freely through. The roots are more comfortable, and the tiny root hairs
can reach the particles of both kinds of mineral food. But the needful
third element is decaying plant and animal substances, called "humus."
These enrich the soil, but they do a more important thing: their decay
hastens the release of plant food from the earthy part of the soil, and
they add to it a sticky element which has a wonderful power to attract
and hold the water that soaks into the earth.
_What is the best garden soil?_ A mixture of sand, clay, and humus is
called "loam." If sand predominates, it is a sandy loam--warm, mellow
soil. If clay predominates, we have a clay loam--a heavy, rich, but
cool soil. All gradations between the two extremes are suited to the
needs of crops, from the melons on sandy soil, to celery that prefers
deep, cool soil, and cranberries that demand muck--just old humus.
_How do plant roots feed in soil?_ By means of delicate root hairs which
come into contact with particles of soil around which a film of soil
water clings. This fluid dissolves the food, and the root absorbs the
fluid. Plants can take no food in solid form. Hence it is of the
greatest importance to have the soil pulverized and spongy, able to
absorb and hold the greatest amount of water. The moisture-coated soil
particles must have air-spaces between them. Air is as necessary to the
roots as to the tops of growing plants.
_Why does the farmer plough and harrow and roll the land?_ To pulverize
the soil; to mellow and lighten it; to mix in thoroughly the manure he
has spread on it, and to reach, if he can, the deeper layers that have
plant food which the roots of his crops have not yet touched. Killing
weeds is but a minor business, compared with tillage.
Later, ploughing or cultivating the surface lightly not only destroys
the weeds, but it checks the loss of water by evaporation from the
cracks that form in dry weather. Raking the garden once a day in dry
weather does more good than watering it. The "dust mulch" acts as a cool
sunguard over the roots.
_The process of soil-making._ If the man chopping wood in the Yosemite
Valley looks about him he can see the soil-making forces at work on a
grand scale. The bald, steep front of El Capitan is of the hardest
granite, but it is slowly crumbling, and its fragments are accumulating
at the bottom of the long slope. Rain and snow fill all crevices in the
rocks. Frost is a wonderful force in widening these cracks, for water
expands when it freezes. The loosened rock masses plough their way down
the steep, gathering, as they go, increasing power to tear away any
rocks in their path.
Wind blows finer rock fragments along, and they lodge in cracks. Fine
dust and the seeds of plants are lodged there. The rocky slopes of the
Yosemite Valley are all more or less covered with trees and shrubs that
have come from wind-sown seeds. These plants thrust their roots deeper
each year into the rock crevices. The feeding tips of roots secrete
acids that eat away lime and other substances that occur in rocks. Dead
leaves and other discarded portions of the trees rot about their roots,
and form soil of increasing depth. The largest trees grow on the rocky
soil deposited at the base of the slope. The tree's roots prevent the
river from carrying it off.
When granite crumbles, its different mineral elements are separated.
Clear, glassy particles of quartz we call sand. Dark particles of
feldspar become clay, and may harden into slate. Sand may become
sandstone. Exposed slate and sandstone are crumbled by exposure to wind
and frost and moving water, and are deposited again as sand-bars and
beds of clay.
The most interesting phase of soil study is the discovery of what a work
the humble earthworm does in mellowing and enriching the soil.
THE WORK OF EARTHWORMS
The farmer and the gardener should expect very poor crops if they
planted seed without first ploughing or spading the soil. Next, its fine
particles must be separated by the breaking of the hard clods. A wise
man ploughs heavy soil in the fall. It is caked into great clods which
crumble before planting time. The water in the clods freezes in winter.
The expansion due to freezing makes this soil water a force that
separates the fine particles. So the frost works for the farmer.
Just under the surface of the soil lives a host of workers which are our
patient friends. They work for their living, and are perhaps unconscious
of the fact that they are constantly increasing the fertility of the
soil. They are the earthworms, also called fishworms, which are
distributed all over the world. They are not generally known to farmers
and gardeners as friendly, useful creatures, and their services are
rarely noticed. We see robins pulling them out of the ground, and we are
likely to think the birds are ridding us of a garden pest. What we need
is to use our eyes, and to read the wonderful discoveries recorded in a
book called "Vegetable Mould and Earthworms," written by Charles
Darwin.
