spinal column extended to the end of the tail, whether the fins were
arranged equally or unequally on the sides; the paired side fins look
like limbs fringed with fins. Every Devonian fish of the gar type seems
to have had a lung to help out its gill-breathing.
In these traits the first fishes were much like the amphibians. They
were the parent stock from which branched later the true fishes and the
amphibians, as a single trunk parts into two main boughs. The trunk is
the connecting link.
The sea bottom was still thronged with crinoids, and lamp shells, and
cup corals. Shells of both clam and snail shapes are plentiful. The
chambered straight-horns are fewer and smaller, and coiled forms of this
type of shell are found. Trilobite forms are smaller, and their numbers
decrease.
The first land plants appeared during this age. Ferns and giant club
mosses and cycads grew in swampy ground. This was the beginning of the
wonderful fern forests that marked the next age, when coal was formed.
The rocks that bear the record of these living things in their fossils,
form strata of great thickness that overlie the Silurian deposits. There
is no break between them. So we understand that the sea changed its
shore-line only when the Silurian deposits rose to the water-level.
The Devonian sea was smaller than the Silurian. A great tract of
Devonian deposits occupies the lower half of the state of New York,
Canada between Lakes Erie and Huron, and the northern portions of
Indiana and Illinois. These older layers of the stratified rock are
covered with the deposits of later periods. Rivers that cut deep
channels reveal the earlier rocks as outcrops along their canyon walk.
The record of the age of fishes is, for the most part, still an unopened
book. The pages are sealed, waiting for the geologist with his hammer to
disclose the mysteries.
KING COAL
In this country, and in this age, who can doubt that coal is king? It is
one of the few necessities of life. In various sections of the country,
layers of coal have been discovered--some near the surface, others deep
underground. These are the storehouses of fuel which the coal miners dig
out and bring to the surface, and the railroads distribute. From
Pennsylvania and Ohio to Alabama stretches the richest coal-basin.
Illinois and Indiana contain another. From Iowa southward to Texas
another broad basin lies. Central Michigan and Nova Scotia each has
isolated coal-basins. All these have been discovered and mined, for they
lie in the oldest part of the country.
In the West, coal-beds have been discovered in several states, but many
regions have not yet been explored. Vast coal-fields, still untouched,
have been located in Alaska. The Government is trying to save this fuel
supply for coming generations. Many of the richest coal-beds from Nova
Scotia southward dip under the ocean. They have been robbed by the
erosive action of waves and running water. Glaciers have ground away
their substance, and given it to the sea. Much that remains intact must
be left by miners on account of the difficulties of getting out coal
from tilted and contorted strata.
As a rule, the first-formed coal is the best. The Western coal-fields
belong to the period following the Carboniferous Age. Although
conditions were favourable to abundant coal formation, Western coal is
not equal to the older, Eastern coal. It is often called _lignite_, a
word that designates its immaturity compared with anthracite.
Coal formed in the Triassic Period is found in a basin near Richmond,
Virginia. There is an abundance of this coal, but it has been subjected
to mountain-making pressure and heat, and is extremely inflammable. The
miners are in constant danger on account of coal gas, which becomes
explosive when the air of the shaft reaches and mingles with it. This
the miner calls "fire damp." North Carolina has coal of the same
formation, that is also dangerous to mine, and very awkward to reach, on
account of the crumpling of the strata.
There are beds of coal so pure that very little ash remains after the
burning. Five per cent, of ash may be reasonably expected in pure coal,
unmixed with sedimentary deposits. Such coal was formed in that part of
the swamp which was not stirred by the inflow of a river. Wherever muddy
water flowed in among the fallen stems of plants, or sand drifted over
the accumulated peat, these deposits remained, to appear later and
bother those who attempt to burn the coal.
