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....
Think of the tremendous eruptions of Vesuvius, of Etna, of Hecla, of
Mauna Loa. Think of whole towns crushed and buried, with their
thousands of living inhabitants. Think of rivers of glowing lava
streaming up from regions below ground, and pouring along the surface
for a distance of forty, fifty, and even sixty miles, as in Iceland
and Hawaii. Think of red-hot cinders flung from a volcano-crater to a
height of ten thousand feet. Think of lakes of liquid fire in other
craters, five hundred to a thousand feet across, huge cauldrons of
boiling rock. Think of showers of ashes from the furnace below of yet
another, borne so high aloft as to be carried seven hundred miles
before they sank to earth again. Think of millions of red-hot stones
flung out in one eruption of Vesuvius. Think of a mass of rock, one
hundred cubic yards in size, hurled to a distance of eight miles or
more out of the crater of Cotopaxi.
[Illustration: HOT WELLS.]
Think also of earthquake-shocks felt through twelve hundred miles of
country. Think of fierce tremblings and heavings lasting in constant
succession through days and weeks of terror. Think of hundreds of
miles of land raised several feet in one great upheaval. Think of the
earth opening in scores of wide-lipped cracks, to swallow men and
beasts. Think of hot mud, boiling water, scalding stream, liquid rock,
bursting from such cracks, or pouring from rents in a mountain-side.
Truly these are signs of a state of things in or below the solid crust
on which we live, that may make us doubt the absolute security of
"Mother Earth."
Different explanations have been put forward to explain this seemingly
fiery state of things underground.
Until lately the belief was widely held that our earth was one huge
globe of liquid fire, with only a slender cooled crust covering her, a
few miles in thickness.
This view was supported by the fact that heat is found to increase as
men descend into the earth. Measurements of such heat-increase have
been taken, both in mines and in borings for wells. The usual rate is
about one degree more of heat, of our common thermometer, for every
fifty or sixty feet of descent. If this were steadily continued, water
would boil at a depth of eight thousand feet below the surface; iron
would melt at a depth of twenty-eight miles; while at a depth of forty
or fifty miles no known substance upon earth could remain solid.
The force of this proof is, however, weakened by the fact that the
rate at which the heat increases differs very much in different
places. Also it is now generally supposed that such a tremendous
furnace of heat--a furnace nearly eight thousand miles in
diameter--could not fail to break up and melt so slight a covering
shell.
Many believe, therefore, not that the whole interior of the earth is
liquid with heat, but that enormous fire-seas or lakes of melted rock
exist here and there, under or in the earth-crust. From these lakes
the volcanoes would be fed, and they would be the cause of earthquakes
and land-upheavals or land-sinkings. There are strong reasons for
supposing that the earth was once a fiery liquid body, and that she
has slowly cooled through long ages. Some hold that her centre
probably grew solid first from tremendous pressure; that her crust
afterwards became gradually cold; and that between the solid crust and
the solid inside or "nucleus," a sea of melted rock long existed, the
remains of which are still to be found in these tremendous fiery
reservoirs.
The idea accords well with the fact that large numbers of extinct or
dead volcanoes are scattered through many parts of the earth. If the
above explanation be the right one, doubtless the fire-seas in the
crust extended once upon a time beneath such volcanoes, but have since
died out or smouldered low in those parts.
A somewhat curious calculation has been made, to illustrate the
different modes of working of these two mighty powers--Fire and Water.
The amount of land swept away each year in mud, and borne to the ocean
by the River Ganges, was roughly reckoned, and also the amount of land
believed to have been upheaved several feet in the great Chilian
earthquake.
It was found that the river, steadily working month by month, would
require some four hundred years to carry to the sea the same weight of
material, which in one tremendous effort was upheaved by the fiery
underground forces.
Yet we must not carry this distinction too far. Fire does not always
work suddenly, or water slowly; witness the slow rising and sinking of
land in parts of the earth, continuing through centuries; and witness
also the effects of great floods and storms.
The crust of the earth is made of rock. But what is rock made of?
