cooled, and are overlaid by the warmer contributions of that tide, and
are thus brought lower and lower in the sea. When they start downward
they have, as observations show, a temperature not much above the
freezing point of salt water. They do not congeal for the reason that
the salt of the ocean lowers the point at which the water solidifies
to near 28° Fahr. The effect of this action is gradually to press down
the surface cold water until it attains the very bottom in all the
circumpolar regions. At the same time this descending water drifts
along the bottom of the ocean troughs toward the equatorial realm. As
this cold water is heavier than that which is of higher temperature
and nearer the surface, it has no tendency to rise. Being below the
disturbing influences of any current save its own, it does not tend,
except in a very small measure, to mingle with the warmer overlying
fluid. The result is that it continues its journey until it may come
within the tropics without having gained a temperature of more than
35° Fahr., the increase in heat being due in small measure to that
which it receives from the earth's interior and that which it acquires
from the overlying warmer water. Attaining the region of the tropical
current, this drift water from the poles gradually rises, to take the
place of that which goes poleward, becomes warm, and again starts on
its surface journey toward the arctic and antarctic regions.
Nothing is known as to the rate of this bottom drift from the polar
districts toward the equator, but, from some computation which he has
made, the writer is of the opinion that several centuries is doubtless
required for the journey from the Arctic Circle to the tropics. The
speed of the movement probably varies; it may at times require some
thousand years for its accomplishment. The effect of the bottom drift
is to withdraw from seas in high latitudes the very cold water which
there forms, and to convey it beneath the seas of middle latitudes to
a realm where it is well placed for the reheating process. If all the
cold water of circumpolar regions had to journey over the surface to
the equator, the perturbing effect of its flow on the climates of
various lands would be far greater than it is at present. Where such
cold currents exist the effect is to chill the air without adding much
to the rainfall; while the currents setting northward not only warm
the regions near which they flow, but by so doing send from the water
surfaces large quantities of moisture which fall as snow or rain. Thus
the Gulf Stream, directly and indirectly, probably contributes more
than half the rainfall about the Atlantic basin. The lack of this
influence on the northern part of North America and Asia causes those
lands to be sterilized by cold, although destitute of permanent ice
and snow upon their surfaces.
We readily perceive that the effect of the oceanic circulation upon
the temperatures of different regions is not only great but widely
contrasted. By taking from the equatorial belt a large part of the
heat which falls within that realm, it lowers the temperature to the
point which makes the district fit for the occupancy of man, perhaps,
indeed, tenable to all the higher forms of life. This same heat
removed to high latitudes tempers the winter's cold, and thus makes a
vast realm inhabitable which otherwise would be locked in almost
enduring frosts. Furthermore, this distribution of temperatures tends
to reduce the total wind energy by diminishing the trades and counter
trades which are due to the variations of heat which are encountered
in passing polarward from the equator. Still further, but for this
circulation of water in the sea, the oceans about the poles would be
frozen to their very bottom, and this vast sheet of ice might be
extended southward to within the parallels of fifty degrees north and
south latitude, although the waters under the equator might at the
same time be unendurably hot and unfit for the occupancy of living
beings.
A large part of the difficulties which geologists encounter in
endeavouring to account for the changes of the past arise from the
evidences of great climatal revolutions which the earth has undergone.
In some chapters of the great stone book, whose leaves are the strata
of the earth, we find it plainly written in the impressions made by
fossils that all the lands beyond the equatorial belt have undergone
changes which can only be explained by the supposition that the heat
and moisture of the countries have been subjected to sudden and
remarkable changes. Thus in relatively recent times thick-leaved
plants which retained their vegetation in a rather tender state
throughout the year have flourished near to the poles, while shortly
afterward an ice sheet, such as now covers the greater part of
Greenland, extended down to the line of the Ohio River at Cincinnati.
