THE EARTH
A PLANET AT CLOSE RANGE
_Diameter--7927 miles_
VOLUMES could not cover the extremely complex history of the earth, nor
picture with justice the exquisite construction of even its smallest
features. It is here that the astronomer puts away his glass and turns
detective with the geologist on his own home ground.
Astronomers, basing their computations of the earth's age upon the rate
at which the sun is shrinking or contracting, have placed it at
25,000,000 years, but geologists claim that this is too short a time to
account for the enormous thickness of stratified deposits, the salinity
of the ocean, and other similar evidence which seems to prove that it is
probably 100,000,000 years old, or even older. Later estimates based
upon other modes of production of solar heat may extend this estimate
several hundred fold.
The moon is believed by some to have once been a part of the earth and
that it was formed through a division of the earth's ball when our world
was in an early stage of evolution. Becoming two separated bodies, they
have remained bound together by mutual attraction, and thus move around
the sun.
According to the Nebular Hypothesis expounded by La Place, the earth and
moon once shone like stars but, through the long ages of time they have
gradually cooled, and, their lights extinguished, two cold-surfaced
globes now wend their way around the sun.
[Figure 155: THE EARTH, SEEN FROM A DISTANCE.
(Used by permission of the publishers, Rand McNally & Company,
from Dodge's "Elementary Geography.")]
The smaller of these two globes is now scarred and old but the other is
still fresh with sparkling waters, green verdure, and is teeming with
life.
According to the later and more generally accepted Planetesimal Theory
of Moulton and Chamberlain, these globes were cold in the beginning but
became hot through compression as the material of which they were
composed was drawn toward their centers by the pull of gravitation.
The four outer planets are mostly gaseous; the four inner ones have
hardened crusts. Let us scan for a moment the various surface features
of the third one in order from the sun,--the Earth, our own globe, and
consider the unique and marvelous manner in which its various features
have come to be.
In the course of millions of years, the earth, through the pull of
gravitation, became gradually more and more dense. Different elements in
its gases (which according to the later theory were squeezed out by
pressure), combined to form rock, water and atmosphere. Hydrogen united
with oxygen formed water; nitrogen, oxygen and small quantities of
carbonic acid gas formed the atmosphere; oxygen mixed with silicon made
quartz, the earth's first rock. This rock, called granite, rose to the
top of the molten liquid material and formed a thin film of crust, for
it was lighter than the metallic substance which sank to form the
earth's core.
This thin, rocky film of crust which covered the hot liquid matter was
at first so hot that the gases which had formed the water could not stay
on its surface but rose in the air as steam. After the temperature
became lower and the rocks cooled, rain poured down in sheets of water,
remained on the crust and formed a warm sea.
For a long time this film was but a slight obstruction to the raging
fiery forces confined beneath it and again and again these forces tore
the crust asunder and made the surface of the earth a scene of wild
commotion. But each time the cooling gained a slight mastery over
the heat, and by cooling the earth's film was continually shrinking thus
forming a thicker and more solid protective crust. After an heroic
struggle between these mighty forces the crust was at last strong enough
even to sink down in places to fit the inner core; the sea then ran into
these hollows and into the wrinkles formed from the shrinking, and the
first ridges or islands appeared above the surface of the water.
These ridges or islands were then attacked by the elements, eroded by
the weather and the rains, and the granite which formed them was
crumbled into quartz, feldspar and mica. The torrents of rain formed
rivers, and these, seeking a level, carried the debris down to the
shores of the ridges,--the finer sediment, the feldspar and mica, being
washed far out from the rocky land where it settled to the bottom of the
sea and in time formed which under pressure turned to clay, while the
clay under greater pressure, turned to slate. The quartz of the granite,
ground to fine particles, stayed near the shore and formed sandy
beaches, which if covered under great pressure, became sandstone. Thus
were some of our various rocks formed from the original rock of granite,
and no matter where one sees sandstone or claybanks, even if on top of a
mountain, such a place was once under water, for that is the only place
where such sedimentary material could form.
"The sound of streams that swift and slow,
Draw down Æonian hills, and sow
The dust of continents to be."
This slow disintegration of the earth's first land continued for
millions of years, the waste accumulating along the shores, and as far
out as the finer sediment could be carried. Finally the pressure on the
edges of this land caused the thin crust to soften and break, the molten
magma poured forth in great masses, and the thick sediments were smashed
together, folded up and thrust upward to form the earth's _first hills_.
[Figure 156: A PORTION OF THE COLUMBIAN LAVA FLOW IN THE STATE
OF WASHINGTON.
Photograph by Asahel Curtis, Seattle.]
