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[Illustration: Pike's Peak, Colorado]
EARTH and SKY EVERY
CHILD SHOULD KNOW
EASY STUDIES OF THE EARTH AND THE
STARS FOR ANY TIME AND PLACE
BY
JULIA ELLEN ROGERS
AUTHOR OF
"THE TREE BOOK," "THE SHELL BOOK," "KEY TO THE NATURE
LIBRARY," "TREES EVERY CHILD SHOULD KNOW."
ILLUSTRATED BY
THIRTY-ONE PAGES OF PHOTOGRAPHS AND DRAWINGS
[Illustration]
NEW YORK
GROSSET & DUNLAP
PUBLISHERS
COPYRIGHT, 1910, BY DOUBLEDAY, PAGE & COMPANY
PUBLISHED, OCTOBER, 1910
ALL RIGHTS RESERVED, INCLUDING THAT OF TRANSLATION
INTO FOREIGN LANGUAGES, INCLUDING THE SCANDINAVIAN
ACKNOWLEDGMENTS
A number of the photographs in this volume are used by permission of the
American Museum of Natural History. The star maps and drawings of the
constellations are by Mrs. Jerome B. Thomas. The poem by Longfellow,
quoted in part, is with the permission of the publishers, Houghton,
Mifflin & Co.
* * * * *
CONTENTS
_PART I. THE EARTH_
PAGE
THE GREAT STONE BOOK 3
THE FOSSIL FISH 6
THE CRUST OF THE EARTH 9
WHAT IS THE EARTH MADE OF? 14
THE FIRST DRY LAND 22
A STUDY OF GRANITE 27
METAMORPHIC ROCKS 31
THE AIR IN MOTION 35
THE WORK OF THE WIND 44
RAIN IN SUMMER, _by Henry W. Longfellow_ 50
WHAT BECOMES OF THE RAIN? 51
THE SOIL IN FIELDS AND GARDENS 58
THE WORK OF EARTHWORMS 63
QUIET FORCES THAT DESTROY ROCKS 68
HOW ROCKS ARE MADE 72
GETTING ACQUAINTED WITH A RIVER 78
THE WAYS OF RIVERS 84
THE STORY OF A POND 90
THE RIDDLE OF THE LOST ROCKS 93
THE QUESTION ANSWERED 96
GLACIERS AMONG THE ALPS 98
THE GREAT ICE SHEET 104
FOLLOWING SOME LOST RIVERS 110
THE MAMMOTH CAVE OF KENTUCKY 114
LAND BUILDING BY RIVERS 121
THE MAKING OF MOUNTAINS 126
THE LAVA FLOOD OF THE NORTHWEST 130
THE FIRST LIVING THINGS 134
AN ANCIENT BEACH AT EBB TIDE 138
THE LIME ROCKS 147
THE AGE OF FISHES 152
KING COAL 155
HOW COAL WAS MADE 160
THE MOST USEFUL METAL 167
THE AGE OF REPTILES 175
THE AGE OF MAMMALS 180
THE HORSE AND HIS ANCESTORS 186
THE AGE OF MAN 194
_PART II. THE SKY_
EVERY FAMILY A "STAR CLUB" 201
THE DIPPERS AND THE POLE STAR 207
CONSTELLATIONS YOU CAN ALWAYS SEE 213
WINTER CONSTELLATIONS 219
ORION, HIS DOGS, AND THE BULL 223
SEVEN FAMOUS CONSTELLATIONS 231
THE TWENTY BRIGHTEST STARS 239
HOW TO LEARN MORE 241
ILLUSTRATIONS
Pike's Peak _Frontispiece_
FACING PAGE
Sand Dunes in Arizona 44
Grand Canon of the Colorado 45
Castles Carved by Rain and Wind 52
Where All the Water Comes From 53
The Richest Gold and Silver Mines 72
Rocks Being Ground to Flour 73
A Pond Made by a Glacier 88
The Struggle Between a Stream and Its
Banks 89
Ripple Marks and Glacial Striae 102
Glacial Grooves and Markings 103
Crinoid and Ammonite 140
Fossil Corals, Coquina, Hippurite Limestone 141
Fossil Fish 152
Meteorite 153
Eocene Fish and Trilobite 156
How Coal Was Made 157
Banded Sandstone. Opalized Wood 176
Allosaurus 177
A Three-horned Dinosaur 178
Remains of Brontosaurus 179
Restoration of Brontosaurus 182
Ornitholestes, a Small Dinosaur 183
A Mammoth 186
An Ancestor of the Horse 187
Orion, His Dogs, and the Bull 214
Other Fanciful Sketches of Constellations 215
The Sky in Winter 244
The Sky in Spring 244
The Sky in Summer 244
The Sky in Autumn 244
PART I
THE EARTH
* * * * *
THE GREAT STONE BOOK
"The crust of our earth is a great cemetery where the rocks
are tombstones on which the buried dead have written their
own epitaphs. They tell us who they were, and when and where
they lived."--_Louis Agassiz._
Deep in the ground, and high and dry on the sides of mountains, belts of
limestone and sandstone and slate lie on the ancient granite ribs of the
earth. They are the deposits of sand and mud that formed the shores of
ancient seas. The limestone is formed of the decayed shells of animal
forms that flourished in shallow bays along those shores. And all we
know about the life of these early days is read in the epitaphs written
on these stone tables.