[Illustration: Eocene fish]
[Illustration: _By permission of the American Museum of Natural History_
Trilobite from the Niagara limestone, Upper Silurian, of Western New
York]
[Illustration: Sigillaria, Stigmaria and Lepidodendron]
[Illustration: _By permission of the American Museum of Natural History_
Coal fern]
You know pure coal, that burns with great heat and leaves but little
ashes. You know also the other kind, that ignites with difficulty, burns
with little flame, gives out little heat, and dying leaves the furnace
full of ashes. You are trying to burn ancient mud that has but a small
proportion of coal mixed with it. The miners know good coal from poor,
and so do the coal dealers. It is not profitable to mine the impure part
of the vein. It costs as much to mine and ship as the best quality, and
it brings a much lower price.
The deeper beds of coal are better than those formed in comparatively
recent time and found lying nearer the surface. In many bogs a layer of
embedded root fibres, called peat, is cut into bricks and dried for
burning. Deeper than peat-beds lie the _lignites_, which are old beds of
peat, on the way to become coal. _Soft coal_ is older than lignite. It
contains thirty to fifty per cent. of volatile matter, and burns
readily, with a bright blaze. The richest of this bituminous coal is
called _fat_, or _fusing coal_. The bitumen oozes out, and the coal
cakes in burning. Ordinary soft coal contains less, but still we can see
the resinous bitumen frying out of it as it burns. There is more heat
and less volatile matter in _steam coal_, so-called because it is the
fuel that most quickly forms steam in an engine. _Hard coal_ contains
but five to ten per cent. of volatile matter. It is slow to ignite and
burns with a small blue blaze.
From peat to anthracite coal I have named the series which increases
gradually in the amount of heat it gives out, and increases and then
decreases in its readiness to burn and in the brightness of its flame.
Anthracite coal has the highest amount of fixed carbon. This is the
reason why it makes the best fuel, for fixed carbon is the substance
which holds the store of imprisoned sunlight, liberated as heat when the
coal burns. Tremendous pressure and heat due to shrinking of the earth's
crust have crumpled and twisted the strata containing coal in eastern
Pennsylvania, and thus changed bituminous coal into anthracite. Ohio
beds, formed at the same time, but undisturbed by heat and pressure, are
bituminous yet.
The coal-beds of Rhode Island are anthracite, but the coal is so hard
that it will not burn in an open fire. The terrible, mountain-making
forces that crumpled these strata and robbed the coal of its volatile
matter, left so little of the gas-forming element, that a very special
treatment is necessary to make the carbon burn. It is used successfully
in furnaces built for the smelting of ores.
The last stage in the coal series is a black substance which we know as
black lead, or graphite. We write with it when we use a "lead" pencil.
This is anthracite coal after all of the volatile matter has been driven
out of it. It cannot burn, although it is solid carbon. The beds of
graphite have been formed out of coal by the same changes in the
earth's crust which have converted soft coal into anthracite.
The tremendous pressure that bears on the coal measures has changed a
part of the carbon into liquid and gaseous form. Lakes of oil have been
tapped from which jets of great force have spouted out. Such
accumulations of oil usually fill porous layers of sandstone and are
confined by overlying and underlying beds of impervious clay. Gas may be
similarly confined until a well is drilled, relieving the pressure, and
furnishing abundant or scanty supply of this valuable fuel. Western
Pennsylvania coal-fields have beds of gas and oil. If mountain-making
forces had broken the strata, as in eastern Pennsylvania, the gas and
the oil would have been lost by evaporation.
This is what happened in the anthracite coal-belt.
HOW COAL WAS MADE
The broad, rounded dome of a maple tree shades my windows from the
intense heat of this August day. The air is hot, and every leaf of the
tree's thatched roof is spread to catch the sunlight. The carbon in the
air is breathed in through openings on the under side of each leaf. The
sap in the leaf pulp uses the carbon in making starch. The sun's heat is
absorbed. It is the energy that enables the leaf-green to produce a
wonderful chemical change. Out of soil water, brought up from the roots,
and the carbonic acid gas, taken in from the air, rich, sugary starch is
manufactured in the leaf laboratory.
This plant food returns in a slow current, feeding the growing cells
under the bark, from leaf tip to root tip, throughout the growing tree.