Although these changes of climate are, as we shall hereafter note,
probably due to entangled causes, we must look upon the modifications
of the ocean streams as one of the most important elements in the
causation. We can the more readily imagine such changes to be due to
the alterations in the course and volume of the ocean current when we
note how trifling peculiarities in the geography of the
shores--features which are likely to be altered by the endless changes
which occur in the form of a continent--affect the run of these
currents. Thus the growth of coral reefs in southern Florida, and, in
general, the formation of that peninsula, by narrowing the exit of the
great current from the Gulf of Mexico, has probably increased its
velocity. If Florida should again sink down, that current would go
forth into the North Atlantic with the speed of about a mile an hour,
and would not have momentum enough to carry its waters over half the
vast region which they now traverse. If the lands about the western
border of the Caribbean Sea, particularly the Isthmus of Darien,
should be depressed to a considerable depth below the ocean level,
the tropical current would enter the Pacific Ocean, adding to the
temperature of its waters all the precious heat which now vitalizes
the North Atlantic region. Such a geographic accident would not only
profoundly alter the life conditions of that part of the world, but it
would make an end of European civilization.
In the chapter on climatal changes further attention will be given to
the action of ocean currents from the point of view of their influence
on the heat and moisture of different parts of the world. We now have
to consider the last important influence of ocean currents--that which
they directly exercise on the development of organic life. The most
striking effect of this nature which the sea streams bring about is
caused by the ceaseless transportation to which they subject the eggs
and seeds of animals and plants, as well as the bodies of the mature
form which are moved about by the flowing waters. But for the
existence of these north and south flowing currents, due to the
presence of the continental barriers, the living tenants of the seas
would be borne along around the earth, always in the same latitude,
and therefore exposed to the same conditions of temperature. In this
state of affairs the influences which now make for change in organic
species would be far less than they are. Journeying in the great
whirlpools which the continental barriers make out of the westward
setting tropical currents, these organic species are ever being
exposed to alterations in their temperature conditions which we know
to be favourable to the creation of those variations on which the
advance of organic life so intimately depends. Thus the ocean currents
not only help to vary the earth by producing changes in the climate of
both sea and land, breaking up the uniformity which would otherwise
characterize regions at the same distance from the equator, but they
induce, by the consequences of the migrations which they enforce,
changes in the organic tenants of the sea.
Another immediate effect of ocean streams arises where their currents
of warm water come against shores or shallows of the sea. At these
points, if the water have a tropical temperature, we invariably find a
vast and rapid development of marine animals and plants, of which the
coral-making polyps are the most important. In such positions the
growth of forms which secrete solid skeletons is so rapid that great
walls of their remains accumulate next the shore, the mass being built
outwardly by successive growths until the realm of the land may be
extended for scores of miles into the deep. In other cases vast mounds
of this organic _débris_ may be accumulated in mid ocean until its
surface is interspersed with myriads of islands, all of which mark the
work due to the combined action of currents and the marine life which
they nourish. Probably more than four fifths of all the islands in the
tropical belt are due in this way to the life-sustaining action of the
currents which the trade winds create.
There are many secondary influences of a less important nature which
are due to the ocean streams. The reader will find on most wall-maps
of the world certain areas in the central part of the oceans which are
noted as Sargassum seas, of which that of the North Atlantic, west and
south of the Azore Islands, is one of the most conspicuous. In these
tracts, which in extent may almost be compared with the continents, we
find great quantities of floating seaweed, the entangled fronds of
which often form a mass sufficiently dense to slightly restrain the
speed of ships. When the men on the caravels of Columbus entered this
tangle, they were alarmed lest they should be unable to escape from
its toils. It is a curious fact that these weeds of the sea while
floating do not reproduce by spores the structures which answer to the
seeds of higher plants, but grow only by budding. It seems certain
that they could not maintain their place in the ocean but for the
action of the currents which convey the bits rent off from the shores
where the plant is truly at home. This vast growth of plant life in
the Sargassum basins doubtless contributed considerable and important
deposits of sediment to the sea floors beneath the waters which it
inhabits. Certain ancient strata, known as the Devonian black shale,
occupying the Ohio valley and the neighbouring parts of North America
to the east and north of that basin, appear to be accumulations which
were made beneath an ancient Sargassum sea.
