underground chambers found there are in the limestone rock which varies
from ten to four hundred feet in thickness, and averages a little less
than two hundred feet. Over this territory the number of sink-holes
average one hundred to the square mile; and the streams that have poured
their water into these basins have made a network of open caverns one
hundred thousand miles in length.
A great many small caverns have been thoroughly explored and are famous
for their beauty. The Diamond Cave is one of the most splendid, for it
is lined with walls and pillars of alabaster that sparkle in the
torchlight with crystals that look like veritable diamonds. Beautiful
springs and waterfalls are found in many caves, but the grandest of all
is the Mammoth Cave, beside which no other is counted worthy to be
compared.
Great tales the miners told of the wonder and the beauty of these
caverns, the walls of which were supported by arching alabaster columns
and wonderful domes, of indescribable beauty of form and colouring. In
1799, the year that Washington died, a pioneer discovered the entrance
to a cave, the size and beauty of which surpassed anything he had seen
before. After exploring it for a short distance he returned home and
took his whole family with him to enjoy the first view of the wonderful
cavern he had discovered. They carried pine knots and a lighted torch,
by which they made their way for some distance, but the torch was
accidentally extinguished and they groped their way in darkness and
missed the entrance. Without anything to guide them, they wandered in
darkness for three days, and were almost dead when at last they stumbled
upon the exit. This is the doorway of the Mammoth Cave of Kentucky, one
of the wonders of the world.
This was a terrible experience. The next persons who attempted to
explore the new cave were better provisioned against the chance of
spending some time underground. The pioneers found rich deposits of
nitre in the "Great Cave," as they called it. Scientists visited it and
explored many of its chambers. The reputation of this cavern has been
spread by thousands of visitors who have come from all over the world to
see it. The cave has not yet been completely explored. The regular
tours, on which the guides conduct visitors, cover but a small part of
the one hundred and fifty miles measured by the two hundred or more
avenues. The passages wind in and out, crossing each other, sometimes at
different levels, and forming a network of avenues in which the
unaccustomed traveller would surely be lost. The old guides know every
inch of their regular course, and their quaint and edifying talk adds
greatly to the pleasure of the visitors.
From the hotel, parties are organized for ten o'clock in the morning and
seven o'clock in the evening. Each visitor is provided with a lard-oil
lamp. The guide carries a flask of oil and plenty of matches. No special
garb is necessary, though people usually dress for comfort, and wear
easy shoes. The temperature of the cave is uniform winter and summer,
varying between fifty-three and fifty-four degrees Fahrenheit.
The cave entrance is an arch of seventy-foot span in the hillside. A
winding flight of seventy stone steps leads the party around a
waterfall, into a great chamber under the rocks. Then the way goes
through a narrow passage, where the guide unlocks an iron gate to let
them in. The visitors now leave all thoughts of daylight behind, for the
breeze that put out their lights as they entered the cave is past, and
they stand in the Rotunda, a vast high-ceilinged chamber, silent and
impressive, with walls of creamy limestone, encrusted with gypsum, which
has been stained black by manganese. From the vestibule on, each passage
and each room has a name, based upon some historic event or some fancied
resemblance. The Giant's Coffin is a great kite-shaped rock lying in one
of the rooms of the cave. The Star Chamber has a wonderful
crystal-studded dome in which the guide produces the effect of a sunrise
by burning coloured lights. Bonfires built at suitable points produce
wonderful shadow effects, which are like nothing else in the world. The
old saltpetre vats which the visitors pass in taking the "Long Route"
through the cave, point them back to the days during the War of 1812,
when this valuable mineral was extracted from the earth in the floor of
the cave. The industry greatly enriched the thrifty owners of the cave,
but the works were abandoned after peace was declared.
It must be a wonderful experience to walk steadily for nine hours over
the Long Route, for so pure is the air and so wonderful is the scenery
that people rarely complain of fatigue when the experience is over.