Certain leading divisions of rocks have been already considered:
The Water-made Rocks;
The Fire-made Rocks, both Plutonic and Volcanic;
The Water-and-Fire-made Rocks.
The first of these--Water-made Rocks--may be subdivided into three
classes. These are,--
I. _Flint Rocks_; II. _Clay Rocks_; III. _Lime Rocks_.
This is not a book in which it would be wise to go closely into the
mineral nature of rocks. Two or three leading thoughts may, however,
be given.
Does it not seem strange that the hard and solid rocks should be in
great measure formed of the same substances which form the thin
invisible air floating around us?
Yet so it is. There is a certain gas called Oxygen Gas. Without that
gas you could not live many minutes. Banish it from the room in which
you are sitting, and in a few minutes you will die.
This gas makes up nearly one-quarter by weight of the atmosphere round
the whole earth.
The same gas plays an important part in the ocean; for more than
three-quarters of water is _oxygen_.
It plays also an important part in rocks; for about half the material
of the entire earth's crust is oxygen.
Another chief material in rocks is _silicon_. This makes up
one-quarter of the crust, leaving only one-quarter to be accounted
for. Silicon mixed with oxygen makes silica or quartz. There are few
rocks which have not a large amount of quartz in them. Common flint,
sandstones, and the sand of our shores, are made of quartz, and
therefore belong to the first class of Silicious or Flint Rocks.
Granites and lavas are about one-half quartz. The beautiful stones,
amethyst, agate, chalcedony, and jasper, are all different kinds of
quartz.
Another chief material in rocks is a white metal called _aluminium_.
United to oxygen it becomes alumina, the chief substance in clay.
Rocks of this kind--such as clays, and also the lovely blue gem,
sapphire--are called Argillaceous Rocks, from the Latin word for clay,
and belong to the second class. Such rocks keep fossils well.
Another is _calcium_. United to oxygen and carbonic acid, it makes
carbonate of lime, the chief substance in limestone; so all limestones
belong to the third class of Calcareous or Lime Rocks.
Other important materials may be mentioned, such as _magnesium,
potassium, sodium, iron, carbon, sulphur, hydrogen, chlorine,
nitrogen_. These, with many more, not so common, make up the remaining
quarter of the earth-crust.
Carbon plays as important a part in animal and vegetable life as
silicon in rocks. Carbon is most commonly seen in three distinct
forms--as charcoal, as black-lead, and as the pure brilliant diamond.
Carbon united, in a particular proportion, to oxygen, forms carbonic
acid; and carbonic acid united, in a particular proportion, to lime,
forms limestone.
_Hydrogen_ united to oxygen forms water. Each of these two gases is
invisible alone, but when they meet and mingle they form a liquid.
_Nitrogen_ united to oxygen and to a small quantity of carbonic acid
gas forms our atmosphere.
Rocks of pure flint, pure clay, or pure lime, are rarely or never met
with. Most rocks are made up of several different substances melted
together.
* * * * *
In the fire-built rocks no remains of animals are found, though in
water-built rocks they abound. Water-built rocks are sometimes divided
into two classes--those which only contain occasional animal remains,
and those which are more or less built up of the skeletons of animals.
[Illustration: AMIBA PRINCEPS, ONE OF THE MANY ORDERS OF THE RHIZOPODA
CLASS, MAGNIFIED ONE HUNDRED TIMES.]
There are some exceedingly tiny creatures inhabiting the ocean, called
Rhizopods. They live in minute shells, the largest of which may be
almost the size of a grain of wheat, but by far the greater number are
invisible as shells without a microscope, and merely show as fine
dust. The rhizopods are of different shapes, sometimes round,
sometimes spiral, sometimes having only one cell, sometimes having
several cells. In the latter case a separate animal lives in each
cell. The animal is of the very simplest as well as the smallest kind.
He has not even a mouth or a stomach but can take in food at any part
of his body.
[Illustration: RHIZOPODS (MAGNIFIED).]
These rhizopods live in the oceans in enormous numbers. Tens of
millions are ever coming into existence, living out their tiny lives,
dying, and sinking to the bottom.