The sap builds solid wood. The maple tree has been built out of muddy
water and carbon gas. It stands a miracle before our eyes. In its tough
wood fibres is locked up all the heat its leaves absorbed from the sun,
since the day the maple seed sprouted and the first pair of leaves
lifted their palms above the ground.
If my maple tree should die, and fall, and lie undisturbed on the
ground, it would slowly decay. The carbon of its solid frame would pass
back into the air, as gas, and the heat would escape so gradually that I
could not notice it at all, unless I thrust my hand into the warm,
crumbling mass.
If my tree should be cut down to-day and chopped into stove wood, it
would keep a fire in my grate for many months.
Burning destroys wood substance a great deal faster than decay in the
open air does, but the processes of rotting and burning are alike in
this: each process releases the carbon, and gives it back to the air. It
gives back also the sun's heat, stored while the tree was growing. There
is left on the ground, and in the ashes on the hearth, only the mineral
substance taken up in the water the roots gathered underground.
If my tree stood in swampy ground and fell over under a high wind, the
water that covered it and saturated its substance would prevent decay.
The carbon would not be allowed to escape as a gas to the air; the woody
substance would become gradually changed into _peat_. In this form it
might remain for thousands of years, and finally be changed into coal.
Whether it was burned while yet in the condition of peat, or millions of
years later, when it was transformed into coal, the heat stored in its
substance was liberated by the burning. The carbon and the heat went
back to the air.
Every green plant we see spreads its leaves to the sun. Every stick of
wood we burn, and every lump of coal, is giving back, in the form of
light and heat, the energy that came from sunshine and was captured by
the green leaves. How long the wood has held this store of heat we may
easily compute, for we can read the age of a tree. But the age of coal
we cannot accurately state. The years probably should be counted by
millions, instead of thousands.
The great inland sea that covered the middle portion of the continent
during the Silurian and the Devonian periods, became shallow by the
deposit of vast quantities of sediment. Along the lines of the deposits
of greatest thickness, a crumpling of the earth's crust lifted the first
fold of the Alleghany Mountains as a great sea wall on the east, and on
the western shore another formed the beginning of the Ozark Mountain
system in Missouri. An island was lifted out of the sea, forming the
elevated ground on which the city of Cincinnati now stands. Various
other ridges and islands divided the ancient sea into much smaller
bodies of water. Hemmed in by land these shallow sea-basins gradually
changed into fresh-water lakes, for they no longer had connection with
the ocean, and all the water they received came from rain. After
centuries of freshets, and of filling in with the rock debris brought by
the streams, they became great marshes, in which grew water-loving
plants. Generation after generation of these plants died, and their
remains, submerged by the water, were converted into peat. In the
course of ages this peat became coal. This is the history of the coal
measures.
There is no guesswork here. The stems of plants do not lose their
microscopic structure in all the ages it has taken to transform them to
coal. A thin section of coal shows under a magnifier the structure of
the stems of the coal-forming plants. Moreover, the veins of coal
preserve above or below them, in shales that were once deposits of mud,
the branching trunks of trees, perfectly fossilized. There are no better
proofs of the vegetable origin of coal than the lumps themselves. But
they are plain to the naked eye, while the coal tells its story to the
man with the microscope.
The fossil remains of the plants that flourished when coal was forming
are gigantic, compared with plants of the same families now living. We
must conclude that the climate was tropical, the air very heavy with
moisture, and charged much more heavily than it is now with carbonic
acid gas.
These conditions produced, in rapid succession, forests of tree ferns
and horsetails and giant club mosses. These are the three types of
plants out of which the coal was made. They were all rich in resin,
which makes the coal burn readily. The ferns had stems as large as tree
trunks. Some have been found that are eighteen inches in diameter. We
know they are ferns, because the leaves are found with their fruits
attached to them in the manner of present-day ferns. The stems show the
well known scar by which fern leaves are joined. And the wood of these
fossil fern stems is tubular in structure, just as the wood of living
ferns is to-day.