There is no dust on the floors of these subterranean chambers, and they
are not damp except near places where water trickles, here and there, in
rivulets and cascades. Pools of water at the bottoms of pits so deep
that a lighted torch requires several seconds to reach the bottom, and
rivers and lakes of considerable size, show where some of the surface
water goes to. A strange underground suction creates whirlpools in some
of these streams. People go in boats holding twenty passengers for a row
on Echo River, and the guide dips up with a net the blind fish and
crayfish and cave lizards which inhabit these subterranean waters. The
echoes in various chambers of the Mammoth Cave are remarkable. In some
of them a song by a single voice comes back with full chords, as if
several voices carried the different parts. The single notes of flute
and cornet are returned with the same beautiful harmonies. A pistol shot
is given back a dozen times, the sound rebounding like a ball from rock
to rock of the arching walls. The vibrations of the water made by the
rower's paddles reecho in sounds like bell notes, and they are
multiplied into harmonies that suggest the chimes in the belfry of a
cathedral.
The walls of various chambers differ from each other according to the
minerals that compose them. Some are creamy white limestone arches, some
are walled with black gypsum, some are hung with great curtains of
stalagmites, solid but suggesting the lightness and grace of folds of
crepe. Under such hangings the floor is built up in stalactites. The
mineral-laden water, the constant drip of which has produced a hanging,
icicle-like stalagmite, has built up the stalactite to meet it.
Probably nothing is more beautiful than the flower-like crystals that
bloom all over the walls of a chamber called "Mary's Bower." The floor,
even, sparkles with jewels that have fallen from the wonderful and
delicate flower clusters built from deposits of the lime-laden water
which goes on building and replacing the bits that fall. "Martha's
Vineyard" is decorated with nodules, like bunches of grapes, that
glisten as if the dew were on them. The white gypsum in some caves makes
the walls look as if they were carved out of snow. Still others have
clear, transparent crystals that make them gleam in the torches' light
as if the walls were encrusted with diamonds.
The cave region of Indiana is also famous. The great Wyandotte Cave in
Crawford County is the most noted of many similar caverns. In some of
the chambers, bats are found clinging to the ceiling, heads downward,
like swarms of bees. The caverns of Luray, in Virginia, are complex and
wonderful in their structure, and famous for the beautiful stalactites
and stalagmites they contain. But there is no cave in this country so
wonderful and so grand in its dimensions as the Mammoth Cave in
Kentucky.
LAND-BUILDING BY RIVERS
Once a year, when the rainy season comes in the mountainous country
south of Egypt, the old Nile floods its banks and spreads its slimy
waters over the land, covering the low plains to the very edge of the
Sahara Desert. The people know it is coming, and are prepared for this
flood. We should think such an overflow of our nearest river a monstrous
calamity, but the Egyptians bless the river which blesses them. They
know that without the Nile's overflow their country would be added to
the Desert of Sahara. In a short time after the overflow, the river
reaches its highest point and begins to ebb. Canals lying parallel to
its course are filled with water which is saved for use in the hot, dry
summer. As the flood goes down, a deposit of slimy mud lies as a rich
fertilizer on the land. It is this and the water which the earth has
absorbed that make Egypt one of the most fertile agricultural countries
in the world.
The region covered by the Nile's overflow is the flood plain of this
river. On this plain the Pyramids, the Sphinx, and other famous
monuments of Egypt stand. The statue of Rameses II. built 3,000 years
ago, has its base buried nine feet deep in the rich soil made of Nile
sediment. A well dug in this region goes through forty feet of this soil
before striking the underlying sand. How many years ago did the first
Nile overflow take place? We may begin our calculation by finding out
the average yearly deposit. It is a slow process that accumulates but
nine feet in 3,000 years. If you were in Egypt when the Nile went back
into its banks, you would see that the scum it leaves in a single
overflow adds not a great deal to the thickness of the soil. Possibly
floods have varied in their deposits from year to year, so that any
calculation of the time it took to build that forty feet of surface soil
must be but a rough estimate. This much we know: it has been an
uninterrupted process which has taken place within the present
geological epoch, "the Age of Man."