Under the stratified rocks, the granite foundations tell nothing of life
on the earth. But the sea rolled over them, and in it lived a great
variety of shellfish. Evidently the earliest fossil-bearing rocks were
worn away, for the rocks that now lie on the granite show not the
beginnings, but the high tide of life. The "lost interval" of which
geologists speak was a time when living forms were few in the sea.
In the muddy bottoms of shallow, quiet bays lie the shells and skeletons
of the creatures that live their lives in those waters and die when they
grow old and feeble. We have seen the fiddler crabs by thousands on
such shores, young and old, lusty and feeble. We have seen the rocks
along another coast almost covered by the coiled shells of little gray
periwinkles, and big clumps of black mussels hanging on the piers and
wharfs. All these creatures die, at length, and their shells accumulate
on the shallow sea bottom. Who has not spent hours gathering dead shells
which the tide has thrown up on the beach? Who has not cut his foot on
the broken shells that lie in the sandy bottom we walk on whenever we go
into the surf to swim or bathe?
Read downward from the surface toward the earth's centre--
TABLE OF CONTENTS
------+----------------------+-----------+-----------------------
Part | _Rock Systems_ | _Dominant | _Dominant Plants_
| | Animals_ |
------+----------------------+-----------+-----------------------
VII. | Recent | Man | Flowering kinds
|{ Quaternary | |
VI. | { Pliocene | Mammals | Early flowering
|{ Tertiary { Miocene | |
| { Eocene | |
V. | Mesozoic | Reptiles | Cycads
IV. | Carboniferous | Amphibians| Ferns and Conifers
III. | Devonian | Fishes | Ferns
II. | Silurian | Molluscs | Seaweeds
I. | Fire-formed | No life | No life
------+----------------------+-----------+-----------------------
It is by dying that the creatures of the sea write their epitaphs. The
mud or sand swallows them up. In time these submerged banks may be left
dry, and become beds of stone. Then some of the skeletons and shells
may be revealed in blocks of quarried stone, still perfect in form after
lying buried for thousands of years.
The leaves of this great stone book are the layers of rock, laid down
under water. Between the leaves are pressed specimens--fossils of
animals and plants that have lived on the earth.
THE FOSSIL FISH
I remember seeing a flat piece of stone on a library table, with the
skeleton of a fish distinctly raised on one surface. The friend who
owned this strange-looking specimen told me that she found it in a stone
quarry. She brought home a large piece of the slate, and a stone-mason
cut out the block with the fish in it, and her souvenir made a useful
and interesting paper-weight.
The story of that fish I heard with wonder, and have never forgotten. I
had never heard of fossil animals or plants until my good neighbour
talked about them. She showed me bits of stone with fern leaves pressed
into them. One piece of hard limestone was as full of little sea-shells
as it could possibly be. One ball of marble was a honeycombed pattern,
and called "fossil coral."
The fossil fish was once alive, swimming in the sea, and feeding on the
things it liked to eat, as all happy fishes do. Near shore a river
poured its muddy water into the sea, and the sandy bottom was covered
with the mud that settled on it. At last the fish grew old, and perhaps
a trifle stupid about catching minnows. It died, and sank to the muddy
floor of the sea. Its horny bones were not dissolved by the water. They
remained, and the mud filtered in and filled all the spaces. Soon the
fish was buried completely by the sediment the river brought.
Years, thousands of them, went by, and the layer of mud was so thick and
heavy above the skeleton of the fish that it bore a weight of tons
there, under the water. The close-packed mud became a stiff clay. After
more thousands of years, the sea no longer came so far ashore, for the
river had built up a great delta of land out of mud. The clay in which
the fish was hidden hardened into slate. Water crept down in the loose
upper layers, dissolving out salt and other minerals, and having harder
work to soak through, the lower it went. The water left some of the
minerals it had accumulated, calcium and silica and iron, in the lower
rock beds, making them harder than they were before, and heavier and
less porous.
When the river gorge was cut through these layers of rock, the colour
and thickness of each kind were laid bare. Centuries after, perhaps
thousands of years, indeed, the quarrymen cut out the layers fit for
building stones, flags for walks and slates for roofing. In the
splitting of a flagstone, the long-buried skeleton of the fish came to
light.
Under our feet the earth lies in layers. Under the soil lie loose beds
of clay and sand and gravel, and under these loose kinds of earth are
close-packed clays, sandstones, limestones, shales, often strangely
tilted away from the horizontal line, but variously fitted, one layer to
another. Under these rocks lie the foundations of the earth--the
fire-formed rocks, like granite. The depth of this original rock is
unknown. It is the substance out of which the earth is made, we think.
All the layered rocks are made of particles of the older ones, stolen by
wind and water, and finally deposited on the borders of lakes and seas.
So our rivers are doing to-day what they have always done--they are
tearing down rocks, grinding and sifting the fragments, and letting them
fall where the current of fresh water meets a great body of water that
is still, or has currents contrary to that of the river.
Do you see a little dead fish in the water? It is on the way to become a
fossil, and the mud that sifts over it, to become a layer of slate.
Every seashore buries its dead in layers of sand and mud.