The benefits of ploughing and spading are the loosening and pulverizing
of the packed earth; the mixing of dead leaves and other vegetation on
and near the surface with the more solid earth farther down; the letting
in of water and air; and the checking of loss of water through cracks
the sun forms by baking the soil dry.
The earthworm is a creature of the dark. It cannot see, but it is
sufficiently sensitive to light to avoid the sun, the rays of which
would shrivel up its moist skin. Having no lungs or gills, the worm uses
the skin as the breathing organ; and it must be kept moist in order to
serve its important use. This is why earthworms are never seen above
ground except on rainy days, and never in the top soil if it has become
dry. In seasons of little rain, they go down where the earth is moist,
and venture to the surface only at night, when dew makes their coming up
possible.
Earthworms have no teeth, but they have a long snout that protrudes
beyond the mouth. Their food is found on and in the surface soil. They
will eat scraps of meat by sucking the juices, and scrape off the pulp
of leaves and root vegetables in much the same way. Much of their
subsistence is upon organic matter that can be extracted from the soil.
Quantities of earth are swallowed. It is rare that an earthworm is dug
up that does not show earth pellets somewhere on their way through the
long digestive canal. The rich juices of plant substance are absorbed
from these pellets as they pass through the body.
Earthworms explore the surface of the soil by night, and pick up what
they can find of fresh food. Nowhere have I heard of them as a nuisance
in gardens, but they eagerly feed on bits of meat, especially fat, and
on fresh leaves. They drag all such victuals into their burrows, and
begin the digestion of the food by pouring on it from their mouths a
secretion somewhat like pancreatic juice.
The worms honeycomb the earth with their burrows, which are long,
winding tubes. In dry or cold weather these burrows may reach eight feet
under ground. They run obliquely, as a rule, from the surface, and are
lined with a layer of the smooth soil, like soft paste, cast from the
body. The lining being spread, the burrow fits the worm's body closely.
This enables it to pass quickly from one end to the other, though it
must wriggle backward or forward without turning around.
At the lower end of the burrow, an enlarged chamber is found, where
hibernating worms coil and sleep together in winter. At the top, a
lining of dead leaves extends downward for a few inches, and in day time
a plug of the same material is the outside door. At night the worm comes
to the surface, and casts out the pellets of earth swallowed. The burrow
grows in length by the amount of earth scraped off by the long snout
and swallowed. The daily amount of excavation done is fairly estimated
by the castings observed each morning on the surface.
One earthworm's work for the farmer is not very much, but consider how
many are at work, and what each one is doing. It is boring holes through
the solid earth, and letting in the surface water and the air. It is
carrying the lower soil up to the surface, often the stubborn subsoil,
that no plough could reach. It is burying and thus hastening the decay
of plant fibre, which lightens heavy soil and makes it rich because it
is porous. Moreover, the earthworms are doing over and over again this
work of fining and turning over the soil, which the plough does but
seldom.
By the continuous carrying up of their castings, the earthworms
gradually bury manures spread on the surface. The collapse of their
burrows and the making of new ones keep the soil constantly in motion.
The particles are being loosened and brought into contact with the soil
water, that dissolves, and thus frees for the use of feeding roots, the
plant food stored in the rock particles that compose the mineral part of
the soil.
The weight of earth brought to the surface by worms in the course of a
year has been carefully estimated. Darwin gives seven to eighteen tons
per acre as the lowest and highest reports, based on careful collecting
of castings by four observers, working on small areas of totally
different soils. In England, earthworms have done a great deal more
toward burying boulders and ancient ruins than any other agency. They
eagerly burrow under heavy objects, the weight of which causes them to
crush the honeycombed earth. Undiscouraged, the earthworms repeat their
work.
