sea was still too hot for living things to exist in it. The land above
the water-line was bare rocks. These were rapidly being crumbled by the
action of the air, which was not the mild, pleasant air we know, but was
full of destructive gases, breathed out through cracks in the thin crust
of the earth from the heated mass below. If you stand on the edge of a
lava lake, like one of those on the islands of the Hawaiian group, the
stifling fumes that rise might make you feel as if you were back at the
beginning of the earth's history, when the solid crust was just a thin
film on an unstable sea of molten rock, and this volcano but one of the
vast number of openings by which the boiling lava and the condensed
gases found their way to the surface. Then the rivers ran black with the
waste of the rocky earth they furrowed, and there was no vegetation to
soften the bleakness of the landscape.
The beginnings of life on the earth are a mystery. Nobody can guess the
riddle. The earliest rocks were subjected to great heat. It is not
possible that life could have existed in the heated ocean or on the
land. Gradually the shores of the seas became filled up with sediment
washed down by the rivers. Layer on layer of this sediment accumulated,
and it was crumpled by pressure, and changed by heat, so that if any
plants or animals had lived along those old shores their remains would
have been utterly destroyed.
Rocks that lie in layers on top of these oldest, fire-scarred
foundations of the earth show the first faint traces of living things.
Limestone and beds of iron ore are signs of the presence of life. The
first animals and plants lived in the ancient seas.
From the traces that are left, we judge that the earliest life forms
were of the simplest kind, like some plants and animals that swim in a
drop of water. Have you ever seen a drop of pond water under a compound
microscope? It is a wonder world you look into, and you forget all the
world besides. You are one of the wonderful higher animals, the highest
on the earth. You focus on a shapeless creature that moves about and
feels and breathes, but hasn't any eyes or mouth or stomach--in fact, it
is the lowest form of animal life, and one of the smallest. It is but
one of many animal forms, all simple in structure, but able to feed and
grow and reproduce their kind.
Gaze out of the window on the garden, now. The flowering plants, the
green grass, and the trees are among the highest forms of plants. In the
drop of water under the microscope tiny specks of green are floating.
They belong to the lowest order of plants. Among the plant and the
animal forms that have been studied and named, are a few living things
the places of which in the scale are not agreed upon. Some say they are
animals; some believe they are plants. They are like both in some
respects. It is probable that the first living things were like these
confusing, minute things--not distinctly plants or animals, but the
parent forms from which, later on, both plants and animals sprang.
The lowest forms of life, plant and animal, live in water to-day. They
are tiny and their bodies are made of a soft substance like the white of
an egg. If these are at all like the living creatures that swarmed in
the early seas, no wonder they left no traces in the rocks of the early
part of the age when life is first recorded by fossils. Soft-bodied
creatures never do.
Some of the animals and the plants in the drop of water under the
microscope have body walls of definite shapes, made of lime, or of a
glassy substance called silica. When they die, these "skeletons" lie at
the bottom of the water, and do not decay, as the living part of the
body does, because they are mineral. Gradually a number of these shells,
or hard skeletons, accumulate. In a glass of pond water they are found
at the bottom, amongst the sediment. In a pond how many thousands of
these creatures must live and their shells fall to the bottom at last,
buried in the mud!
So it is easy to understand why the first creatures on earth left no
trace. The first real fossils found in the rocks are the hard shells or
skeletons of the first plants and animals that had hard parts.
AN ANCIENT BEACH AT EBB TIDE
When the tide is out, the rocks on the Maine coast have plenty of living
creatures to prove this northern shore inhabited. Starfishes lurk in the
hollows, and the tent-shaped shells of the little periwinkle encrust the
wet rocks. Mussels cling to the rocks in clumps, fastened to each other
by their ropes of coarse black hair. The furry coating of sea mosses
that encrust the rocks is a hiding-place for many kinds of living
things, some soft-bodied, some protected by shells. The shallow water is
the home of plants and animals of many different kinds. As proof of this
one finds dead shells and fragments of seaweeds strewn on the shore
after a storm.
Along the outer shores of the Cape Cod peninsula and down the Jersey
coast, the sober colouring of the shells of the north gives way to a
brighter colour scheme. In the warmer waters, life becomes gayer, if we
may judge by the rich tints that ornament the shells. The kinds of
living creatures change. They are larger and more abundant. The seaweeds
are more varied and more luxuriant in growth.
