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Young Folks' Library, Volume XI (of 20) by Various

V >> Various >> Young Folks\' Library, Volume XI (of 20)




And, strange to say, we shall find that this same ocean, so busily
engaged with the help of its tributary rivers in pulling down land, is
no less busily engaged with their help in building it up.

You have sometimes seen directions upon a vial of medicine to "shake"
before taking the dose. When you have so shaken the bottle the clear
liquid grows thick; and if you let it stand for awhile the thickness
goes off, and a fine grain-like or dust-like substance settles down at
the bottom--the settlement or _sediment_ of the medicine. The finer
this sediment, the slower it is in settling. If you were to keep the
liquid in gentle motion, the fine sediment would not settle down at
the bottom. With coarser and heavier grains the motion would have to
be quicker to keep them supported in the water.

Now it is just the same thing with our rivers and streams. Running
water can support and carry along sand and earth, which in still water
would quickly sink to the bottom; and the more rapid the movement of
the water, the greater is the weight it is able to bear.

This is plainly to be seen in the case of a mountain torrent. As it
foams fiercely through its rocky bed it bears along, not only mud and
sand and gravel, but stones and even small rocks, grinding the latter
roughly together till they are gradually worn away, first to rounded
pebbles, then to sand, and finally to mud. The material thus swept
away by a stream, ground fine, and carried out to sea--part being
dropped by the way on the river-bed--is called _detritus_, which
simply means _worn-out_ material.

[Illustration: A MOUNTAIN TORRENT.]

The tremendous carrying-power of a mountain torrent can scarcely be
realized by those who have not observed it for themselves. I have seen
a little mountain-stream swell in the course of a heavy thunderstorm
to such a torrent, brown and turbid with earth torn from the
mountainside, and sweeping resistlessly along in its career a shower
of stones and rock-fragments. That which happens thus occasionally
with many streams is more or less the work all the year round of many
more.

As the torrent grows less rapid, lower down in its course, it ceases
to carry rocks and stones, though the grinding and wearing away of
stones upon the rocky bed continues, and coarse gravel is borne still
upon its waters. Presently the widening stream, flowing yet more
calmly, drops upon its bed all such coarser gravel as is not worn away
to fine earth, but still bears on the lighter grains of sand. Next the
slackening speed makes even the sand too heavy a weight, and that in
turn falls to line the river-bed, while the now broad and placid
stream carries only the finer particles of mud suspended in its
waters. Soon it reaches the ocean, and the flow being there checked by
the incoming ocean-tide, even the mud can no longer be held up, and it
also sinks slowly in the shallows near the shore, forming sometimes
broad mud-banks dangerous to the mariner.

This is the case only with smaller rivers. Where the stream is
stronger, the mud-banks are often formed much farther out at sea; and
more often still the river-detritus is carried away and shed over the
ocean-bed, beyond the reach of our ken. The powerful rush of water in
earth's greater streams bears enormous masses of sand and mud each
year far out into the ocean, there dropping quietly the gravel, sand,
and earth, layer upon layer at the bottom of the sea. Thus pulling
down and building up go on ever side by side; and while land is the
theatre oftentimes of decay and loss, ocean is the theatre oftentimes
of renewal and gain.


Did you notice the word "sediment" used a few pages back about the
settlement at the bottom of a medicine-vial?

There is a second name given to the Stratified Rocks, of which the
earth's crust is so largely made up. They are called also _Sedimentary
Rocks_.

The reason is simply this. The Stratified Rocks of the present day
were once upon a time made up out of the sediment stolen first from
land and then allowed to settle down on the sea-bottom.

Long, long ago, the rivers, the streams, the ocean, were at work, as
they are now, carrying away rock and gravel, sand and earth. Then, as
now, all this material, borne upon the rivers, washed to and fro by
the ocean, settled down at the mouths of rivers or at the bottom of
the sea, into a sediment, one layer forming over another, gradually
built up through long ages. At first it was only a soft, loose, sandy
or muddy sediment, such as you may see on the seashore, or in a
mud-bank. But as the thickness of the sediment increased, the weight
of the layers above gradually pressed the lower layers into firm hard
rocks; and still, as the work of building went on, these layers were,
in their turn, made solid by the increasing weight over them. Certain
chemical changes had also a share in the transformation from soft mud
to hard rock, which need not be here considered.