There upon the ocean-floor gather their remains, a heaped-up multitude
of minute skeletons or shells, layer forming over layer.
It was long suspected that the white chalk cliffs of England were
built up in some such manner as this through past ages. And now at
length proof has been found, in the shape of mud dredged up from the
ocean-bottom--mud entirely composed of countless multitudes of these
little shells, dropping there by myriads, and becoming slowly joined
together in one mass.
Just so, it is believed, were the white chalk cliffs built--gradually
prepared on the ocean-floor, and then slowly or suddenly upheaved, so
as to become a part of the dry land.
Think what the enormous numbers must have been of tiny living
creatures, out of whose shells the wide reaches of white chalk cliffs
have been made. Chalk cliffs and chalk layers extend from Ireland,
through England and France, as far as to the Crimea. In the south of
Russia they are said to be six hundred feet thick. Yet one cubic inch
of chalk is calculated to hold the remains of more than one million
rhizopods. How many countless millions upon millions must have gone to
the whole structure! How long must the work of building up have
lasted!
[Illustration: THREE POLYPS OF CORAL.]
These little shells do not always drop softly and evenly to the
ocean-floor, to become quietly part of a mass of shells. Sometimes,
where the ocean is shallow enough for the waves to have power below,
or where land currents can reach, they are washed about, and thrown
one against another, and ground into fine powder; and the fine powder
becomes in time, through different causes, solid rock.
[Illustration: CORAL POLYP.]
Limestone is made in another way also. In the warm waters of the South
Pacific Ocean there are many islands, large and small, which have
been formed in a wonderful manner by tiny living workers. The workers
are soft jelly-like creatures, called polyps, who labor together in
building up great walls and masses of coral.
[Illustration: CORAL ISLAND.]
[Illustration: YOUNG CORAL POLYP ATTACHED TO A ROCK AND EXPANDED.]
They never carry on their work above the surface of the water, for in
the air they would die. But the waves break the coral, and heap it up
above high-water mark, and carry earth and seeds to drop there till at
length a small low-lying island is formed.
The waves not only heap up broken coral, but they grind the coral into
fine powder, and from this powder limestone rock is made, just as it
is from the powdered shells of rhizopods. The material used by the
polyps in building the coral is chiefly lime, which they have the
power of gathering out of the water, and the fine coral-powder,
sinking to the bottom, makes large quantities of hard limestone. Soft
chalk is rarely, if ever, found near the coral islands.
[Illustration: 1. WHITE CORAL. 2. PORTION OF A BRANCH (MAGNIFIED).]
Limestones are formed in the same manner from the grinding up of other
sea-shells and fossils, various in kind; the powder becoming gradually
united into solid rock.
There is yet another way in which limestone is made, quite different
from all these. Sometimes streams of water have a large quantity of
lime in them; and these as they flow will drop layers of lime which
harden into rock. Or a lime-laden spring, making its way through the
roof of an underground cavern, will leave all kinds of fantastic
arrangements of limestone wherever its waters can trickle and drip.
Such a cavern is called a "stalactite cave."
* * * * *
So there are different kinds of fossil rock-making. There may be rocks
made of other materials, with fossil simply buried in them. There may
be rocks made entirely of fossils, which have gathered in masses as
they sank to the sea-bottom, and have there become simply and lightly
joined together. There may be rocks made of the ground-up powder of
fossils, pressed into a solid substance or united by some other
substance.
Rocks are also often formed of whole fossils, or stones, or shells,
bound into one by some natural soft sticky cement, which has gathered
round them and afterwards grown hard, like the cement which holds
together the stones in a wall.
The tiny rhizopods (meaning root foot) which have so large a share in
chalk and limestone making, are among the smallest and simplest known
kinds of animal life.
There are also some very minute forms of vegetable life, which exist
in equally vast numbers, called Diatoms. For a long while they were
believed to be living animals, like the rhizopods. Scientific men are
now, however, pretty well agreed that they really are only vegetables
or plants.
The diatoms have each one a tiny shell or shield, not made of lime
like the rhizopod-shells, but of flint. Some think that common flint
may be formed of these tiny shells.