THE CRUST OF THE EARTH
It is hard to believe that our solid earth was once a ball of seething
liquid, like the red-hot iron that is poured out of the big clay cups
into the sand moulds at an iron foundry. But when a mountain like
Vesuvius sets up a mighty rumbling, and finally a mass of white-hot lava
bursts from the centre and streams down the sides, covering the
vineyards and olive orchards, and driving the people out of their homes
in terror, it seems as if the earth's crust must be but a thin and frail
affair, covering a fiery interior, which might at any time break out.
The people who live near volcanoes might easily get this idea.
But they do not. They go back as soon as the lava streams are cooled,
and rebuild their homes, and plant more orchards and vineyards. "It is
_so_ many years," say they to one another, "since the last bad eruption.
Vesuvius will probably sleep now till we are dead and gone."
This is good reasoning. There are few active volcanoes left on the
earth, compared with the number that were once active, and long ago
became extinct. And the time between eruptions of the active ones grows
longer; the eruptions less violent. Terrible as were the recent
earthquakes of San Francisco and Messina, this form of disturbance of
the earth's crust is growing constantly less frequent. The earth is
growing cooler as it grows older; the crust thickens and grows stronger
as centuries pass. We have been studying the earth only a few hundred
years. The crust has been cooling for millions of years, and
mountain-making was the result of the shrinking of the crust. That
formed folds and clefts, and let masses of the heated substance pour out
on the surface.
My first geography lesson I shall never forget. The new teacher had very
bright eyes and _such_ pretty hands! She held up a red apple, and told
us that the earth's substance was melted and burning, inside its crust,
which was about as thick, in proportion to the size of the globe, as the
skin of the apple. I was filled with wonder and fear. What if we
children jumped the rope so hard as to break through the fragile shell,
and drop out of sight in a sea of fiery metal, like melted iron? Some of
the boys didn't believe it, but they were impressed, nevertheless.
The theory of the heated interior of the earth is still believed, but
the idea that flames and bubbling metals are enclosed in the outer layer
of solid matter has generally been abandoned. The power that draws all
of its particles toward the earth's centre is stated by the laws of
gravitation. The amount of "pull" is the measure of the weight of any
substance. Lift a stone, and then a feather pillow, much larger than
the stone. One is strongly drawn to the earth; the other not. One is
_heavy_, we say, the other _light_.
If a stone you can pick up is heavy, how much heavier is a great boulder
that it takes a four-horse team to haul. What tremendous weight there is
in all the boulders scattered on a hillside! The hill itself could not
be made level without digging away thousands of tons of earth. The
earth's outer crust, with its miles in depth of mountains and level
ground, is a crushing weight lying on the heated under-substance. Every
foot of depth adds greatly to the pressure exerted upon the mass, for
the attraction of gravitation increases amazingly as the centre of the
earth is approached.
It is now believed that the earth is solid to its centre, though heated
to a high degree. Terrific pressure, which causes this heat, is exerted
by the weight of the crust. A crack in the crust may relieve this
pressure at some point, and a mass of substance may be forced out and
burst into a flaming stream of lava. Such an eruption is familiar in
volcanic regions. The fact that red-hot lava streams from the crater of
Vesuvius is no proof that it was seething and bubbling while far below
the surface.
Volcanoes, geysers, and hot springs prove that the earth's interior is
hot. The crust is frozen the year around in the polar regions, and never
between the Tropics of Cancer and Capricorn. The sun's rays produce our
different climates, but they affect only the surface. Underground, there
is a rise of a degree of temperature for every fifty feet one goes
down. The lowest mine shaft is about a mile deep. That is only one
four-thousandth of the distance to the earth's centre.
By an easy computation we could locate the known melting-point for
metals and other rock materials. But one degree for each fifty feet of
depth below the surface may not be correct for the second mile, as it is
for the first. Again, the melting-point is probably a great deal higher
for substances under great pressure. The weight of the crust is a burden
the under-rocks bear. Probably the pressure on every square inch reaches
thousands of tons. Could any substance become liquid with such a weight
upon it, whatever heat it attained? Nobody can answer this question.
The theory that volcanoes are chimneys connecting lakes of burning lava
with the surface of the earth is discredited by geologists. The weight
of the overlying crust would, they think, close such chambers, and
reduce liquids to a solid condition.
Since the first land rose above the sea, the crust of the earth has
gradually become more stable, but even now there is scarcely a day when
the instruments called seismographs do not record earthquake shocks in
some part of the earth; and the outbreaks of Vesuvius and AEtna, the
constant boiling of lava in the craters of the Hawaiian Islands and
other volcanic centres, prove that even now the earth's crust is very
thin and unstable. The further back in time we go, the thinner was the
crust, the more frequent the outbursts of volcanic activity, the more
readily did wrinkles form.
The shores of New Jersey and of Greenland are gradually sinking, and the
sea coming up over the land. Certain parts of the world are gradually
rising out of the sea. In earlier times the rising or the sinking of
land over large areas happened much more frequently than now.
WHAT IS THE EARTH MADE OF?
"Baking day" is a great institution in the comfortable farm life of the
American people. The big range oven is not allowed to grow cold until
rows of pies adorn the pantry shelves, and cakes, tarts, and generous
loaves of bread are added to the store. Cookies, perhaps, and a big pan
full of crisp, brown doughnuts often crown the day's work. No gallery of
art treasures will ever charm the grown-up boys and girls as those
pantry shelves charmed the bright-eyed, hungry children, who were
allowed to survey the treasure-house, and sample its good things while
they were still warm.