All this has through thousands of years been going on. The land is
perpetually crumbling away; and fresh land under the sea is being
perpetually built up, from the very same materials which the sea and
the rivers have so mercilessly stolen from continents and islands.
This is the way, if geologists rightly judge, in which a very large
part of the enormous formations of Stratified or Sedimentary Rocks
have been made.

[Illustration: VIEW IN A CANON.]

So far is clear. But now we come to a difficulty.

The Stratified Rocks, of which a very large part of the continents is
made, appear to have been built up slowly, layer upon layer, out of
the gravel, sand, and mud, washed away from the land and dropped on
the shore of the ocean.

[Illustration: SEA CLIFFS SHOWING A SERIES OF STRATIFIED ROCKS.]

You may see these layers for yourself as you walk out into the
country. Look at the first piece of bluff rock you come near, and
observe the clear pencil-like markings of layer above layer--not often
indeed lying _flat_, one over another, and this must be explained
later, but however irregularly slanting, still plainly visible. You
can examine these lines of stratification on the nearest cliff, the
nearest quarry, the nearest bare headland, in your neighborhood.

But how can this be? If all these stratified rocks are built on the
floor of the ocean out of material taken _from_ the land, how can we
by any possibility find such rocks _upon_ the land? In the beds of
rivers we might indeed expect to see them, but surely nowhere else
save under ocean waters.

Yet find them we do. Through England, through the two great
world-continents, they abound on every side. Thousands of miles in
unbroken succession are composed of such rocks.

Stand with me near the seashore, and let us look around. Those white
chalk cliffs--they, at least, are not formed of sand or earth. True,
and the lines of stratification are in them very indistinct, if seen
at all; yet they too are built up of sediment of a different kind,
dropping upon ocean's floor. See, however, in the rough sides of
yonder bluff the markings spoken of, fine lines running alongside of
one another, sometimes flat, sometimes bent or slanting, but always
giving the impression of layer piled upon layer. Yet how can one for a
moment suppose that the ocean-waters ever rose so high?

Stay a moment. Look again at yonder white chalk cliff, and observe a
little way below the top a singular band of shingles, squeezed into
the cliff, as it were, with chalk below and earth above.

That is believed to be an old sea-beach. Once upon a time the waters
of the sea are supposed to have washed those shingles, as now they
wash the shore near which we stand, and all the white cliff must have
lain then beneath the ocean.

Geologists were for a long while sorely puzzled to account for these
old sea-beaches, found high up in the cliffs around our land in many
different places.

They had at first a theory that the sea must once, in far back ages,
have been a great deal higher than it is now. But this explanation
only brought about fresh difficulties. It is quite impossible that the
level of the sea should be higher in one part of the world than in
another. If the sea around England were then one or two hundred feet
higher than it is now, it must have been one or two hundred feet
higher in every part of the world where the ocean-waters have free
flow. One is rather puzzled to know where all the water could have
come from, for such a tremendous additional amount. Besides, in some
places remains of sea-animals are found in mountain heights, as much
as two or three thousand feet above the sea-level--as, for instance,
in Corsica. This very much increases the difficulty of the above
explanation.

So another theory was started instead, and this is now generally
supposed to be the true one. What if instead of the whole ocean having
been higher, parts of the land were lower? England at one time, parts
of Europe at another time, parts of Asia and America at other times,
may have slowly sunk beneath the ocean, and after long remaining there
have slowly risen again.

This is by no means so wild a supposition as it may seem when first
heard, and as it doubtless did seem when first proposed. For even in
the present day these movements of the solid crust of our earth are
going on. The coasts of Sweden and Finland have long been slowly and
steadily rising out of the sea, so that the waves can no longer reach
so high upon those shores as in years gone by they used to reach. In
Greenland, on the contrary, land has long been slowly and steadily
sinking, so that what used to be the shore now lies under the sea.
Other such risings and sinkings might be mentioned, as also many more
in connection with volcanoes and earthquakes, which are neither slow
nor steady, but sudden and violent.