"Long before man existed, the land was regularly ploughed, and continues
still to be ploughed by earthworms. It may be doubted whether there are
many other animals which have played so important a part in the history
of the world as have these lowly organized creatures."
After years of study, Charles Darwin came to this conclusion. The more
we study the lives of these earth-consuming creatures, the more fully do
we believe what the great nature student said. The fertile soil is made
of rock meal and decayed leaves and roots. Only recently have ploughs
been invented. But the great forest crops have grown in soil made mellow
by the earthworm's ploughing.
QUIET FORCES THAT DESTROY ROCKS
Wind and water are the blustering active agents we see at work tearing
down rocks and carrying away their particles. They do the most of this
work of levelling the land; but there are quiet forces at work which
might not attract our attention at all, and yet, without their help,
wind and running water would not accomplish half the work for which they
take the credit.
The air contains certain destructive gases which by their chemical
action separate the particles of the hardest rocks, causing them to
crumble. Now the wind blows away these crumbling particles, and the
solid unchanged rock beneath is again exposed to the crumbling agencies.
The changes in temperature between day and night cause rocks to contract
and expand, and these changes put a strain upon the mineral particles
that compose them. Much scaling of rock surfaces is due to these causes.
Building a fire on top of a rock, and then dashing water upon the heated
mass, shatters it in many directions. This process merely intensifies
the effect produced by the mild changes of winter and summer. Water is
present in most rocks, in surprising quantities, often filling the
spaces in porous rocks like sandstones.
When winter brings the temperature down to the freezing point, the water
near the surface of the rock first feels it. Ice forms, and every
particle of water is swollen by the change. A strain is put upon the
mineral particles against which the particles of ice crowd for more
room. Frost is a very powerful agent in the crumbling of rocks, as well
as of stubborn clods of earth. In warm climates, and in desert regions
where there is little moisture in the rocks, this destructive action of
freezing water is not known. In cold countries, and in high altitudes,
where the air is heavy with moisture, its greatest work is done.
Some kinds of rock decay when they become dry, and resist crumbling
better when they absorb a certain amount of moisture. Alternate wetting
and drying is destructive to certain rocks.
One of the unnoticed agents of rock decay is the action of lowly plants.
Mosses grow upon the faces of rocks, thrusting their tiny root processes
into pits they dig deeper by means of acids secreted by the delicate
tips. You have seen shaded green patches of lichens, like little rugs,
of different shapes, spread on the surface of rocks. But you cannot see
so well the work these growths are doing in etching away the surface,
and feeding upon the decaying mineral substance.
Mosses and lichens do a mighty work, with the help of water, in
reducing rocks to their original elements, and thus forming soil. No
plants but lichens and mosses can grow on the bare faces of rocks. As
their root-like processes lengthen and go deeper into the rock face,
particles are pried off, and the under-substance is attacked. Higher
plants then find a footing. Have you not seen little trees growing on a
patch of moss which gets its food from the air and the rock to which it
clings? The spongy moss cushion soaks up the rain and holds it against
the rock face. A streak of iron in the rock may cause the water to
follow and rust it out, leaving a distinct crevice. Now the roots of any
plant that happens to be growing on the moss may find a foot-hold in the
crack. Streaks of lime in a rock readily absorb water, which gradually
dissolves and absorbs its particles, inviting the roots to enter these
new passages and feed upon the disintegrating minerals. Dead leaves
decay, and the acids the trickling water absorbs from them are
especially active in disintegrating lime rocks.
From such small beginnings has resulted the shattering of great rock
masses by the growth of plants upon them. Tree roots that grow in rock
crevices exert a power that is irresistible. The roots of smaller plants
do the same great work in a quieter way.
When a hurricane or a flood tears down the mountain-side, sweeping
everything before it, trees, torn out by the roots, drag great masses
of rock and soil into the air, and fling them down the slope. Wind and
water thus finish the destruction which the humble mosses and lichens
began. What seemed an impregnable fortress of granite has crumbled into
fragments. Its particles are reduced to dust, or are on the way to this
condition. The plant food locked up in granite boulders becomes
available to hungry roots. Forests, grain-fields, and meadows cover the
work of destructive agencies with a mantle of green.