You could count a dozen different kinds of cakes and pies, rolls and
cookies on those pantry shelves, yet several of them were made out of
the same dough. Instead of a loaf of bread, mother could make two or
three kinds of coffee cake, or cinnamon rolls, or currant buns, or
Parker-House rolls. Even the pastry, which made the pies and tarts, was
not so different from the bread dough, for each was made of flour, and
contained, besides the salt, "shortening," which was butter or lard.
Sugar was used in everything, from the bread, which had a
table-spoonful, to the cookies, which were finished with a sifting of
sugar on top.
How much of the food we eat is made of a very few staple
foodstuffs,--starch, sugar, fats! So in the wonderful earth and all that
grows out of it and lives upon it. Only seventy different elements have
been discovered, counting, besides the earth, the water and the air, and
even the strange wandering bodies, called meteorites, that fall upon the
earth out of the sky. Like the flour in the different cakes and pies,
the element carbon is found in abundance and in strangely different
combinations. As a gas, in combination with oxygen, it is breathed out
of our lungs, and out of chimneys where coal and wood are burned. It
forms a large part of the framework of trees and other plants, and
remains as charcoal when the wood is slowly burned under a close
covering. There is a good proportion of carbon in animal bodies, in the
bones as well as the soft parts, and carbon is plentiful in the mineral
substances of the earth.
The chemist is the man who has determined for us the existence and the
distribution of the seventy elements. He finds them in the solid
substances of the globe and in the water that covers four-fifths of its
surface; in the atmosphere that covers sea and land, and in all the
living forms of plants and animals that live in the seas and on the
land. By means of an instrument called the spectroscope, the heavenly
bodies are proved to be made of the same substances that are found in
the rocks. The sun tells what it is made of, and one proof that the
earth is a child of the sun is in the fact that the same elements are
found in the substance of both.
Of the seventy elements, the most important are these: Oxygen, silicon,
aluminum, iron, manganese, calcium, magnesium, potassium, sodium,
carbon, hydrogen, phosphorus, sulphur, chlorine, nitrogen.
_Oxygen_ is the most plentiful and the most important element. One-fifth
of the air we breathe is oxygen; one-third of the water we drink. The
rock foundations of the earth are nearly one-half oxygen. No fire can
burn, no plant or animal can grow, or even decay after it dies, unless
oxygen is present and takes an active part in each process. Strangely
enough, this wonderful element is invisible. We open a window, and pure
air, rich in oxygen, comes in and takes the place of the bad air but we
cannot see the change. Water we see, but if the oxygen and the hydrogen
which compose the colourless liquid were separated, each would become at
once an invisible gas. The oxygen of solid rocks exists only in
combination with other elements.
_Silicon_ is the element which, united with oxygen, makes the rock
called quartz. On the seashore the children are busy with their pails
and shovels digging in the white, clean sand. These grains are of
quartz,--fine crystals of a rock which forms nearly three-quarters of
the solid earth's substance. Not only in rocks, but out here in the
garden, the soil is full of particles of sand. You cannot get away from
it.
_Aluminum_ is a light, bluish-white metal which we know best in
expensive cooking utensils. It is more abundant even than iron, but
processes of extracting it from the clay are still expensive. It is
oftenest found in combination with oxygen and silicon. While nearly
one-tenth of the earth's crust is composed of the metal aluminum,
four-fifths and more is composed of the minerals called silicates of
aluminum--oxygen, silicon, and aluminum in various combinations. It is
more plentiful than any other substance in rocks and in the clays and
ordinary soils, which are the finely ground particles of rock material.
_Iron_ is one of the commonest of elements. We know it by its red
colour. A rusty nail is covered with oxide of iron, a combination which
is readily formed wherever iron is exposed to the action of water or
air. You have seen yellowish or red streaks in clefts of the rocks. This
shows where water has dissolved out the iron and formed the oxide. The
red colour of New Jersey soil is due to the iron it contains. Indeed,
the whole earth's crust is rich in iron which the water easily
dissolves. The roots of plants take up quantities of iron in solution
and this mounts to the blossoms, leaves, and fruit. The red or yellow
colour of autumn leaves, of apples, of strawberries, of tulips, and of
roses, is produced by iron. The rosy cheeks of children are due to iron
in the food they eat and in the water they drink. The doctor but follows
the suggestion of nature when he gives a pale and listless person a
tonic of iron to make his blood red.
Iron is rarely found free, but it forms about five per cent. of the
crust of the earth, and it is believed to form at least one-fifth of the
unknown centre of the earth, the bulk of the globe, the weight of which
we know, but concerning the substance of which we can say little that is
positive.
_Manganese_ is not a conspicuous element, but is found united with
oxygen in purplish or black streaks on the sides of rocks. It is
somewhat like iron, but much less common.
_Calcium_ is the element that is the foundation of limestones. The
skeletons and shells of animals are made of calcite, a common mineral
formed by the uniting of carbon, oxygen, and calcium. Marbles are,
perhaps, the most permanent form of the limestone rocks. "Hard" water
has filtered through rocks containing calcite, and absorbed particles of
this mineral. From water thus impregnated, all animal life on the earth
obtains its bone-building and shell-building materials.
_Carbon_ forms a large part of the tissues of plants and animals, and in
the remains of these it is chiefly found in the earth's crust. When
these burn or decay, the carbon remains as charcoal or escapes to the
air in union with oxygen as the well known carbonic acid gas. This is
one of the most important foods of plants. Joined with calcium it forms
the mineral calcite, or carbonate of lime.