The ocean currents have greatly favoured and in many instances
determined the migrations not only of marine forms, but of land
creatures as well. Floating timber may bear the eggs and seeds of many
forms of life to great distances until the rafts are cast ashore in a
realm where, if the conditions favour, the creatures may find a new
seat for their life. Seeds of plants incased in their often dense
envelopes may, because they float, be independently carried great
distances. So it comes about that no sooner does a coral or other
island rise above the waters of the sea than it becomes occupied by a
varied array of plants. The migrations of people, even down to the
time of the voyages which discovered America, have in large measure
been controlled by the run of the ocean streams. The tropical set of
the waters to the westward helped Columbus on his way, and enabled him
to make a journey which but for their assistance could hardly have
been accomplished. This same current in the northern part of the Gulf
Stream opposed the passage of ships from northern Europe to the
westward, and to this day affects the speed with which their voyages
are made.
THE CIRCUIT OF THE RAIN.
We have now to consider those movements of the water which depend upon
the fact that at ordinary temperatures the sea yields to the air a
continued and large supply of vapour, a contribution which is made in
lessened proportion by water in all stages of coldness, and even by
ice when it is exposed to dry air. This evaporation of the sea water
is proportional to the temperature and to the dryness of the air where
it rests upon the ocean. It probably amounts on the average to
somewhere about three feet per annum; in regions favourably situated
for the process, as on the west coast of northern Africa, it may be
three or four times as much, while in the cold and humid air about the
poles it may be as little as one foot. When contributed to the air,
the water enters on the state of vapour, in which state it tends to
diffuse itself freely through the atmosphere by virtue of the motion
which is developed in particles when in the vaporous or gaseous state.
The greater part of the water evaporated from the seas probably finds
its way as rain at once back into the deep, yet a considerable portion
is borne away horizontally until it encounters the land. The
precipitation of the water from the air is primarily due to the
cooling to which it is subjected as it rises in the atmosphere. Over
the sea the ascent is accomplished by the simple diffusion of the
vapour or by the uprise through the aërial shaft, such as that near
the equator or over the centres of the whirling storms. It is when the
air strikes the slopes of the land that we find it brought into a
condition which most decidedly tends to precipitate its moisture.
Lifted upward, the air as it ascends the slopes is brought into cooler
and more rarefied conditions. Losing temperature and expanding, it
parts with its water for the same reason that it does in the ascending
current in the equatorial belt or in the chimneys of the whirl storms.
A general consequence of this is that wherever moisture-laden winds
from the sea impinge upon a continent they lay down a considerable
part of the water which they contain.
If all the lands were of the same height, the rain would generally
come in largest proportion upon their coastal belt, or those portions
of the shore-line districts over which the sea winds swept. But as
these winds vary in the amount of the watery vapour which they
contain, and as the surface of the land is very irregular, the
rainfall is the most variable feature in the climatal conditions of
our sphere. Near the coasts it ranges from two or three inches in arid
regions--such as the western part of the Sahara and portions of the
coast regions of Chili and Peru--to eight hundred inches about the
head waters of the Brahmapootra River in northern India, where the
high mountains are swept over by the moisture-laden airs from the
neighbouring sea. Here and there detached mountainous masses produce a
singular local increase in the amount of the rainfall. Thus in the
lake district in northwestern England the rainfall on the seaward side
of mountains, not over four thousand feet high, is very much greater
than it is on the other slope, less than a score of miles away. These
local variations are common all over the world, though they are but
little observed.
In general, the central parts of continents are likely to receive much
less rainfall than their peripheral portions. Thus the central
districts of North America, Asia, and Australia--three out of the five
continental masses--have what we may call interior deserts. Africa has
one such, though it is north of the centre, and extends to the shores
of the Mediterranean and the Atlantic. The only continent without this
central nearly rainless field is South America, where the sole
characteristic arid district is situated on the western slope of the
Cordilleran range. In this case the peculiarity is due to the fact
that the strong westerly setting winds which sweep over the country
encounter no high mountains until they strike the Andean chain. They
journey up a long and rather gradual slope, where the precipitation is
gradually induced, the process being completed when they strike the
mountain wall. Passing over its summit, they appear as dry winds on
the Pacific coast.