Not all the rich sediment the Nile brings down is left on the level
flood plain along its course. A vast quantity is dumped at the river's
mouth, where the tides of the Mediterranean check the river's current.
Thus the great delta is formed. The broad river splits into many mouths
that spread out like a fan and build higher and broader each year the
mud-banks between the streams. Upper Egypt consists of river swamps.
Lower Egypt, from Cairo to the sea, is the delta built by the river
itself on sea bottom. From the head of the delta, where the river
commences to divide, to the sea, is an area of 10,000 square miles made
out of material contributed by upper Egypt, and built by the river.
Layer upon layer, it is constantly forming, but most rapidly during the
season of floods.
Coming closer home, let us look at the map of the Mississippi Valley.
Begin as far north as St. Louis. For the rest of its course the
Mississippi River flows through a widening plain of swamp land, flooded
in rainy seasons. Through this swampy flood plain the river meanders;
its current, heavily loaded with sediment, swings from one side to the
other of the channel, building up here, wearing away there, and
straightening its course when the curves become so sharp that their
sides meet. Then the current breaks through the thin wall, and a bayou
of still water is left behind.
Below Baton Rouge the Mississippi breaks into many mouths, that spread
and carry the water of the great river into the Gulf of Mexico. The Nile
delta is triangular, like delta, [Greek: D], the fourth letter of the
Greek alphabet; but the Mississippi's delta is very irregular. The main
mouth of the river flows fifty miles out into the Gulf between
mud-banks, narrow and low. At the tip it branches into several streams.
From the mouth of the Ohio to the Gulf, the Mississippi flood plain
covers 30,000 square miles. Over this area, sediment to an average depth
of fifty feet has been laid down. In earlier times the river flooded
this whole area, when freshets swelled its tributaries in the spring.
The flood plain then became a sea, in the middle of which the river's
current flowed swiftly. The slow-flowing water on each side of the main
current let go of its burden of sediment and formed a double ridge.
Between these two natural walls the main river flowed. When its level
fell, two side streams, running parallel with the main river drained the
flood plains on each side into the main tributaries to right and left.
These natural walls deposited when the river was in flood are called
_levees_. Each heavy flood builds them higher, and the bed of the stream
rises by deposits of sediment. So it happens that the level of the river
bed is higher than the level of its flood plain.
This is an interesting fact in geology. But the people who have taken
possession of the rich flood plain of the Mississippi River, who have
built their homes there, drained and cultivated the land, and built
cities and towns on the areas reclaimed from swamps, recognize the
elevation of the river bed as the greatest danger that threatens them.
Suppose a flood should come. Even if it does not overflow the levees, it
may break through the natural banks and thus overflow the cities and the
farm lands to left and right.
Instead of living in constant fear of such a calamity, the people of the
Mississippi flood plain have sought safety by making artificial levees,
to make floods impossible. These are built upon the natural levees. As
the river bed rises by the deposit of mud, the levees are built higher
to contain the rising waters. No longer does the rich soil of the
Mississippi flood plain receive layers of sediment from the river's
overflow. The river very rarely breaks through a levee. The United
States Government has spent great sums in walling in the river, and each
state along its banks does its share toward paying for this
self-protection.
By means of _jetties_ the river's current is directed into a
straightened course, and its power is expended upon the work of
deepening its own channel and carrying its sediment to the Gulf. Much as
the river has been forced to do in cleaning its own main channel,
dredging is needed at various harbours to keep the river deep enough for
navigation. The forests of the mountain slopes in Colorado are being
slaughtered, and the headwaters of the Missouri are carrying more and
more rocky debris to choke the current of the Mississippi. Colorado soil
is stolen to build land in the vast delta, which is pushing out into the
Gulf at the rate of six miles in a century--a mile in every sixteen
years. The Mississippi delta measures 14,000 square miles. With the
continued denuding of mountain slopes, we shall expect the rate of delta
growth to be greatly increased, until reforesting checks the destructive
work of wind and water.