HOW ROCKS ARE MADE
The granite shaft is made out of the original substance of the earth's
crust. Its minerals are the elements out of which all of the rock masses
of the earth are formed, no matter how different they look from granite.
Sandstone is made of particles of quartz. Clay and slate are made out of
feldspar and mica. Iron ore comes from the hornblende in granite. The
mineral particles, reassembled in different proportions, form all of the
different rocks that are known.
Here in my hand is a piece of pudding-stone. It is made of pebbles of
different sizes, each made of different coloured minerals. The pebbles
are cemented together with a paste that has hardened into stone. This
kind of rock the geologists call _conglomerate_. Pudding-stone is the
common name, for the pebbles in the pasty matrix certainly do suggest
the currants and the raisins that are sprinkled through a Christmas
pudding.
Under the seashores there are forming to-day thick beds of sand. The
rivers bring the rock material down from the hills, and it is sorted and
laid down. The moving water drops the heaviest particles near shore, and
carries the finer ones farther out before letting them fall.
[Illustration: The town of Cripple Creek, Colorado, which has grown up
like magic since 1891, covers the richest gold and silver mines in the
world]
[Illustration: The level valley is filled up with fine rock flour washed
from the sides of the neighboring mountains]
The hard water, that comes through limestone rocks, adds lime in
solution to the ocean water. All the shellfish of the sea, and the
creatures with bony skeletons, take in the bone-building, shell-making
lime with their food. Generations of these inhabitants of the sea have
died, and their shells and bones have accumulated and been transformed
into thick beds of limestone on the ocean floor. This is going on
to-day; but the limestone does not accumulate as rapidly as when the
ocean teemed with shell-bearing creatures of gigantic size. Of these we
shall speak in another chapter.
The fine dust that is blown into the ocean from the land, and that makes
river water muddy, accumulates on the sea bottom as banks of mud, which
by the burden of later deposits is converted into clay. Sandstone is but
the compressed sand-bank.
In the study of mountains, geologists have discovered that old seashores
were thrown up into the first great ridges that form the backbone of a
mountain system. The Rocky Mountains, and the Appalachian system on the
east, were made out of thick strata of rocks that had been formed by
accumulations of mud and sand--the washings of the land--on the opposite
shores of a great mid-continental sea, that stretched from the crest of
one great mountain system across to the other, and north and south from
the Laurentian Hills to the Gulf of Mexico. The great weight of the
accumulating layers of rock materials on one side, and the wasted land
surfaces on the other, made the sea border a line of greatest weakness
in the crust of the earth. The shrinking of the globe underneath caused
the break; mashing and folding followed, throwing the ridge above
sea-level, and making dry land out of rock waste which had been
accumulating, perhaps for millions of years, under the sea. The
wrinkling of the earth's crust was the result of crushing forces which
produced tremendous heat.
Streams of lava sprang out through the fissures and poured streams of
melted rock down the sides of the fold, quite burying, in many places,
the layers of limestone, sandstone, and clay. Between the strata of
water-formed rocks there were often created chimney-like openings, into
which molten rock from below was forced, forming, when cool, veins and
dikes of rock material, specimens of the substance of the earth's
interior.
Tremendous pressure and heat, acting upon stratified rocks saturated
with water transform them into very different kinds of rock. Limestone,
subjected to these forces, is changed into marble. Clays are transformed
into slates. Sandstone is changed into quartzite, the sand grains being
melted so as to become no longer visible to the naked eye. The
anthracite coal of the Pennsylvania mountains is the result of heat and
pressure acting upon soft coal. Associated with these beds of hard coal
are beds of black lead, or graphite, the substance used in making "lead"
pencils. We believe that the same forces that operated to transform clay
rocks into slate, and limestone into marble, transformed soft coal into
hard, and hard coal into graphite, in the days when the earth was young.