_Hydrogen_ is one of the two gases that unite to form water. Oxygen is
the other. Many kinds of rock contain a considerable amount of water.
Surface water sinks into porous soils and rocks, and accumulates in
pockets and veins which feed springs, and are the reserve water supply
that keeps our rivers flowing, even through dry weather. More water is
held by absorption in the earth's solid crust than in all the oceans and
seas and great lakes.
Hydrogen, combined with carbon, occurs in solid rocks where the remains
of plants and animals have slowly decayed. From such processes the
so-called hydrocarbons, rock oil and natural gas, have accumulated. When
such decay goes on above ground, these valuable products escape into the
air. Marsh gas, whose feeble flame above decaying vegetation is the
will-o'-the-wisp of swamps, is an example.
_Magnesium_, _potassium_, and _sodium_ are found in equal quantities in
the earth's crust, but never free. In union with chlorine, each forms a
soluble salt, and is thus found in water. Common salt, chloride of
sodium, is the most abundant of these. Water dissolves salt out of the
rocks, and carries it into the sea. Clouds that rise by the evaporation
of ocean water leave the salt behind, hence the seas are becoming more
and more salty, for the rivers carry salt to the oceans, which hold fast
all they get.
_Phosphorus_ is an element found united with oxygen in the tissues of
both plants and animals. It is most abundant in bones. Rocks containing
fossil bones are rich in lime phosphates, which are important commercial
fertilizers for enriching the soil. Beds of these rocks are found and
mined in South Carolina and elsewhere.
_Sulphur_ is well known as a yellow powder found most plentifully in
rocks that are near volcanoes. It is a needed element in plant and
animal bodies. It occurs in rocks, united with many different elements.
In union with oxygen and a metal it forms the group of minerals called
sulphates. In union with iron it forms sulphide of iron. The "fool's
gold" which Captain John Smith's colonists found in the sand at
Jamestown, was this worthless iron pyrites.
_Chlorine_ is a greenish, yellow gas, very heavy, and dangerous to
inhale. If it gets into the lungs, it settles into the lowest levels,
and one must stand on one's head to get it out. As an element of the
earth's crust it is not very plentiful, but it is a part of all the
chlorides of sodium, magnesium, and potassium. In salt, it forms two per
cent. of the sea water. It is much less abundant in the rocks.
To these elements we might add _nitrogen_, that invisible gas which
forms nearly four-fifths of our atmosphere, and is a most important
element of plant food in the soil. Most of the seventy elements are very
rare. Many are metals, like gold and iron and silver. Some are not
metals. Some are solid. A few are liquid, like the metal mercury, and
several are gaseous. Some are free and pure, and show no disposition to
unite with others. Nuggets of gold are examples of this. Some exist only
in union with other elements. This is the common rule among the
elements. Changes are constantly going on. The elements are constantly
abandoning old partnerships and forming new ones. Growth and decay of
plant and animal life are but parts of the great programme of constant
change which is going on and has been in progress since the world
began.
THE FIRST DRY LAND
When the earth's crust first formed it was still hot, though not so hot
as when it was a mass of melted, glowing substance. As it moved through
the cold spaces of the sky, it lost more heat and its crust became
thicker. At length the cloud masses became condensed enough to fall in
torrents of water, and a great sea covered all the land. This was before
any living thing, plant or animal, existed on our planet. Can you
imagine the continents and islands that form the land part of a map or
globe suddenly overwhelmed by the oceans, the names and boundaries of
which you have taken such pains to learn in the study of geography? The
globe would be one blank of blue water, and geography would be
abolished--and there would be nobody to study it. Possibly the fishes in
the sea might not notice any change in the course of their lives, except
when they swam among the ruins of buried cities and peered into the
windows of high buildings, or wondered what new kind of seaweed it was
when they came upon a submerged forest.
In that old time of the great sea that covered the globe, we are told
that there was a dense atmosphere over the face of the deep. So things
were shaping themselves for the far-off time when life should exist,
not only in the sea, where the first life did appear, but on land. But
it took millions of years to fit the earth for living things.
The cooling of the earth made it shrink, and the crust began to be
folded into gentle curves, as the skin of a shrunken apple becomes
wrinkled on the flesh. Some of these creases merely changed the depth of
water on the sea bottom; but one ridge was lifted above the water. The
water parted and streamed down its sloping sides, and a granite reef,
which shone in the sunshine, became the first dry land. It lay east and
west, and stretched for many miles. It is still dry land and is a part
of our own continent. Now it is but a small part of the country, but it
is known by geologists, who can tell its boundaries, though newer land
joins it on every side. It is named the Laurentian Hills, on geological
maps. Its southern border reaches along the northern boundary of the
Great Lakes to the head-waters of the Mississippi River.
From this base, two ridges are lifted, forming a colossal V. One extends
northeast to Nova Scotia; the other northwest to the Arctic seas. The V
encloses Hudson Bay.
Besides this first elongated island of bare rocks, land appeared in a
strip where now the Blue Ridge Mountains stretch from New England to
Georgia. The other side of the continent lifted up two folds of the
crust above sea level. They are the main ridges of the Colorado and the
Wasatch Mountains. Possibly the main ridge of the Sierra Nevada rose
also at this time. The Ozark group of mountains, too, showed as a few
island peaks above the sea.