Again, there is a kind of rock called Mountain Meal, which is entirely
made up of the remains of diatoms. Examined under the microscope,
thousands of minute flint shields of various shapes are seen. This
rock, or earth, is very abundant in many places, and is sometimes used
as a polishing powder. In Bohemia there is a layer of it no less than
fourteen feet thick. Yet so minute are the shells of which it is
composed, that one square inch of rock is said to contain about four
thousand millions of them. Each one of these millions is a separate
distinct fossil....
* * * * *
[Illustration: SUCCESSION OF BURIED COAL-GROWTHS AND ERECT
TREE-STUMPS. SYDNEY, CAPE BRETON.
_a._ Sandstone, _b._ Shales, _c._ Coal-seams, _d._ Bed containing
Roots and Stumps _in situ_.]
If you examine carefully a piece of coal, you will find, more or less
clearly, markings like those which are seen in a piece of wood.
Sometimes they are very distinct indeed. Coal abounds in impressions
of leaves, ferns, and stems, and fossil remains of plants and
tree-trunks are found in numbers in coal-seams.
Coal is a vegetable substance. The wide coal-fields of Britain and
other lands are the _fossil_ remains of vast forests.
Long ages ago, as it seems, broad and luxuriant forests flourished
over the earth. In many parts generation after generation of trees
lived and died and decayed, leaving no trace of their existence,
beyond a little layer of black mould, soon to be carried away by wind
and water. Coal could only be formed where there were bogs and
quagmires.
But in bogs and quagmires, and in shallow lakes of low-lying lands,
there were great gatherings of slowly-decaying vegetable remains,
trees, plants, and ferns all mingling together. Then after a while the
low lands would sink and the ocean pouring in would cover them with
layers of protecting sand or mud; and sometimes the land would rise
again, and fresh forests would spring into life, only to be in their
turn overwhelmed anew, and covered by fresh sandy or earthy deposits.
These buried forests lay through the ages following, slowly hardening
into the black and shining coal, so useful now to man.
The coal is found thus in thin or thick seams, with other rock-layers
between, telling each its history of centuries long past. In one place
no less than sixteen such beds of coal are found, one below another,
each divided from the next above and the next underneath by beds of
clay or sand or shale. The forests could not have grown in the sea,
and the earth-layers could not have been formed on land, therefore
many land-risings and sinkings must have taken place. Each bed
probably tells the tale of a succession of forests....
* * * * *
Before going on to a sketch of the early ages of the Earth's
history--ages stretching back long long before the time of Adam--it is
needful to think yet for a little longer about the manner in which
that history is written, and the way in which it has to be read.
For the record is one difficult to make out, and its style of
expression is often dark and mysterious. There is scarcely any other
volume in the great Book of Nature, which the student is so likely to
misread as this one. It is very needful, therefore, to hold the
conclusions of geologists with a light grasp, guarding each with a
"perhaps" or a "may be." Many an imposing edifice has been built, in
geology, upon a rickety foundation which has speedily given way.
In all ages of the world's history up to the present day, rock-making
has taken place--fire-made rocks being fashioned underground, and
water-made rocks being fashioned above ground though under water.
Also in all ages different kinds of rocks have been fashioned side by
side--limestone in one part of the world, sandstone in another, chalk
in another, clay in another, and so on. There have, it is true, been
ages when one kind seems to have been the _chief_ kind--an age of
limestone, or an age of chalk. But even then there were doubtless more
rock-buildings going on, though not to so great an extent. On the
other hand, there may have been ages during which no limestone was
made, or no chalk, or no clay. As a general rule, however, the various
sorts of rock-building have probably gone on together. This was not so
well understood by early geologists as it is now.
The difficulty is often great of disentangling the different strata,
and saying which was earlier and which later formed.
Still, by close and careful study of the rocks which compose the
earth's crust, a certain kind of order is found to exist, more or less
followed out in all parts of the world. _When_ each layer was formed
in England or in America, the geologist cannot possibly say. He can,
however, assert, in either place, that a certain mass of rock was
formed before a certain other mass in that same place, even though
the two may seem to lie side by side; for he knows that they were so
placed only by upheaval, and that once upon a time the one lay beneath
the other.
