time, and many
strange creatures appeared that do not live at present.