Even while the winds frequently blow in from the sea, as along the
western coast of the Americas, they may come over water which is
prevailingly colder than the land. This is characteristically the case
on the western faces of the American continent, where the sea is
cooled by the currents setting toward the equator from high latitudes.
Such cool sea air encountering the warm land has its temperature
raised, and therefore does not tend to lay down its burden of
moisture, but seeks to take up more. On this account the rainfall in
countries placed under such conditions is commonly small.
By no means all the moisture which comes upon the earth from the
atmosphere descends in the form of rain or snow. A variable, large,
though yet undetermined amount falls in the form of dew. Dew is a
precipitation of moisture which has not entered the peculiar state
which we term fog or cloud, but has remained invisible in the air. It
is brought to the earth through the radiation of heat which
continually takes place, but which is most effective during the
darkened half of the day, when the action is not counterbalanced by
the sun's rays. While the sun is high and the air is warm there is a
constant absorption of moisture in large part from the ground or from
the neighbouring water areas, probably in some part from those
suspended stores of water, the clouds, if such there be in the
neighbourhood. We can readily notice how clouds drifting in from the
sea often melt into the dry air which they encounter. Late in the
afternoon, even before the sun has sunk, the radiation of heat from
the earth, which has been going on all the while, but has been less
considerable than the incurrent of temperature, in a way overtakes
that influx. The air next the surface becomes cooled from its contact
with the refrigerating earth, and parts with its moisture, forming a
coating of water over everything it touches. At the same time the
moisture escaping from the warmed under earth likewise drops back upon
its cooled surface almost as soon as it has escaped. The thin sheet of
water precipitated by this method is quickly returned to the air when
it becomes warmed by the morning sunshine, but during the night
quantities of it are absorbed by the plants; very often, indeed, with
the lowlier vegetation it trickles down the leaves and enters the
earth about the base of the stem, so that the roots may appropriate
it. Our maize, or Indian corn, affords an excellent example of a plant
which, having developed in a land of droughts, is well contrived,
through its capacities for gathering dew, to protect itself against
arid conditions. In an ordinary dew-making night the leaves of a
single stem may gather as much as half a pint of water, which flows
down their surfaces to the roots. So efficient is this dew supply,
this nocturnal cloudless rain, that on the western coast of South
America and elsewhere, where the ordinary supply of moisture is almost
wanting, many important plants are able to obtain from it much of the
water which they need. The effect is particularly striking along
seashores, where the air, although it may not have the humidity
necessary for the formation of rain, still contains enough to form
dew.
It is interesting to note that the quantity of dew which falls upon an
area is generally proportioned to the amount of living vegetation
which it bears. The surfaces of leaves are very efficient agents of
radiation, and the tangle which they make offers an amount of
heat-radiating area many times as great as that afforded by a surface
of bared earth. Moreover, the ground itself can not well cool down to
the point where it will wring the moisture out of the air, while the
thin membranes of the plants readily become so cooled. Thus vegetation
by its own structure provides itself with means whereby it may be in a
measure independent of the accidental rainfall. We should also note
the fact that the dewfall is a concomitant of cloudless skies. The
quantity which is precipitated in a cloudy night is very small, and
this for the reason that when the heavens are covered the heat from
the earth can not readily fly off into space. Under these conditions
the temperature of the air rarely descends low enough to favour the
precipitation of dew.
Having noted the process by which in the rain circuit the water
leaves the sea and the conditions of distribution when it returns to
the earth, we may now trace in more detail the steps in this great
round. First, we should take note of the fact that the water after it
enters the air may come back to the surface of the earth in either of
two ways--directly in the manner of dewfall, or in a longer circuit
which leads it through the state of clouds. As yet we are not very
well informed as to the law of the cloud-making, but certain features
in this picturesque and most important process have been tolerably
well ascertained.