These first rocks were rapidly eaten away, for the atmosphere was not
like ours, but heavily charged with destructive gases, which did more,
we believe, to disintegrate the exposed rock surfaces than did the two
other forces, wind and water, combined. The sediment washed down to the
sea by rains, accumulated along the shores, filling the shallows and
thus adding to the width of the land areas. The ancient granite ridge of
the Laurentian Hills is now low, and slopes gently. This is true of all
very old mountains. The newer ones are high and steep. It takes time to
grind down the peaks and carry off the waste material loosened by
erosion.
Far more material than could have been washed down the slopes of the
first land ridges came directly from the interior of the earth, and
spread out in vast, submarine layers upon the early crust. Volcanic
craters opened under water, and poured out liquid mineral matter, that
flowed over the sea bottom before it cooled. Imagine the commotion that
agitated the water as these submerged chimneys blew off their lids, and
discharged their fiery contents! It was long before the sea was cool
enough to be the home of living things.
The layers of rock that formed under the sea during this period of the
earth's history are of enormous thickness. They were four or five miles
deep along the Laurentian Hills. They broadened the original granite
ridge by filling the sea bottom along the shores. The backbones of the
Appalachian system and the Cordilleras were built up in the same
way--the oldest rocks were worn away, and their debris built up newer
ones in strata.
When these layers of rock became dry land, the earth's crust was much
more stable and cool than it had ever been before. The vast
rock-building of that era equals all that has been done since. The
layers of rocks formed since then do not equal the total thickness of
these first strata. So we believe that the time required to build those
Archaean rock foundations equals or surpasses the vast period that has
elapsed since the Archaean strata were formed.
The northern part of North America has grown around those old granite
ridges by the gradual rising of the shores. The geologist may walk along
the Laurentian Hills, that parted the waters into a northern and a
southern ocean. He crosses the rocky beds deposited upon the granite;
then the successive beds formed as the land rose and the ocean receded.
Age after age is recorded in the rocks. Gradually the sea is crowded
back, and the land masses, east, west, and north, meet to form the
continent. Nowhere on the earth are the steps of continental growth
shown in unbroken sequence as they are in North America.
How long ago did those first islands appear above the sea? Nobody
ventures a definite answer to this question. No one has the means of
knowing. But those who know most about it estimate that at the least one
hundred million years have passed since then--one hundred thousand
thousand years!
A STUDY OF GRANITE
In Every village cemetery it is easy to find shafts of gray or speckled
granite, the polished surfaces of which show that the granite is made of
small bits of different coloured minerals, cemented together into solid
rock. Outside the gate you will usually find a place where monuments and
gravestones may be bought. Here there is usually a stonecutter chipping
away on a block with his graving tools. He is a man worth knowing, and
because his work is rather monotonous he will probably be glad to talk
to a chance visitor and answer questions about the different kinds of
stone on which he works.
There are bits of granite lying about on the ground. If you have a
hand-glass of low power, such as the botany class uses to examine the
parts of flowers, it will be interesting to look through it and see the
magnified surface of a flake of broken granite. Here are bits of glassy
quartz, clear and sparkling in the sun. Black and white may be all the
colours you make out in this specimen, or it may be that you see specks
of pink, dark green, gray, and smoky brown, all cemented together with
no spaces that are not filled. The particles of quartz are of various
colours, and are very hard. They scratch glass, and you cannot scratch
them with the steel point of your knife, as you can scratch the other
minerals associated with the grains of quartz.
Granite is made of quartz, feldspar, and mica, sometimes with added
particles of hornblende. Feldspar particles have as wide a range of
colour as quartz, but it is easy to tell the two apart. A knife will
scratch feldspar, as it is not so hard as quartz. The crystals of
feldspar have smooth faces, while quartz breaks with a rough surface as
glass does. Feldspar loses its glassy lustre when exposed to the
weather, and becomes dull, with the soft lustre of pearl.
Mica may be clear and glassy, and it ranges in colour from transparency
through various shades of brown to black. It has the peculiarity of
splitting into thin, leaf-like, flexible sheets, so it is easy to find
out which particles in a piece of granite are mica. One has only to use
one's pocket knife with a little care. Hornblende is a dark mineral
which contains considerable iron. It is found in lavas and granites,
where it easily decays by the rusting of the iron. It is not unusual to
see a rough granite boulder streaked with dark red rust from this cause.
The crumbling of granite is constantly going on as a result of the
exposure of its four mineral elements to the air. Quartz is the most
stable and resistant to weathering. Soil water trickling over a granite
cliff has little effect on the quartz particles; but it dissolves out
some of the silicon. The bits of feldspar are even more resistant to
water than quartz is, but the air causes them to decay rapidly, and
finally to fall away in a sort of mealy clay. Mica, like feldspar,
decays easily. Its substance is dissolved by water and carried away to
become a kind of clay. The hornblende rusts away chiefly under the
influence of moist air and trickling water.
We think of granite as a firm, imperishable kind of rock, and use it in
great buildings like churches and cathedrals that are to stand for
centuries. But the faces that are exposed to the air suffer, especially
in regions having a moist climate. The signs of decay are plainly
visible on the outer surfaces of these stones. Fortunate it is that the
weathering process cannot go very deep.
The glassy polish on a smooth granite shaft is the silicon which acts as
a cement to bind all the particles together. It is resistant to the
weather. A polished shaft will last longer than an unpolished one.