Among the ferns which dominated these old marsh forests grew one kind,
the scaly leaves of which covered the stems and bore their fruits on the
branching tips. These giants, some of them with trunks four feet in
diameter, belong to the same group of plants as our creeping club
mosses, but in the ancient days they stood up among the other ferns as
trees forty or fifty feet high.
The giant scouring rushes, or horsetails, had the same general
characteristics as the little reed-like plants we know by those names
to-day.
The highest plants of the coal period were leafy trees with nut-like
fruits, that resemble the yew trees of the present. These gigantic trees
were the first conifers upon the earth. They foreshadowed the pines and
the other cone-bearing evergreens. Their leaves were broad and their
fruits nut-like. The Japanese ginkgo, or maidenhair fern tree, is an
old-fashioned conifer somewhat like those first examples of this family.
Trunks sixty to seventy feet long, crowned with broad leaves and a spike
of fruit, have been found lying in the upper layers of the coal-seams,
and in sandstone strata that lie between the strata of coal. Peculiar
circular discs, which the microscope reveals along the sides of the wood
fibres of these fossil trees, prove the wood structure to be like that
of modern conifers.
Generation after generation of forests lived and died in the vast
spreading swamps of this era. The land sank, and freshets came here and
there, drowning out all plant life, and covering the layers of peat with
beds of sand or mud. When the water went down, other forests took
possession, and a new coal-bed was started. It is plainly seen that
flooding often put an end to coal formation. Fifteen seams of coal, one
above another, is the greatest number that have been found. The veins
vary from one inch to forty feet in thickness. These are separated by
layers of sandstone or shale, which accumulated as sediment, covering
the stumps of dead tree ferns and other growths, and preserving them as
fossils to tell the story of those bygone ages as plainly as any other
record could have done.
Fresh-water animals succeeded those of salt water in the swamps that
formed the coal measures. Overhead, the first insects flitted among the
branches of the tree ferns. Dragon-flies darted above the surface and
dipped in water as they do to-day. Spiders, scorpions, and cockroaches,
all air-breathing insects, were represented, but none of the higher,
nectar-loving insects, like flies and bees and butterflies, were there.
Flowering plants had not yet appeared on the earth. Snakelike
amphibians, some fishlike, some lizard-like, and huge crocodilian forms
appeared for the first time. These air-breathing swamp-dwellers could
not have lived in salt water.
Fresh-water molluscs and land shells appear for the first time as
fossils in the rocks of the coal measures. On the shores of the ocean,
the rocks of this period show that trilobites, horseshoe crabs, and
fishes still lived in vast numbers, and corals continued to form
limestone. The old types of marine animals changed gradually, but the
coal measures show strikingly different fossils. These rocks bear the
first record of fresh-water and land animals.
THE MOST USEFUL METAL
It is fortunate for us all that, out of the half-dozen so-called useful
metals, iron, which is the most useful of them all to the human race,
should be also the most plentiful and the cheapest. Aluminum is abundant
in the common clay and soil under our feet. But separating it is still
an expensive process; so that this metal is not commercially so
plentiful as iron is, nor is it cheap.
All we know of the earth's substance is based on studies of the
superficial part of its crust, a mere film compared with the eight
thousand miles of its diameter. Nobody knows what the core of the
earth--the great globe under this surface film--is made of; but we know
that it is of heavier material than the surface layer; and geologists
believe that iron is an important element in the central mass of the
globe.
One thing that makes this guess seem reasonable is the great abundance
of iron in the earth's crust. Another thing is that meteors which fall
on the earth out of the sky prove to be chiefly composed of iron. All of
their other elements are ones which are found in our own rocks. If we
believe that the earth itself is a fragment of the sun, thrown off in a
heated condition and cooling as it flew through space, we may consider
it a giant meteor, made of the substances we find in the chance meteor
that strikes the earth.