When we reach the shores of the West Indian islands and the keys of
Florida the greatest abundance and variety of living forms are found.
The submerged rocks blossom with flower-like sea anemones of every
colour. Corals, branching like trees and bushes on the sea floor, form
groves under water. Among them brilliant-hued fishes swim, and highly
ornamented shells glide, as people know who have gazed through the glass
bottoms of the boats built especially to show visitors the wonderful sea
gardens at Nassau, Bahama Islands, and at Santa Catalina Island,
southern California.
On every beach the skeletons of animals which die help to build up the
land; though the process is not so rapid in the north as on the shores
that approach the tropics. The coast of Florida has a rim of island
reefs around it built out of coral limestone. Indeed, the peninsula was
built by coral polyps. Houses in St. Augustine are built of coquina
rock, which is simply a mass of broken shells held together by a lime
cement. Every sea beach is packed with shells and other remnants of
animals and plants that live in the shallow waters. Deeper and deeper
year by year the sand buries these skeletons, and many of them are
preserved for all time.
Thus what is sandy beach to-day may, a few million years from now, be
uncovered as a ledge of sandstone with the prints of waves distinctly
shown, and fossil shells of molluscs, skeletons of fishes, and branches
of seaweed--all of them different from those then growing upon the
earth.
In the neighbourhood of Cincinnati there have been uncovered banks of
stone accumulated along the border of an ancient sea. From the sides of
granite highlands streams brought down the sand built into these oldest
sandstone rocks. The fine mud which now appears as beds of slate was the
decay of feldspar and hornblende in the same granite. Limestone beds are
full of the fossil shells of creatures that lived in the shallow seas.
Their skeletons, accumulating on the bottom, formed deep layers of
limestone mud. These rocks preserve, by the fossils they contain, a
great variety of shellfish which had limy skeletons. The sea fairly
swarmed along its shallow margin with these creatures. We might not
recognize many of the shells and other curious fossils we find in the
rock uncovered by the workmen who are cutting a railroad embankment.
They are not exactly like the living forms that grow along our beaches
to-day, but they are enough like them for us to know that they lived
along the seashore, and if we had time to examine all the rocks of this
kind preserved in a museum we should decide that seashore life was quite
as abundant then as it is now. The pressed specimens of plants of those
earliest seashores are mere imprints showing that they were pulpy and
therefore gradually decayed. Only their shape is recorded by dark stains
made by each branching part. The decay of the vegetable tissue painted
the outline on the rock which when split apart shows us what those
ancient seaweeds looked like. They belonged to the group of plants we
call kelp, or tangle, which are still common enough in the sea, though
the forms we now have are not exactly like the old ones. Seaweeds belong
to the very lowest forms of plants.
[Illustration: Crinoid from Indiana]
[Illustration: _By permission of the American Museum of Natural History_
Ammonite from Jurassic of England]
[Illustration: _By permission of the American Museum of Natural History_
Fossil corals Coquina, Hippurite limestone]
The limestones are full of fossils of corals. Indeed, there must have
been reefs like those that skirt Florida to-day built by these
lime-building polyps. Their forms are so well preserved in the rocks
that it is possible to know just how they looked when they grew in the
shallows.
One very common kind is called a cup coral, because the polyp formed a
skeleton shaped like a cup. The body wall surrounded the skeleton, and
the arms or tentacles rose from the centre of the funnel-like depression
in the top. Little cups budded off from their parents, but remained
attached, and at length the skeletons of all formed great masses of limy
rock. Some cup corals grew in a solid mass, the new generation forming
an outer layer, thus burying the parent cups.
A second type of corals in these oldest limestones is the honeycomb
group. The colonies of polyps lived in tubes which lengthened gradually,
forming compact, limy cylinders like organ pipes, fitted close together.
The living generation always inhabited the upper chambers of the tubes.
A third type is the chain coral, made of tubes joined in rows, single
file like pickets of a fence. But these walls bend into curious
patterns, so that the cross-section of a mass of them looks like a
complex pattern of crochet-work, the irregular spaces fenced with chain
stitches. Each open link is a pit in which a polyp lived.