The word _sedimentary_ is applied to rocks which were originally laid
down under water, as sediment, brought by running water, or by wind, or
by the decay of organic substances. _Stratified_ rocks are those which
are arranged in layers. Sedimentary rocks will fall into this class.
_Aqueous_ rocks are those which are formed under water. Most of the
stratified and sedimentary rocks, but not all, may be included under
this term. Rocks that are made out of fragments of other rocks torn down
by the agencies of erosion are called _fragmental_. Wind, water, and ice
are the three great agencies that wear away the land, bring rock
fragments long distances, and deposit them where aqueous rocks are being
formed. Volcanic eruptions bring material from the earth's interior.
This material ranges all the way from huge boulders to the finest
impalpable dust, called volcanic ashes. Rivers of ice called glaciers
crowd against their banks, loosening rock masses and carrying away
fragments of all sizes, in their progress down the valley. Brooks and
rivers carry the pebbles and the larger rock masses they are able to
loosen from their walls and beds, and grind them smooth as they move
along toward lower levels.
The air itself causes rocks to crumble; percolating water robs them of
their soluble salts, reducing even solid granite to a loose mass of
quartz grains and clay. Plants and animals absorb as food the mineral
substances of rocks, when they are dissolved in water. They transform
these food elements into their own body substance, and finally give back
their dead bodies, the mineral substances of which are freed by decay to
return to the earth, and become elements of rock again.
The decay of rock is well shown by the materials that accumulate at the
base of a cliff. Angular fragments of all sizes, but all more or less
flattened, come from strata of shaly rock, that can be seen jutting out
far above. A great deal of this sort of material is found mingled with
the soil of the Northeastern States. Round pebbles in pudding-stone have
been formed in brook beds and deposited on beaches where they have
become caked in mud and finally consolidated into rock. If the beach
chanced to be sandy instead of muddy, a matrix of sandy paste holds the
larger pebbles in place. Limestone paste cements together the pebbles of
limestone conglomerates.
In St. Augustine many of the houses are built of coquina rock, a mass of
broken shells which have become cemented together by lime mud, derived
from their own decay. On the slopes of volcanoes, rock fragments of all
kinds are cemented together by the flowing lava. So we see that there
are pudding-stones of many kinds to be found. If some solvent acid is
present in the water that percolates through these rocks it may soften
the cement and thus free the pebbles, reducing the conglomerate again to
a mere heap of shell fragments, or gravel, or rounded pebbles.
The story of rock formation tells how fire and water, and the two
combined, have made, and made over, again and again, the substance of
the earth's crust. Chemical and physical changes constantly tear down
some portions of the earth to build up others. The constant, combined
effort of wind and water is to level the earth and fill up the ocean
bed. Rocks are constantly being formed; the changes that have been going
on since the world began are still in progress. We can see them all
about us on any and every day of our lives.
GETTING ACQUAINTED WITH A RIVER
I have two friends whose childhood was spent in a home on the banks of a
noble eastern river. Their father taught the boy and the girl to row a
boat, and later each learned the more difficult art of managing a canoe.
On holidays they enjoyed no pleasure so much as a picnic on the
river-bank at some point that could be reached by rowing. As they grew
older, longer trips were planned, and the river was explored as far as
it was navigable by boat or canoe. Last summer when vacation came, these
two carried out a long-cherished plan to find the beginning of the
river--to follow it to its source. So they left home, and canoed
up-stream, until the stream became a brook, so shallow they could go no
farther. Then they followed it on foot--wading, climbing, making little
detours, but never losing the little river. At last they came to the
beginning of it--a tiny rivulet trickled out of the side of a hill,
filling a wooden keg that formed a basin, where thirsty passers-by could
stoop and drink. They decided to mark the spring, so that people who
found it later, and were refreshed by its clear water, might know that
here was born the greatest river of a great state. But they were not the
original discoverers. Above the spring, a board was nailed to a tree,
saying that this is the headwater of the river with the beautiful Indian
name, Susquehanna.