So it becomes no impossible matter to believe that, in the course of
ages past, all those wide reaches of our continents and islands, where
sedimentary rocks are to be found, were each in turn, at one time or
another, during long periods, beneath the rolling waters of the
ocean....

* * * * *

These built-up rocks are not only called "Stratified," and
"Sedimentary." They have also the name of _Aqueous Rock_, from the
Latin word _aqua, water_; because they are believed to have been
formed by the action of the water.

They have yet another and fourth title, which is, _Fossiliferous
Rocks_.

Fossils are the hardened remains of animals and vegetables found in
rocks. They are rarely, if ever, seen in unstratified rocks; but many
layers of stratified rocks abound in these remains. Whole skeletons as
well as single bones, whole tree-trunks as well as single leaves, are
found thus embedded in rock-layers, where in ages past the animal or
plant died and found a grave. They exist by thousands in many parts of
the world, varying in size from the huge skeleton of the elephant to
the tiny shell of the microscopic animalcule.

[Illustration: FOSSIL OF CARBONIFEROUS FERN.]

Fossils differ greatly in kind. Sometimes the entire shell or bone is
changed into stone, losing all its animal substance, but retaining its
old outline and its natural markings. Sometimes the fossil is merely
the hardened impress of the outside of a shell or leaf, which has
dented its picture on soft clay, and has itself disappeared, while the
soft clay has become rock, and the indented picture remains fixed
through after-centuries. Sometimes the fossil is the cast of the
inside of a shell; the said shell having been filled with soft mud,
which has taken its exact shape and hardened, while the shell itself
has vanished. The most complete description of fossil is the first of
these three kinds. It is wonderfully shown sometimes in fossil wood,
where all the tiny cells and delicate fibres remain distinctly marked
as of old, only the whole woody substance has changed into hard stone.

[Illustration: FOOTPRINTS FROM TRIASSIC SANDSTONE OF CONNECTICUT.]

But although the fossil remains of quadrupeds and other land-animals
are found in large quantities, their number is small compared with the
enormous number of fossil sea-shells and sea-animals.

[Illustration: FOSSIL FOOTPRINTS.]

Land-animals can, as a rule, have been so preserved, only when they
have been drowned in ponds or rivers, or mired in bogs and swamps, or
overtaken by frost, or swept out to sea.

Sea-animals, on the contrary, have been so preserved on land whenever
that land has been under the sea; and this appears to have been the
case, at one or another past age, with the greater part of our
present continents. These fossil remains of sea-animals are
discovered in all quarters of the world, not only on the seashore but
also far inland, not only deep down underground but also high up on
the tops of lofty mountains--a plain proof that over the summits of
those mountains the ocean must once have rolled, and this not for a
brief space only, but through long periods of time. And not on the
mountain-summit only are these fossils known to abound, but sometimes
in layer below layer of the mountain, from top to bottom, through
thousands of feet of rock.

[Illustration: FOSSIL SHELLS.]

This may well seem puzzling at first sight. Fossils of sea-creatures
on a mountain-top are startling enough; yet hardly so startling as the
thought of fossils _inside_ that mountain. How could they have found
their way thither?

The difficulty soon vanishes, if once we clearly understand that all
these thousands of feet of rock were built up slowly, layer after
layer, when portions of the land lay deep under the sea. Thus _each
separate layer_ of mud or sand or other material became in its turn
the _top layer_, and was for the time the floor of the ocean, until
further droppings of material out of the waters made a fresh layer,
covering up the one below.

While each layer was thus in succession the top layer of the building,
and at the same time the floor of the ocean, animals lived and died
in the ocean, and their remains sank to the bottom, resting upon the
sediment floor. Thousands of such dead remains disappeared, crumbling
into fine dust and mingling with the waters, but here and there one
was caught captive by the half-liquid mud, and was quickly covered and
preserved from decay. And still the building went on, and still layer
after layer was placed, till many fossils lay deep down beneath the
later-formed layers; and when at length, by slow or quick upheaval of
the ground, this sea-bottom became a mountain, the little fossils were
buried within the body of that mountain. So wondrously the matter
appears to have come about.