Granites differ in colouring because the minerals that compose them, the
feldspars, quartzes, micas, and hornblendes, have each so wide a range
of colour. Again, the proportions of the different mineral elements vary
greatly in different granites. A banded granite the colours of which
give it a stratified appearance is called a gneiss.
We have spoken before of the seventy elements found in the earth's
crust. A mineral is a union of two or more of these different elements;
and we have found four minerals composing our granite rock. It may be
interesting to go back and inquire what elements compose these four
minerals. Quartz is made of silicon and oxygen. Feldspar is made of
silicon, oxygen, and aluminum. Mica is made of silicon, oxygen, and
carbon, with some mingling of potassium and iron and other elements in
differing proportions. Hornblende is made of silicon, oxygen, carbon,
and iron.
The crumbling of a granite rock separates the minerals that compose it,
reducing some to the condition of clay, others to grains of sand. Some
of the elements let go their union and become free to form new unions.
Water and wind gather up the fragments of crumbling granite and carry
them away. The feldspar and mica fragments form clay; the quartz
fragments, sand. All of the sandstones and slates, the sand-banks and
sand beaches, are made out of crumbled granite, the rocky foundations of
the earth.
METAMORPHIC ROCKS
In the dawn of life on the earth, soft-bodied creatures, lowest in the
scale of being, inhabited the sea. The ancient volcanoes the
subterranean eruptions of which had spread layers of mineral substance
on the ocean floor, and heated the water to a high degree, had subsided.
The ocean was sufficiently cool to maintain life. The land was being
worn down, and its debris washed into the ocean. The first sand-banks
were accumulating along sandy shores. The finer sediment was carried
farther out and deposited as mud-banks. These were buried under later
deposits, and finally, by the rising of the earth's crust, they became
dry land. Time and pressure converted the sand-banks into sandstones;
the mud-banks into clay. The remains of living creatures utterly
disappeared, for they had no hard parts to be preserved as fossils.
The shrinking of the earth's crust had crumpled into folds of the utmost
complexity those horizontal layers of lava rock poured out on the ocean
floor. Next, the same forces attacked the thick rock layers formed out
of sediment--the aqueous or water-formed sandstones and clays.
The core of the globe contracts, and the force that crumples the crust
to fit the core generates heat. The alkaline water in the rocks joins
with the heat produced by the crumpling and crushing forces, acting
downward, and from the sides, to transform pure sandstone into glassy
quartzite, and clay into slate. In other words, water-formed rocks are
baked until they become fire-formed rocks. They are what the geologist
calls _metamorphic_, which means _changed_.
In many mountainous regions there are breaks through the strata of
sandstone and slates and limestones, through which streams of lava have
poured forth from the heated interior. Along the sides of these fissures
the hot lava has changed all the rocks it touched. The heat of the
volcanic rock matter has melted the silica in the sand, which has
hardened again into a crystalline substance like glass.
Have you ever visited a brick-yard? Here men are sifting clay dug out of
a pit or the side of a hill, adding sand from a sand-bank, and in a big
mixing box, stirring these two "dry ingredients" with water into a thick
paste. This dough is moulded into bricks, sun-dried, and then baked in
kilns themselves built of bricks. At the end of the baking, the soft,
doughy clay block is transformed into a hard, glassy, or dull brick.
From aqueous rock materials, fire has produced a metamorphic rock.
Volcanic action is imitated in this common, simple process of
brickmaking.
Milwaukee brick is made of clay that has no iron in it. For this reason
the bricks are yellow after baking. Most bricks are red, on account of
the iron in the clay, which is converted into a red oxide, or rust, by
water and heat.
Common flower pots and the tiles used in draining wet land are not
glazed, as hard-burned bricks are. The baking of these clay things is
done with much less heat. They are left somewhat porous. But the tiles
of roofs are baked harder, and get a surface glaze by the melting of the
glassy particles of the sand.
As bricks vary in colour and quality according to the materials that
compose them, so the metamorphic rocks differ. The white sand one sees
on many beaches is largely quartz. This is the substance of pure, white
sandstone. Metamorphism melts the silica into a glassy liquid cement;
the particles are bound close together on cooling. The rock becomes a
white, granular quartzite, that looks like loaf sugar. If banded, it is
called gneiss. Such rocks take a fine polish.
Pure limestone is also white and granular. When metamorphosed by heat,
it becomes white marble. The glassy cement that holds the particles of
lime carbonate shows as the glaze of the polished surface. It is silica.
One sees the same mineral on the face of polished granite.
Clays are rarely pure. Kaolin is a white clay which, when baked, becomes
porcelain. China-ware is artificially metamorphosed kaolin. In the
early rocks the clay beds were transformed by heat into jasper and
slates. In beds where clay mingled with sand, in layers, gneiss was
formed. If mica is a prominent element, the metamorphic rock is easily
parted into overlapping, scaly layers. It is a mica schist. If
hornblende is the most abundant mineral, the same scaly structure shows
in a dark rock called hornblende schist, rich in iron. A schist
containing much magnesia is called serpentine.
The bricks of the wall, the tiles on the roof, the flower pots on the
window sill, and the dishes on the breakfast table, are examples of
metamorphic rocks made by man's skill, by the use of fire and water
acting on sand and clay. Pottery has preserved the record of
civilization, from the making of the first crude utensils by cave men to
the finest expression of decorative art in glass and porcelain.