Rising upward from the sea, the vapour of water commonly remains
transparent and invisible until it attains a considerable height above
the surface, where the cooling tends to make it assume again the
visible state of cloud particles. The formation of these cloud
particles is now believed to depend on the fact that the air is full
of small dust motes, exceedingly small bits of matter derived from the
many actions which tend to bring comminuted solid matter into the air,
as, for instance, the combustion of meteoric stones, which are greatly
heated by friction in their swift course through the air, the
ejections of volcanoes, the smoke of forest and other fires, etc.
These tiny bits, floating in the air, because of their solid nature
radiate their heat, cool the air which lies against them, and thereby
precipitate the water in the manner of dew, exactly as do the leaves
and other structures on the surface of the earth. In fact, dew
formation is essentially like cloud formation, except that in the one
case the water is gathered on fixed bodies, and in the other on
floating objects. Each little dust raft with its cargo of condensed
water tends, of course, to fall downward toward the earth's surface,
and, except for the winds which may blow upward, does so fall, though
with exceeding slowness. Its rate of descent may be only a few feet a
day. It was falling before it took on the load of water; it will fall
a little more rapidly with the added burden, but even in a still air
it might be months or years before it would come to the ground. The
reason for this slow descent may not at first sight be plain, though a
little consideration will make it so.
If we take a shot of small size and a feather of the same weight, we
readily note that their rate of falling through the air may vary in
the proportion of ten to one or more. It is easy to conceive that this
difference is due to the very much less friction which the smaller
body encounters in its motion by the particles of air. With this point
in mind, the student should observe that the surface presented by
solid bodies in relation to their solid contents is the greater the
smaller the diameter. A rough, though not very satisfactory, instance
of this principle may be had by comparing the surface and interior
contents of two boxes, one ten feet square and the other one foot
square. The larger has six hundred feet of surface to one thousand
cubic feet of interior, or about half a square foot of outer surface
to the cubic foot of contents; while the smaller box has six feet of
surface for the single cubic foot of interior, or about ten times the
proportion of exterior to contents. The result is that the smaller
particles encounter more friction in moving toward the earth, until,
in the case of finely divided matter, such as the particles of carbon
in the smoke from an ordinary fire, the rate of down-falling may be so
small as to have little effect in the turbulent conditions of
atmospheric motion.
[Illustration: _Pocket Creek, Cape Ann, Massachusetts. Note the
relatively even size of the pebbles, and the splash wave which sets
them in motion._]
The little drops of water which gather round dust motes, falling but
slowly toward the earth, are free to obey the attractions which they
exercise upon each other--impulses which are partly gravitative and
partly electrical. We have no precise knowledge concerning these
movements, further than that they serve to aggregate the myriad little
floats into cloud forms, in which the rafts are brought near together,
but do not actually touch each other. They are possibly kept apart by
electrical repulsion. In this state of association without union the
divided water may undergo the curiously modified aggregations which
give us the varied forms of clouds. As yet we know little as to the
cause of cloud shapes. We remark the fact that in the higher of these
agglomerations of condensed vapour, the clouds which float at an
elevation of from twenty to thirty thousand feet or more, the masses
are generally thin, and arranged more or less in a leaflike form,
though even here a tendency to produce spherical clouds is apparent.