* * * * *

Another difficulty with respect to the stratified rocks has to be
thought of. All these layers or deposits of gravel, sand, or earth, on
the floor of the ocean, would naturally be horizontal--that is, would
lie flat, one upon another. In places the ocean-floor might slant, or
a crevice or valley or ridge might break the smoothness of the
deposit. But though the layers might partake of the slant, though the
valley might have to be filled, though the ridge might have to be
surmounted, still the general tendency of the waves would be to level
the dropping deposits into flat layers.

Then how is it that when we examine the strata of rocks in our
neighborhood, wherever that neighborhood may be, we do not find them
so arranged? Here, it is true, the lines for a space are nearly
horizontal, but there, a little way farther on, they are
perpendicular; here they are bent, and there curved; here they are
slanting, and there crushed and broken.

This only bears out what has been already said about the Book of
Geology. It _has_ been bent and disturbed, crushed and broken.

Great powers have been at work in this crust of our earth. Continents
have been raised, mountains have been upheaved, vast masses of rock
have been scattered into fragments. Here or there we may find the
layers arranged as they were first laid down; but far more often we
discover signs of later disturbance, either slow or sudden, varying
from a mere quiet tilting to a violent overturn.

[Illustration: EXAMPLE OF DISTURBANCE OF THE EARTH'S LAYERS.]

So the Book of Geology is a torn and disorganized volume, not easy to
read.

Yet, on the other hand, these very changes which have taken place are
a help to the geologist.

It may seem at first sight as if we should have an easier task, if the
strata were all left lying just as they were first formed, in smooth
level layers, one above another. But if it were so, we could know very
little about the lower layers.

We might indeed feel sure, as we do now, that the lowest layers were
the oldest and the top layers the newest, and that any fossils found
in the lower layers must belong to an age farther back than any
fossils found in the upper layers.

So much would be clear. And we might dig also and burrow a little way
down, through a few different kinds of rock, where they were not too
thick. But that would be all. There our powers would cease.

Now how different. Through the heavings and tiltings of the earth's
crust, the lower layers are often pushed quite up to the surface, so
that we are able to examine them and their fossils without the least
difficulty, and very often without digging underground at all.

You must not suppose that the real order of the rocks is changed by
these movements, for generally speaking it is not. The lower kinds are
rarely if ever found placed _over_ the upper kinds; only the ends of
them are seen peeping out above ground.

It is as if you had a pile of copy-books lying flat one upon another,
and were to put your finger under the lowest and push it up. All those
above would be pushed up also, and perhaps they would slip a little
way down, so that you would have a row of _edges_ showing side by
side, at very much the same height. The arrangement of the copy-books
would not be changed, for the lowest would still be the lowest in
actual position; but a general tilting or upheaval would have taken
place.

Just such a tilting or upheaval has taken place again and again with
the rocks forming our earth-crust. The edges of the lower rocks often
show side by side with those of higher layers.

But geologists know them apart. They are able to tell confidently
whether such and such a rock, peeping out at the earth's surface,
belongs really to a lower or a higher kind. For there is a certain
sort of order followed in the arrangement of rock-layers all over the
earth, and it is well known that some rocks are never found below some
other rocks, that certain particular kinds are never placed above
certain other kinds. Thus it follows that the fossils found in one
description of rock, must be the fossils of animals which lived and
died before the animals whose fossil remains are found in another
neighboring rock, just because this last rock-layer was built upon the
ocean-floor above and therefore later than the other.

All this is part of the foreign language of geology--part of the
piecing and arranging of the torn volume. Many mistakes are made; many
blunders are possible; but the mistakes and blunders are being
gradually corrected, and certain rules by which to read and understand
are becoming more and more clear.

It has been already said that unstratified rocks are those which have
been at some period, whether lately or very long ago, in a liquid
state from intense heat, and which have since cooled, either quickly
or slowly, crystallizing as they cooled.

Unstratified Rocks may be divided into two distinct classes.

[Illustration: SECTION OF A LAVA BOMB.]

First.--Volcanic Rocks, such as lava. These have been quickly cooled
at the surface of the earth, or not far below it.

Secondly.--Plutonic Rocks, such as granite. These have been slowly
cooled deep down in the earth under heavy pressure.