The geologist can go further. He can often declare that a certain mass
of rock in America and a certain mass of rock in England, quite
different in kind, were probably built up at about the same time. How
long ago that time was he would be rash to attempt to say; but that
the two belong to the same age he has good reason for supposing.
We find rocks piled upon rocks in a certain order, so that we may
generally be pretty confident that the lower rocks were first made,
and the upper rocks the latest built. Further than this, we find in
all the said layers of water-built rocks signs of past life.
As already stated, much of this life was ocean-life, though not all.
Below the sea, as the rock-layers were being formed, bit by bit, of
earth dropping from the ocean to the ocean's floor, sea-creatures
lived out their lives and died by thousands, to sink to that same
floor. Millions passed away, dissolving and leaving no trace behind;
but thousands were preserved--shells often, animals sometimes.
Nor was this all. For now and again some part of the sea-bottom was
upheaved, slowly or quickly, till it became dry land. On this dry land
animals lived again, and thousands of them, too, died, and their bones
crumbled into dust. But here and there one was caught in bog or frost,
and his remains were preserved till, through lapse of ages, they
turned to stone.
Yet again that land would sink, and over it fresh layers were formed
by the ocean-waters, with fresh remains of sea-animals buried in with
the layers of sand or lime; and once more the sea-bottom would rise,
perhaps then to continue as dry land, until the day when man should
discover and handle these hidden remains.
Now note a remarkable fact as to these fossils, scattered far and wide
through the layers of stratified rock.
In the uppermost and latest built rocks the animals found are the
same, in great measure, as those which now exist upon the earth.
Leaving the uppermost rocks, and examining those which lie a little
way below, we find a difference. Some are still the same, and others,
if not quite the same, are very much like what we have now; but here
and there a creature of a different form appears.
Go deeper still, and the kinds of animals change further. Fewer and
fewer resemble those which now range the earth; more and more belong
to other species.
Descend through layer after layer till we come to rocks built in
earliest ages and not one fossil shall we find precisely the same as
one animal living now.
So not only are the rocks built in successive order, stratum after
stratum belonging to age after age in the past, but fossil-remains
also are found in successive order, kind after kind belonging to past
age after age.
Although in the first instance the succession of fossils was
understood by means of the succession of rock-layers, yet in the
second place the arrangement of rock-layers is made more clear by the
means of these very fossils.
A geologist, looking at the rocks in America, can say which there were
first-formed, which second-formed, which third-formed. Also, looking
at the rocks in England, he can say which there were first-formed,
second-formed, third-formed. He would, however, find it very
difficult, if not impossible, to say which among any of the American
rocks was formed at about the same time as any particular one among
the English rocks, were it not for the help afforded him by these
fossils.
Just as the regular succession of rock-strata has been gradually
learned, so the regular succession of different fossils is becoming
more and more understood. It is now known that some kinds of fossils
are always found in the oldest rocks, and in them only; that some
kinds are always found in the newest rocks, and in them only; that
some fossils are rarely or never found lower than certain layers; that
some fossils are rarely or never found higher than certain other
layers.
So this fossil arrangement is growing into quite a history of the
past. And a geologist, looking at certain rocks, pushed up from
underground, in England and in America, can say: "These are very
different kinds of rocks, it is true, and it would be impossible to
say how long the building up of the one might have taken place before
or after the other. But I see that in both these rocks there are
exactly the same kinds of fossil-remains, differing from those in the
rocks above and below. I conclude therefore that the two rocks belong
to about the same great age in the world's past history, when the
same animals were living upon the earth."
Observing and reasoning thus, geologists have drawn up a general plan
or order of strata; and the whole of the vast masses of water-built
rocks throughout the world have been arranged in a regular succession
of classes, rising step by step from earliest ages up to the present
time.
[Illustration]
AMERICA THE OLD WORLD
(FROM GEOLOGICAL SKETCHES.)
BY L. AGASSIZ.
[Illustration]