Among the corals are sprays of a fine feathery growth embedded in the
limestone. Fine, straight, splinter-like branches are saw-toothed on one
or both edges. These limy fossils might not be seen at all, were they
not bedded in shales, which are very fine-grained. Here again are the
skeletons of animals. Each notch on each thread-like branch was the home
of a tiny animal, not unlike a sea anemone and a coral polyp.
To believe this story it is necessary only to pick up a bit of dead
shell or floating driftwood on which a feathery growth is found. These
plumes, like "sea mosses," as they are called, are not plants at all,
but colonies of polyps. Each one lived in a tiny pit, and these pits
range one above the other, so as to look like notches on the thread-like
divisions of the stem. Put a piece of this so-called "sea moss" in a
glass of sea water, and in a few moments of quiet you will see, by the
use of a magnifying glass, the spreading arms of the polyp thrust out of
each pit.
The ancient seas swarmed with these living hydrozoans, and their remains
are found preserved as fossils in the shales which once were beds of
soft mud.
The hard shells of sea urchins and starfishes are made of lime. In the
ancient seas, starfishes were rare and sea urchins did not exist, but
all over the sea bottom grew creatures called crinoids, the soft parts
of which were enclosed in limy protective cases and attached to rocks on
the sea bottom by means of jointed stems. No fossils are more plentiful
in the early limestones than these wonderful "stone lilies." Indeed, the
crinoidal limestone seemed to be built of the skeletons of these
animals. The lily-like body was flung open, as a lily opens its calyx,
when the creature was feeding. But any alarm caused the tentacles to be
drawn in, and the petal-like divisions of the body wall to close tightly
together, till that wall looked like an unopened bud.
On the bottom of the Atlantic, near the Bahama Islands, these stone
lilies are still found growing. Their jointed stems and body parts are
as graceful in form and motion as any lily. The creature's mouth is in
the centre of the flower-like top, and it feeds like the sea urchin, on
particles obtained in the sea water.
The old limestones contain great quantities of "lamp shells," which are
old-fashioned bivalves. Their shells remind us of our bivalve clams and
scallops, but the internal parts were very different. The gills of clams
and oysters are soft parts. Inside of the lamp shells are coiled, bony
arms, supporting the fringed gills.
It is fortunate for us that a few lamp shells still live in the seas. By
studying the soft parts of these living remnants of a very old race we
can know the secrets of the lives of those ancient lamp shells, the
soft parts of which were all washed away, and the fossil shells of which
are preserved. Gradually the lamp shells died out, and the modern
bivalves have come to take their places. Just so, the ancient crinoids
are now almost extinct; the sea urchins and the starfishes have
succeeded them.
The chambered nautilus has its shell divided by partitions and it lives
in the outer chamber, a many-tentacled creature, that is a close
relative of the soft-bodied squid. In the ancient seas the same family
was represented by huge creatures the shells of which were chambered,
but not coiled. Their abundance and great size are proved by the rocks
in which their fossils are preserved. Some of them must have been the
rulers of the sea, as sharks and whales are to-day. Fossil specimens
have been found more than fifteen feet long and ten inches in diameter
in the ancient rocks of some of the Western States. It is possible to
read from the lowest rock formations upward, the rise of these sea
giants and their gradual decline. Certain strata of limestone contain
the last relics of this race, after which they became extinct. As the
straight-chambered forms diminished, great coiled forms became more
abundant, but all died out.
One of the most abundant fossil animals in ancient rocks is called a
trilobite. Its body is divided by two grooves into three parts, a
central ridge extending the whole length of the body and two side
ridges. The front portion of the shell formed the head shield, and
behind the main body part was a little tail shield. The skeleton was
formed of many movable jointed plates, and the creature had eyes set in
the head shield just as the king crab's are set. Jointed legs in pairs
fringed each side of the body. Each leg had two branches, one for
walking, the other for swimming. A pair of feelers rose from the head.
The body could be rolled into a ball when danger threatened, by bringing
head and tail together.