Iron is found, not only in the soil, but in all plant and animal bodies
that take their food from the soil. The red colour in fruits and
flowers, and in the blood of the higher animals, is a form in which iron
is familiar to us. It does more, perhaps, to make the world beautiful
than any other mineral element known.
But long before these benefits were understood, iron was the backbone of
civilization. It is so to-day. Iron, transformed by a simple process
into steel, sustains the commercial supremacy of the great civilized
nations of the world. The railroad train, the steel-armoured battleship,
the great bridge, the towering sky-scraper, the keen-edged tool, the
delicate mechanism of watches and a thousand other scientific
instruments--all these things are possible to-day because iron was
discovered and has been put to use.
It was probably one of the cave men, poking about in his fire among the
rocks, who discovered a lump of molten metal which the heat had
separated from the rest of the rocks. He examined this "clinker" after
it cooled, and it interested him. It was a new discovery. It may have
been he, or possibly his descendants, who learned that this metal could
be pounded into other shapes, and freed by pounding from the pebbles
and other impurities that clung to it when it cooled. The relics of
iron-tipped spears and arrows show the skill and ingenuity of our early
ancestors in making use of iron as a means of killing their prey. The
earliest remains of this kind have probably been lost because the iron
rusted away.
Man was pretty well along on the road to civilization before he learned
where iron could be found in beds, and how it could be purified for his
use. We now know that certain very minute plants, which live in quiet
water, cause iron brought into that water to be precipitated, and to
accumulate in the bottom of these boggy pools. In ancient days these bog
deposits of iron often alternated with coal layers. Millions of years
have passed since these two useful substances were laid down. To-day the
coal is dug, along with the bog iron. The coal is burned to melt the
iron ore and prepare it for use. It is a fortunate region that produces
both coal and iron.
Bituminous coal is plentiful, and scattered all over the country, while
anthracite is scarce. The cheapest iron is made in Alabama, which has
its ore in rich deposits in hillsides, and coal measures close by,
furnishing the raw material for coke. The result is that the region of
Birmingham has become the centre of great wealth through the development
of iron and coal mines.
Where water flows over limestone rock, and percolates through layers of
this very common mineral, it causes the iron, gathered in these rock
masses, to be deposited in pockets. All along the Appalachian Mountains
the iron has been gathered in beds which are being mined. These beds of
ore are usually mixed with clay and other earthy substances from which
the metal can be separated only by melting. The ore is thrown into a
furnace where the metal melts and trickles down, leaving behind the
non-metallic impurities. It is drawn off and run into moulds, where it
cools in the form of "pig" iron.
The first fuel used in the making of pig iron from the ore was charcoal.
In America the early settlers had no difficulty in finding plenty of
wood. Indeed, the forests were weeds that had to be cut down and burned
to make room for fields of grain. The finding of iron ore always started
a small industry in a colony. If there was a blacksmith, or any one else
among the small company who understood working in iron, he was put in
charge.
To make the charcoal, wood was cut and piled closely in a dome-shaped
heap, which was tightly covered with sods, except for a small opening
near the ground. In this a fire was built, and smothered, but kept going
until all the wood within the oven was charred.
This fuel burned readily, with an intense heat, and without ashes.
Sticks of charcoal have the form of the wood, and they are stiff enough
to hold up the ore of iron so that it cannot crush out the fire. For a
long time American iron was supplied by little smelters, scattered here
and there. The workmen beat the melted metal on the forge, freeing it
from impurities, and shaping the pure metal into useful articles.
Sometimes they made it into steel, by a process learned in the Old
World.
The American iron industry, which now is one of the greatest in the
world, centres in Pittsburg, near which great deposits of iron and coal
lie close together. The making of coke from coal has replaced the
burning of charcoal for fuel. When the forests were cut away by
lumbermen, the supply of charcoal threatened to give out, and
experiments were made in charring coal, which resulted in the successful
making of coke, a fuel made from coal by a process similar to the making
of charcoal from wood. The story of the making of coke out of hard and
soft coal is a long one, for it began as far back as the beginning of
the nineteenth century.