The crust was then so thin and easily broken that only low hills
appeared but after it became thicker it was not so easily rent and
_mountain ranges_ were born with an accompanying roar of tremendous
volcanoes. Geologists, studying the ages of the different ranges in the
United States, have concluded that the first to appear were the
Appalachian mountains; later the Rocky mountains in the west, and still
later the Coast ranges. Mountain ranges are invariably found along lines
of very thick sediments. The same strata which along the Appalachian
range are 40,000 feet thick, thin out, when traced westward, to only
4000 feet at the Mississippi River.[1] The high tablelands in portions
of Idaho, Oregon and Washington were formed by great streams of lava
which poured through many of the fissures of the coast ranges and buried
the whole country above the hill tops. This lava cooled into thick
massive layers of solid rock--an interesting sight to this very day.
Although mountain ranges appear amid violence and their growth, in a
geologic sense, is rapid, _plains_ and _plateaus_ are illustrations of
sea-bottoms which have been covered with sediment carried down from the
mountains,--mainly pebbles, sand and clay,--pressed and cemented
together under the water and then gently raised to the surface.
Sometimes the sea-bottom is exposed as a vast plain; again the strata
are upheaved, bent or broken, forming faults and ridges. Many of the
elevated plains and plateaus are later sculptured into hills by rains,
winds, frosts and rivers. The hills are then gently torn down, the
valleys filled up and the sediment again deposited by the rivers upon
the bed of the ocean.
"For every valley shall be exalted and every mountain shall be laid low."
A few million years passed by and these same deposits may again be
raised and become dry land. _The great stretches of land between
the mountains of the eastern and western portion of America_ were formed
from the sediment washed down from these mountains and deposited in the
sea which lay between.
The crust of the earth is still rising and sinking, the land gaining on
the water, but as this is usually such a slow movement, it can only be
traced through a period of many years.
"The notion that the ground is naturally steadfast is an error. The idea
of terra firma belongs with the ancient belief that the earth was the
center of the universe. It is, indeed, by their mobility that the
continents survive the increasing assaults of the ocean waves, and the
continuous down-wearing which the rivers and glaciers bring about."
--_Professor Shaler._
Nearly the whole of Sweden and Norway, for instance, has been rising for
thousands of years. This is proved by the fact that old sea-beaches full
of shells of species now living in neighboring seas, are found from
fifty to seventy miles inland, and several hundred feet above the
present sea-level on both sides of the Scandinavian peninsula.[2] Such
sea-beaches are found high above the present water level in Chile and
Patagonia, and the chalk cliffs of England were formed under the waves
of the ocean. Le Conte says that one of the most evident proofs of
crustal movements in ancient geologic times is the great thickness of
shallow water sediments (sandstones, shales and limestones) over the
greater portions of all of the continents, which represent areas of slow
subsidence; and the great breaks or unconformities between the series
of strata, which represent areas of uplift and atmospheric erosion.
Although sand and gravel are distributed over the bottom of the ocean
near the land, dredging expeditions reaching down their metal cylinders
for samples of the sea-floor at greater depths in various sections of
the globe, have discovered that it is of a different composition,
one-third of its area being covered with red clay. _Red clay_ is
composed of iron and silica and represents all that is left of the
minute shells which cover the tiny sea-animals which flourish in such
countless multitudes throughout the ocean. These shells, after the
animal dies, drift downward in ceaseless showers and the lime, of which
they are largely composed, is dissolved during the long journey to the
bottom. "_Ooze_" is found in water less deep and is formed by the
accumulation of minute shells of different species. Minute shells of the
same species have formed great thicknesses of limestone or of chalk even
as grains of sand build in time their banks of sandstone. Such
information as this is, of course, disclosed by the penetrating eye of
the microscope. Chalk cliffs, now dry land, form thousands of miles of
habitable land in Europe, while coral sand pressed into rock forms the
unique foundation of the Bahama Islands, the Bermudas, many islands of
the Pacific, and the Great Barrier Reef extending for a thousand miles
along the shores of northern Australia!
Looking again about our own country on the land that lies cradled
between the eastern and the western mountains of the United States, one
cannot help but regard with awe the magnificent stretches of time it
must have taken for this great area to have been formed beneath the sea.
This alone proves the great age of the earth. Because it was raised
slowly, the strata lie there little disturbed, one over the other, "like
the leaves of a great stone book," containing the history of the earth.
A good view of the river strata may be obtained in the canyon of the
Colorado river, for this mighty river has carved a bed through the
stratified deposits to a depth of from 3000 to 6000 feet.