In this high realm floating water is probably in the frozen state,
answering to the form of dew, which we call hoar frost. The lower
clouds, gathering in the still air, show very plainly the tendency to
agglomerate into spheres, which appears to be characteristic of all
vaporous material which is free to move by its own impulses. It is
probable that the spherical shape of clouds is more or less due to the
same conditions as gathered the stellar matter from the ancient
nebular chaos into the celestial spheres. Upon these spherical
aggregations of the clouds the winds act in extremely varied ways. The
cloud may be rubbed between opposite currents, and so flattened out
into a long streamer; it may take the same form by being carried off
by a current in the manner of smoke from a fire; the spheres may be
kept together, so as to form the patchwork which we call "mackerel"
sky; or they may be actually confounded with each other in a vast
common cloud-heap. In general, where the process of aggregation of two
cloud bodies occurs, changes of temperature are induced in the masses
which are mixed together. If the temperature resulting from this
association of cloud masses is an average increase, the cloud may
become lighter, and in the manner of a balloon move upward. Each of
the motes in the cloud with its charge of vapour may be compared with
the ballast of the balloon; if they are warmed, they send forth a part
of their load of condensed water again to the state of invisible
vapour. Rising to a point where it cools, the vapour gathers back on
the rafts and tends again to weight the cloud downward. The ballast of
an ordinary balloon has to be thrown away from its car; but if some
arrangement for condensing the moisture from the air could be
contrived, a balloon might be brought into the adjustable state of a
cloud, going up or down according as it was heated or cooled.
When the formation of the drop of water or snowflake begins, the mass
is very small. If in descending it encounters great thickness of
cloud, the bit may grow by further condensation until it becomes
relatively large. Generally in this way we may account for the
diversities in the size of raindrops or snowflakes. It often happens
that the particles after taking on the form of snowflakes encounter in
their descent air so warm that they melt into raindrops, or, if only
partly melted, reach the surface as sleet. Or, starting as raindrops,
they may freeze, and in this simple state may reach the earth, or
after freezing they may gather other frozen water about them, so that
the hailstone has a complicated structure which, from the point of
view of classification, is between a raindrop and a snowflake.
In the process of condensation--indeed, in the steps which precede the
formation of rain and snow--there is often more or less trace of
electrical action; in fact, a part of the energy which was involved in
the vapourization of water, on its condensation, even on the dust
motes appears to be converted into electrical action, which probably
operates in part to keep the little aggregates of water asunder. When
they coalesce in drops or flakes, this electricity often assumes the
form of lightning, which represents the swift passage of the electric
store from a region where it is most abundant to one where it is less
so. The variations in this process of conveying the electricity are
probably great. In general, it probably passes, much as an electric
current is conveyed, through a wire from the battery which produces
the force. In other cases, where the tension is high, or, in other
words, where the discharge has to be hastened, we have the phenomena
of lightning in which the current burns its way along its path, as it
may traverse a slender wire, vapourizing it as it goes. In general,
the lightning flash expends its force on the air conductors, or lines
of the moist atmosphere along which it breaks its path, its energy
returning into the vapour which it forms or the heat which it produces
in the other parts of the air. In some cases, probably not one in the
thousand of the flashes, the charge is so heavy that it is not used up
in its descent toward the earth, and so electrifies, or, as we say,
strikes, some object attached to the earth, through which it passes to
the underlying moisture, where it finds a convenient place to take on
a quiet form. Almost all these hurried movements of electrical energy
which intensely heat and light the air which they traverse fly from
one part of a cloud to another, or cross from cloud sphere to cloud
sphere; of those which start toward the earth, many are exhausted
before they reach its surface, and even those that strike convey but a
portion of their original impulse to the ground.
The wearing-out effect of lightning in its journey along the air
conductors in its flaming passages is well illustrated by what happens
when the charge strikes a wire which is not large enough freely to
convey it. The wire is heated, generally made white hot, often melted,
and perhaps scattered in the form of vapour. In doing this work the
electricity may, and often is, utterly dissipated--that is, changed
into heat. It has been proposed to take advantage of this principle in
protecting buildings from lightning by placing in them many thin
wires, along which the current will try to make its way, being
exhausted in melting or vaporizing the metal through which it passes.