These remarkable, extinct trilobites were the first crustaceans. Their
nearest living relative to-day is the horseshoe crab. The fresh-water
crayfish and the lobster are more distant relatives: so are the shrimps
and the prawns. No such abundance of these creatures exists to-day as
existed when the trilobites thronged the shallows. So well preserved are
these skeletons that, although there are no living trilobites for
comparison, it is possible to find out from the fossil enough about
their structure to know how they fed and lived their lives along with
the straight-horns which were the scavengers of those early seas and the
terror of smaller creatures. The trilobites throve, and, dying, left
their record in the rocks; then disappeared entirely. We find their
fossils in a great variety of forms, shapes, and sizes. The smallest is
but a fraction of an inch long, the largest twenty inches long.
The ancient rocks, in which these lower forms of life have left their
fossils, are known as the Silurian system. The time in which these rocks
were accumulating under the seas covers a vast period. We call it the
Age of Invertebrates, because these soft-bodied, hard-shelled animals,
the crinoids, the molluscs, and the trilobites, with bony external
skeletons and no backbones, were the most abundant. They overshadowed
all other forms of life. The rocks of this wonderful series were formed
on the shores of a great inland sea that covered the central portion of
North America. In the ages that followed, these rocks were covered
deeply with later sediments. But the upheavals of the crust have broken
open and erosion has uncovered these strata in different regions.
Geologists have found written there, page upon page, the record of life
as it existed in the early seas.
THE LIME ROCKS
"Hard" water and "soft" water are very different. The rain that falls
and fills our cisterns is not softer or more delightful to use than the
well water in some favoured regions. In it, soap makes beautiful, creamy
suds, and it is a real pleasure to put one's hands into it. But in hard
water soap seems to curdle, and some softening agent like borax has to
be added or the water will chap the hands. There is little satisfaction
in using water of this kind for any purpose.
Hard water was as soft as any when it fell from the sky; but the rain
water trickled into the ground and passed through rocks containing lime.
Some of this mineral was absorbed, for lime is readily soluble in water.
Clear though it may be, water that has lime in it has quite a different
feeling from rain water. Blow the breath into a basin of hard water, and
a milky appearance will be noted. The carbonic acid gas exhaled from the
lungs unites with the invisible lime, causing it to become visible
particles of carbonate of lime, which fall to the bottom of the basin.
Nearly all well water is hard. So is the water of lakes and rivers and
the ocean, for limestone is one of the most widely distributed rocks in
the surface of the earth. Rain water makes its way into the earth's
crust, absorbs mineral substances, and collects in springs which feed
brooks and rivers and lakes. Wells are holes in the ground which bore
into water-soaked strata of sand.
We gain something from the lime dissolved in hard water, for it is an
essential part of our food. We must drink a certain amount of water each
day to keep the body in perfect health. The lime in this water goes
chiefly to the building of our bones. Plant roots take up lime in the
water that mounts as sap through the plant bodies. We get some of the
lime we need in vegetable foods we eat.
All of the kingdom of vertebrate animals, from the lowest forms to the
highest, all of the shell-bearing animals of sea and land, require lime.
Many of the lower creatures especially these in the sea, such as corals
and their near relatives, encase themselves in body walls of lime. They
absorb the lime from the sea water, and deposit it as unconsciously as
we build the bony framework of our bodies.
All the bone and shell-bearing creatures that die on the earth and in
the sea restore to the land and to the water the lime taken by the
creatures while they lived. Carbonic acid gas in the water greatly
hastens the dissolving of dead shells. Carbonic acid gas, whether free
in the air, or absorbed by percolating water, hastens the dissolving of
skeletons of creatures that die upon land. Then the raw materials are
built again into lime rocks underground.
The lime rocks are the most important group in the list of rocks that
form the crust of the earth. They are made of the elements calcium,
carbon, and oxygen, yet the different members of this calcite group
differ widely in composition and appearance. So do oyster shells and
beef bones, though both contain quantities of carbonate of lime.
Calcite is a soft mineral, light in weight, sometimes white, but oftener
of some other colour. It may be found crystallized or not. Whenever a
drop of acid touches it, a frothy effervescence occurs. The drop of acid
boils up and gives off the pungent odour of carbonic acid gas.
The reason that calcite is hard to find in rocks is that percolating
water, charged with acids, is constantly stealing it, and carrying it
away into the ocean. The rocks that contain it crumble because the limy
portions have been dissolved out.