The choicest material of the builder and the sculptor is limestone baked
by the fires under the earth's crust into marble. The most enduring of
all the rocks are the foundation granite, and the metamorphic rocks that
lie next to them. Over these lie thick layers of sedimentary rocks laid
down by water. In them the record of life on the earth is written in
fossils.
THE AIR IN MOTION
Most of the beautiful things that surround us and make our lives full of
happiness appeal to one or more of our five senses. The green trees we
can see, the bird songs we hear, the perfume of honey-laden flowers we
smell, the velvety smoothness of a peach we feel, and its rich pulp we
taste. But over all and through all the things we see and feel and hear
and taste and smell, is the life-giving air, that lies like a blanket,
miles in depth, upon the earth. The substance which makes the life of
plants and animals possible is, when motionless, an invisible,
tasteless, odourless substance, which makes no sound and is not
perceptible to the touch.
Air fills the porous substance of the earth's crust for a considerable
distance, and even the water has so much air in it that fishes are able
to breathe without coming to the surface. It is not a simple element,
like gold, or carbon, or calcium, but is made up of several elements,
chief among which are nitrogen and oxygen. Four-fifths of its bulk is
nitrogen and one-fifth oxygen. There is present in air more or less of
watery vapour and of carbon dioxide, the gas which results from the
burning or decay of any substance. Although no more than one per cent.
of the air that surrounds us is water, yet this is a most important
element. It forms the clouds that bear water back from the ocean and
scatter it in rain upon the thirsty land. Solid matter in the form of
dust, and soot from chimneys, accumulates in the clouds and does a good
work in condensing the moisture and causing it to fall.
It is believed that the air reaches to a height of one hundred to two
hundred miles above the earth's surface. If a globe six feet in diameter
were furnished with an atmosphere proportionately as deep as ours, it
would be about an inch in depth. At the level of the sea the air reaches
its greatest density. Two miles above sea-level it is only two-thirds as
dense. On the tops of high mountains, four or five miles above sea
level, the air is so rarefied as to cause the blood to start from the
nostrils and eyelids of explorers. The walls of the little blood-vessels
are broken by the expansion of the air that is inside. At the sea-level
air presses at the rate of fifteen pounds per square foot in all
directions. As one ascends to higher levels, the air pressure becomes
less and less.
The barometer is the instrument by which the pressure of air is
measured. A glass tube, closed at one end, and filled with mercury, the
liquid metal often called quicksilver, is inverted in a cup of the same
metal, and so supported that the metal is free to flow between the two
vessels. The pressure of air on the surface of the mercury in the cup is
sufficient at the sea-level to sustain a column of mercury thirty
inches high in the tube. As the instrument is carried up the side of a
mountain the mercury falls in the tube. This is because the air pressure
decreases the higher up we go. If we should descend into the shaft of
the deepest mine that reaches below the sea level, the column of air
supported by the mercury in the cup would be a mile higher, and for this
reason its weight would be correspondingly greater. The mercury would
thus be forced higher in the tube than the thirty-inch mark, which
indicates sea-level.
Another form of barometer often seen is a tube, the lower and open end
of which forms a U-shaped curve. In this open end the downward pressure
of the air rests upon the mercury and holds it up in the closed end,
forcing it higher as the instrument is carried to loftier altitudes. At
sea level a change of 900 feet in altitude makes a change of an inch in
the height of the mercury in the column. The glass tube is marked with
the fractions of inches, or of the metre if the metric system of
measurements is used.
It is a peculiarity of air to become heated when it is compressed, and
cooled when it is allowed to expand again. It is also true that when the
sun rises, the atmosphere is warmed by its rays. This is why the hottest
part of the day is near noon when the sun's rays fall vertically. The
earth absorbs a great deal of the sun's heat in the daytime and through
the summer season. When it cools this heat is given off, thus warming
the surrounding atmosphere. In the polar regions, north and south, the
air is far below freezing point the year round. In the region of the
Equator it rarely falls below 90 degrees, a temperature which we find
very uncomfortable, especially when there is a good deal of moisture in
the air.
If we climb a mountain in Mexico, we leave the sultry valley, where the
heat is almost unbearable, and very soon notice a change. For every
three hundred feet of altitude we gain there is a fall of one degree in
the temperature. Before we are half way up the slope we have left behind
the tropical vegetation, and come into a temperate zone, where the
plants are entirely different from those in the lower valley. As we
climb, the vegetation becomes stunted, and the thermometer drops still
lower. At last we come to the region of perpetual snow, where the
climate is like that of the frozen north.
So we see that the air becomes gradually colder as we go north or south
from the Equator, and the same change is met as we rise higher and
higher from the level of the sea.
It is only when air is in motion that we can feel and hear it, and there
are very few moments of the day, and days of the year, when there is not
a breeze. On a still day fanning sets the air in motion, and creates a
miniature breeze, the sound of which we hear in the swishing of the fan.
The great blanket of air that covers the earth is in a state of almost
constant disturbance, because of the lightness of warm air and the
heaviness of cold air. These two different bodies are constantly
changing places. For instance, the heated air at the Equator is
constantly being crowded upward by cold air which settles to the level
of the earth. Cold streams of air flow to the Tropics from north and
south of the Equator, and push upward the air heated by the sun.