THE MAKING OF MOUNTAINS
The gradual thickening and shrinking of the earth's crust as it cools
have made the wrinkles we call mountain systems. Through millions of
years the globe has been giving off heat to the cold sky spaces through
which it swings in its orbit around the sun. The cooling caused the
contraction of the outer layer to fit the shrinking of the mass. When a
plump peach dries on its pit, the skin wrinkles down to fit the dried
flesh. The fruit shrinks by loss of water, just as the face of an old
person shrinks by loss of fat. The skin becomes wrinkled in both cases.
The weakest places in the earth's crust were the places to crumple,
because they could not resist the lateral pressure that was exerted by
the shrinking process. Along the shores of the ancient seas the rivers
piled great burdens of sediment. This caused the thin crust to sink and
to become a basin alongside of a ridge. The wearing away of the land in
certain places lightened and weakened the crust at these places, so that
it bent upward in a ridge.
Perhaps the first wrinkles were not very high and deep. The gradual
cooling must have exerted continued pressure, and the wrinkles have
become larger. It is not likely that new wrinkles would be formed as
long as the old ones would crumple and draw up into narrower, steeper
slopes, in response to the lateral crushing.
We can imagine those first mountains rising as folds under the sea.
Gradually their bases were narrowed, and their crests lifted out of the
water. They rose as long, narrow islands, and grew in size as time went
on.
Why is the trend of the great mountain systems almost always north and
south? Study the map of the continents and see how few cross ranges are
shown, and how short they are, compared with the others. The molten
globe bulged at its equator, as it rotated on its axis. The moon added
its strong pulling force to make it bulge still more. As the crust
thickened, it became less responsive to the two forces that caused it to
bulge. The shrinkage was greatest where the globe had been most pulled
out of shape. The rate of the earth's rotation is believed to have
diminished. Every change tended to let the earth draw in its (imaginary)
belt, a notch at a time. The forces of contraction acted along the line
of the equator, and formed folds running toward the poles. In this early
time the great mountain systems were born, and they grew in size
gradually, from small beginnings.
These mountains of upheaval, made by the bending of the earth's crust,
and the formation of alternating ridges and depressed valleys, are many.
The earth is old and much wrinkled. Other mountains have been formed by
forces quite different. Volcanic mountains have been far more numerous
in ages gone than they are now.
Mt. Hood and Mt. Rainier are peaks built up by the materials thrown out
of the craters of volcanoes dead these thousands of years. Vesuvius is
at present showing us how volcanic mountains are made. Each eruption
builds larger the cone--that is, the chimney through which the molten
rocks, the ashes, and the steam are ejected. Side craters may open, the
main cone be broken and its form changed, but the mass of lava and
stones and ashes grows with each eruption. The mountain grows by the
additions it receives. AEtna is a mountain built of lava.
A third mountain system grew, not by addition, but by subtraction. The
Catskills illustrate this type. This group of mountains is the remnant
of a table-land made of level layers of red sandstone. The rest of the
high plain has been cut down and carried away, leaving these picturesque
hills, the survival of which is as much a mystery as the disappearance
of the balance of the plateau of which they were once a part.
The fold that forms a typical mountain ridge has a cone of granite, the
original rock foundation of the earth, and on this are layers of
stratified rock, ancient deposits of sediment carried to the sea by
streams. When exposed to wind and rain, the ridge is gradually worn
down. In some places the water cuts away the soft rock and forms a
stream-bed, that cuts deeper and deeper, using the rock fragments as its
tools. Often the layers of aqueous rocks are cut through, and the
granite exposed.
Sometimes the hardest stratified rock-beds resist the water and the wind
and are left as a series of ridges along the sides of the main range.
The crumpling forces may crack the ridge open for its whole length, and
one side of the chasm may slip down and the other go up. The result is a
sheer wall of exposed rock strata, layers of which correspond with those
that lie far below the top of the portion that slid down in the great
upheaval and subsidence that parted them. These slips are known as
_faults_.