There is also a class of rocks, called metamorphic rocks, including
some kinds of marble. These are, strictly speaking, crystalline rocks,
and yet they are arranged in something like layers. The word
"metamorphic" simply means "transformed." They are believed to have
been once stratified rocks, perhaps containing often the remains of
animals; but intense heat has later transformed them into crystalline
rocks, and the animal remains have almost or quite vanished.

[Illustration: LAVA-STREAM ON VESUVIUS.]

Just as the different kinds of Stratified Rocks are often called
Aqueous Rocks, or rocks formed by the action of water--so these
different kinds of Unstratified Rocks are often called Igneous Rocks,
or rocks formed by the action of fire--the name being taken from the
Latin word for fire. The Metamorphic Rocks are sometimes described as
"Aqueo-igneous," since both water and fire helped in the forming of
them.

It was at one time believed, as a matter of certainty, that granite
and such rocks belonged to a period much farther back than the periods
of the stratified rocks. That is to say, it was supposed that
fire-action had come first and water-action second; that the fire-made
rocks were all formed in very early ages, and that only water-made
rocks still continued to be formed. So the name of Primary Rocks, or
First Rocks, was given to the granites and other such rocks, and the
name of Secondary Rocks to all water-built rocks; while those of the
third class were called Transition Rocks, because they seemed to be a
kind of link or stepping-stone in the change from the First to the
Second Rocks.

The chief reason for the general belief that fire-built rocks were
older than water-built ones was, that the former are as a rule found
to lie _lower_ than the latter. They form, as it were, the basement of
the building, while the top-stories are made of water-built rocks.

Many still believe that there is much truth in the thought. It is most
probable, so far as we are able to judge, that the _first-formed_
crust of rocks all over the earth was of cooled and crystallized
material. As these rocks were crumbled and wasted by the ocean,
materials would have been supplied for the building-up of rocks, layer
upon layer.

But this is conjecture. We cannot know with any certainty the course
of events so far back in the past. And geologists are now able to
state with tolerable confidence that, however old many of the granites
may be, yet a large amount of the fire-built rocks are no older than
the water-built rocks which lie over them.

So by many geologists the names of Primary, Transition, and Secondary
Formations are pretty well given up. It has been proposed to give
instead to the crystallized rocks of all kinds the name of Underlying
Rocks (Hypogene Rocks).

But if they really do lie under, how can they possibly be of the same
age? One would scarcely venture to suppose, in looking at a building,
that the cellars had not been finished before the upper floors.

True. In the first instance doubtless the cellars were first made,
then the ground-floor, then the upper stories.

When, however, the house was so built, alterations and improvements
might be very widely carried on above and below. While one set of
workmen were engaged in remodelling the roof, another set of workmen
might be engaged in remodelling the kitchens and first floor, pulling
down, propping up, and actually rebuilding parts of the lower walls.

This is precisely what the two great fellow-workmen, Fire and Water,
are ever doing in the crust of our earth. And if it be objected that
such alterations too widely undertaken might result in slips, cracks,
and slidings, of ceilings and walls in the upper stories, I can only
say that such catastrophes _have_ been the result of underground
alterations in that great building, the earth's crust....

We see therefore clearly that, although the earliest fire-made rocks
may very likely date farther back than the earliest water-made rocks,
yet the making of the two kinds has gone on side by side, one below
and the other above ground, through all ages up to the present moment.

And just as in the present day water continues its busy work above
ground of pulling down and building up, so also fire continues its
busy work underground of melting rocks which afterwards cool into new
forms, and also of shattering and upheaving parts of the earth-crust.

For there can be no doubt that fiery heat does exist as a mighty power
within our earth, though to what extent we are not able to say.

These two fellow-workers in nature have different modes of working.
One we can see on all sides, quietly progressing, demolishing land
patiently bit by bit, building up land steadily grain by grain. The
other, though more commonly hidden from sight, is fierce and
tumultuous in character, and shows his power in occasional terrific
outbursts.

We can scarcely realize what the power is of the imprisoned fiery
forces underground, though even we are not without some witness of
their existence. From time to time even our firm land has been felt to
tremble with a thrill from some far-off shock; and even in our country
is seen the marvel of scalding water pouring unceasingly from deep
underground....

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