"To quarry the heart of the earth
Till, in the rocks red rise,
Its age and birth, through an awful girth
Of strata, should show the wonder-worth
Of patience to all eyes."
The elevated sea-bottom is a fertile field for the fossil hunter,
affording evidence that enables the geologist to reconstruct the
evolutionary development of life on our planet in its divinely ordained
stages. There is often but little resemblance in the fossils found in
the lower strata to the plants and animals now in existence, but traced
from stratum to stratum, the connection is obvious. The "fire-formed"
rocks of the Archæan Age, which support all the other strata, contain
no evidence of life; neither do the first sedimentary rocks which were
deposited above them. The first fossils are found in the strata
overlying the first sedimentary rock.
Geologists have divided the history of the earth into seven ages:
1. _The Archæan Age_, represented by the Archæan system of
rocks.
2. _The Age of Invertebrates_, represented by the Cambrian,
the Lower Silurian and the Upper Silurian rocks.
3. _The Age of Fishes_, represented by the Devonian rocks.
4. _The Age of Amphibians_, represented by the Carboniferous
rocks.
5. _The Age of Reptiles_, represented by the Mesozoic rocks.
6. _The Age of Mammals_, represented by the Cenozoic rocks.
7. _The Age of Man_, represented by recent rocks.
All of these Ages, which represent the life history of the world, left
their impression on the rocks of their times, and each Age is connected
with the preceding and the succeeding Age like links in a long, long
chain.
In regard to the distinctness and importance of the last great era, the
Age of Man, Le Conte, in "Elements of Geology," says that there are two
views which will ever divide geologists, depending on the two views
regarding the relation of man to Nature:
"From a purely structural and animal point of view, man is very closely
united with the animal kingdom. He has no department of his own, but
belongs to the vertebrate department, along with the quadrupeds, birds,
reptiles and fishes. He has no class of his own, but belongs to
the class Mammalia, along with the quadrupeds. Neither has he an order
of his own, but belongs to the order of the Primates, along with the
monkeys, lemurs, etc. Even a family of his own, the Hominidae, is
grudgingly admitted by some. But from a psychical point of view it is
simply impossible to overestimate the space which separates man from all
lower things. Man must be set off not only against the whole animal
kingdom, but against the whole book of Nature besides, as an equivalent:
Nature the _Book_--the revelation--and man the _interpreter_. So in the
history of the earth: from one point of view the era of man is not
equivalent to an era, nor to an age, nor to a period, nor even to an
epoch. But from another point of view it is the equivalent of the whole
geological history of the earth besides. For the history of the earth
_finds its consummation, and its interpreter, and its significance, in
man_."
Ah--how exhilarating the thought! Once again with the astronomer, we
gaze out on the huge planets of the solar system,--the groups and great
galaxies of stars. Does the earth really seem so insignificant in its
smallness?
The beautiful little earth-ball, with its precious freight of human
beings, softly flies about the sun. Amply protected in a transparent
case of atmosphere, it completes its tremendous journey of 576,000,000
miles once every year. During this year its temperature is varied by
four seasons which are caused by the earth being inclined 23½ degrees
on its axis.
"Some say, he bid his angels turn askance
The poles of earth twice ten degrees or more
From the sun's axle; they with labor push'd
Oblique the centric globe."
--_Milton's Paradise Lost._
During this journey around the sun, the earth rotates 366¼ times on its
axis, making one revolution every 23 hours, 56 minutes and 4.09 seconds.
This causes each side to be alternately warmed and lighted every few
hours. Cupped close to the darkened hemisphere lies a huge cone of
shadow which extends out in space past the moon. When the moon
rolls through this shadow it is eclipsed for it has come into our night.
[Figure 157]
This cone of the earth's shadow is our personal night and ours alone,
and, although the stars seem enmeshed in its velvet sides, it is not the
night of earth that holds the stars,--its darkness merely shuts out the
glare of the sun and the stars of the universe therefore become visible.
This remarkable view gave to man a science set in beauty so keen that in
the world's young days it was thought that it was music too sweet to be
heard by any but the gods. But now we know that all Nature is like that,
and there is not a cliff, rock, shell, mountain, valley, water, land or
any living thing in the botanical or zoological kingdom, no matter how
common or how lowly, which does not carry, even as the stars, its quota
of science and song.
[Footnote 1: Le Conte.]
[Footnote 2: De Greer, Bulletin of the Geological Society of America,
vol. III, 1892.]
[Figure 158: THE SOUTHERN PORTION OF THE MOON AT LAST QUARTER.
Photograph by Mount Wilson Observatory through the 100-inch
Hooker telescope.]