THE LAVA FLOOD OF THE NORTHWEST
We know little about the substance that occupies the four thousand miles
of distance between the surface and the centre of our earth. We know
that the terrible weight borne by the central mass compresses it, so
that the interior must grow denser as the core is approached. Scientists
have weighed the earth, and tell us that the crust is lighter than the
rest. The supposition is that there is a great deal of iron in the
interior, and possibly precious metals, too.
Our deepest wells and mines go down about a mile, then digging stops, on
account of the excessive heat. But the crumpling of the crust, and the
wearing away of the folded strata by wind and running water, have laid
bare rocks several miles in thickness on the slopes of mountains, and
exposed the underlying granite, on which the first sedimentary rocks
were deposited. On this granite lie stratified rocks, which are
crystalline in texture. These are the beds, sometimes miles in depth,
called _metamorphic_ rocks, formed by water, then transformed by heat.
The wearing away of rocks by wind and water has furnished the materials
out of which the aqueous rocks have been made. Layers upon layers of
sandstone, shales, limestone, and the like, are exposed when a river
cuts a canyon through a plateau. The layered deposits of debris at the
mouth of the river make new aqueous rocks out of old. Every sandy beach
is sandstone in the making. This work is never ended.
In the early days the earth's crust often gaped open in a mighty crater
and let a flood of lava overspread the surface. The ocean floor often
received this flood of melted rock. In many places the same chimney
opened again and again, each time spreading a new layer of lava on top
of the old, so that the surface has several lava sheets overlying the
aqueous strata.
If the hardened lava sheet proved a barrier to the rising tide of molten
lava in the chimney it was often forced out in sheets between the layers
of aqueous rocks. Wherever the heated material came into contact with
aqueous rocks it transformed them, for a foot or more, into crystalline,
metamorphic rocks.
A chimney of lava is called a _dike_. In mountainous countries dikes are
common. Sometimes small, they may also be hundreds of feet across, often
standing high above the softer strata, which rains have worn away. Dikes
often look like ruined walls, and may be traced for miles where they
have been overturned in the mountain-making process.
The great lava flood of the Northwest happened when the Coast Range was
born. Along the border of the Pacific Ocean vast sedimentary deposits
had accumulated during the Cretaceous and Tertiary Periods. Then the
mighty upheaval came, the mountain ridge rose at the end of the Miocene
epoch and stretched itself for hundreds of miles through the region
which is now the coast of California and Oregon. Great fissures opened
in the folded crust, and floods of lava overspread an area of 150,000
square miles. A dozen dead craters show to-day where those immense
volcanic chimneys were. The depth of the lava-beds is well shown where
the Columbia River has worn its channel through. Walls of lava three
thousand feet in thickness rise on each side of the river. They are made
of columns of basalt, fitted together, like cells of a honeycomb, and
jointed, forming stone blocks laid one upon another. The lava shrinks on
cooling and forms prisms. In Ireland, the Giants' Causeway is a famous
example of basaltic formation. In Oregon, the walls of the Des Chutes
River show thirty lava layers, each made of vertical basalt columns. The
palisades of the Hudson, Mt. Tom, and Mt. Holyoke are examples on the
eastern side of the continent of basaltic rocks made by lava floods.
Northern California, northwestern Nevada, and large part of Idaho,
Montana, Oregon, and Washington are included in the basin filled with
lava at the time of the great overflow, which extended far into British
Columbia. It is probable that certain chimneys continued to discharge
until comparatively recent times. Mt. Rainier, Mt. Shasta, and Mt. Hood
are among dead volcanoes.
Quite a different history has the great Deccan lava-field of India,
which covers a larger area than the basin of our Northwest, and is in
places more than a mile in depth. It has no volcanoes, nor signs of any
ever having existed. The floods alone overspread the region, which shows
no puny "follow-up system" of scattered craters, intermittently in
eruption.
THE FIRST LIVING THINGS