Some limestones resist the destructive action of water. When they are
impregnated with silica they become transformed into marble, which takes
a high polish like granite. Acids must be strong to make any impression
on marble.
The thick beds of pure limestone that underlie the surface soil in
Kentucky and in parts of Virginia sometimes measure several hundred feet
in thickness, a single stratum often being twenty feet thick. They are
all horizontal, for they were formed on sea bottom, and have not been
crumpled in later time. The dead bodies of sea creatures contributed
their shells and skeletons to the lime deposit on the sea bottom. Who
can estimate the time it took to form those thick, solid layers of lime
rock? The animals were corals, crinoids, and molluscs. Little sand and
clay show in the lime rock of this period, before the marshes of the
Carboniferous Age took the place of the ancient inland sea of the
Subcarboniferous Period, the sedimentary accumulations of which we are
now talking about.
The living corals one sees in the shallow water of the Florida coast
to-day are building land by building up their limy skeletons. The reefs
are the dead skeletons of past generations of these tiny living things.
They take in lime from the water, and use it as we use lime in building
our bones. In each case it is an unconscious process of animal
growth--not a "building process" like a mason's building of a wall. Many
people think that the coral polyp builds in this way. They give it
credit for patience in a great undertaking. All the polyp does is to
feed on whatever the water supplies that its digestive organs can use.
It is like a sea anemone in appearance and in habits of life. It is not
at all like an insect. Yet it is common to hear people speak of the
"coral insect"! Do not let any one ever hear you repeat such a mistake.
Southern Florida is made out of coral rock, but thinly covered with
soil. It was made by the growth of reef after reef, and it is still
growing.
The Cretaceous Period of the earth's eventful history is named for the
lime rock which we know as chalk. Beds of this recent kind of limestone
are found in England and in France, pure white, made of the skeletons of
the smallest of lime-consuming creatures, Foraminifera. They swarmed in
deep water, and so did minute sponge animalcules and plant forms called
Diatoms that took silica from the water, and formed their hard parts of
this glassy substance. The result is seen in the nodules of flint found
in the soft, snow-white chalk. Did you ever use a piece of chalk that
scratched the black-board? The flint did it. Have you ever seen the
chalk cliffs of Dover? When you do see them, notice how they gleam white
in the sun. See how the rains have sculptured those cliffs. The
prominences left standing out are strengthened by the flint they
contain. Chalk beds occur in Texas and under our great plains; but the
principal rocks of the age in America were sandstones and clays.
THE AGE OF FISHES
The first animal with a backbone recorded its existence among the
fossils found in rocks of the upper Silurian strata. It is a fish; but
the earliest fossils are very incomplete specimens. We know that these
old-fashioned fishes were somewhat like the sturgeons of our rivers.
Their bodies were encased in bony armour of hard scales, coated with
enamel. The bones of the spine were connected by ball and socket joints,
and the heads were movable. In these two particulars the fishes
resembled reptiles. The modern gar-pike has a number of the same
characteristics.
Another backboned creature of the ancient seas was the ancestral type of
the shark family. In some points this old-fashioned shark reminds us of
birds and turtles. These early fishes foreshadowed all later
vertebrates, not yet on the earth. After them came the amphibians, then
the reptiles, then the birds, and latest the mammals.
The race of fishes began, no doubt, with forms so soft-boned that no
fossil traces are preserved in the rocks. When those with harder bones
appeared, the fossil record began, and it tells the story of the passing
of the early, unfish-like forms, and the coming of new kinds, great in
size and in numbers, that swarmed in the seas, and were tyrants over all
other living things. They conquered the giant straight-horns and
trilobites, former rulers of the seas.
[Illustration: _By permission of the American Museum of Natural History_
A sixteen-foot fossil fish from Cretaceous of Kansas, with a modern
tarpon]
[Illustration: _By permission of the American Museum of Natural History_
Canon Diablo meteorite from Arizona]
One of these giant fishes fifteen to twenty feet long, three feet wide,
had jaws two feet long, set with blade-like teeth. Devonian rocks in
Ohio have yielded fine fossils of gigantic fishes and sharks.