There are certain other forms of lightning, or at least of electrical
discharges, which produce light and which may best be described in
this connection. It occasionally happens that the earth becomes so
charged that the current proceeds from its surface to the clouds. More
rarely, and under conditions which we do not understand, the electric
energy is gathered into a ball-like form, which may move slowly along
the surface until it suddenly explodes. It is a common feature of all
these forms of lightning which we have noted that they ordinarily make
in their movement considerable noise. This is due to the sudden
displacement of the air which they traverse--displacement due to the
action of heat in separating the particles. It is in all essential
regards similar to the sounds made by projectiles, such as meteors or
swift cannon shots, as they fly through the air. It is even more
comparable to the sound produced by exploding gunpowder. The first
sound effect from the lightning stroke is a single rending note, which
endures no longer--indeed, not as long--as the explosion of a cannon.
Heard near by, this note is very sharp, reminding one of the sound
made by the breaking of glass. The rolling, continuous sound which we
commonly hear in thunder is, as in the case of the noise produced by
cannon, due to echo from the clouds and the earth. Thunder is
ordinarily much more prolonged and impressive in a mountainous country
than in a region of plains, because the steeps about the hearer
reverberate the original single crash.
The distribution of thunderstorms is as yet not well understood, but
it appears in many cases that they are attendants on the advancing
face of cyclones and hurricanes, the area in front of these great
whirlstorms being subjected to the condensation and irregular air
movements which lead to the development of much electrical energy.
There are, however, certain parts of the earth which are particularly
subjected to lightning flashes. They are common in the region near the
equator, where the ascending currents bring about heavy rains, which
mean a rapid condensation and consequent liberation of electrical
energy. They diminish in frequency toward the arctic regions. An
observer at the pole would probably fail ever to perceive strong
flashes. For the same reason thunderstorms are more frequent in
summer, the time when the difference in temperature between the
surface and the upper air is greatest, when, therefore, the uprushes
of air are likely to be most violent. They appear to be more common in
the night than in the daytime, for the reason that condensation is
favoured by the cooling which occurs in the dark half of the day. It
is rare, indeed, that a thunderstorm occurs near midday, a period when
the air is in most cases taking up moisture on account of the swiftly
increasing heat.
There are other forms of electrical discharges not distinctly
connected with the then existing condensation of moisture. What the
sailors call St. Elmo's fire--a brush of electric light from the mast
tops and other projections of the ship--indicates the passage of
electrical energy between the vessel and the atmosphere. Similar
lights are said sometimes to be seen rising from the surface of the
water. Such phenomena are at present not satisfactorily explained.
Perhaps in the same group of actions comes the so-called
"Jack-o'-lantern" or "Will-o'-the-wisp" fires flashing from the earth
in marshy places, which are often described by the common people, but
have never been observed by a naturalist. If this class of
illuminations really exists, we have to afford them some other
explanation than that they are emanations of self-inflamed
phosphoretted hydrogen, a method of accounting for them which
illogically finds a place in many treatises on atmospheric phenomena.
A gas of any kind would disperse itself in the air; it could not dance
about as these lights are said to do, and there is no chemical means
known whereby it could be produced in sufficient purity and quantity
from the earth to produce the effects which are described.[3]
[Footnote 3: The present writer has made an extended and careful study
of marsh and swamp phenomena, and is very familiar with the aspect of
these fields in the nighttime. He has never been able to see any sign of
the Jack-o'-lantern light. Looking fixedly into any darkness, such as is
afforded by the depths of a wood, the eye is apt to imagine the
appearance of faint lights. Those who have had to do with outpost duty
in an army know how the anxious sentry, particularly if he is new to the
soldier's trade, will often imagine that he sees lights before him.
Sometimes the pickets will be so convinced of the fact that they see
lights that they will fire upon the fiction of the imaginations. These
facts make it seem probable that the Jack-o'-lantern and his companion,
the Will-o'-the-wisp, are stories of the overcredulous.]
In the upper air, or perhaps even beyond the limits of the field
which deserves the name, in the regions extending from the poles to
near the tropics, there occur electric glowings commonly known as the
aurora borealis. This phenomenon occurs in both hemispheres. These
illuminations, though in some way akin to those of lightning, and
though doubtless due to some form of electrical action, are peculiar
in that they are often attended by glows as if from clouds, and by
pulsations which indicate movements not at electric speed. As yet but
little is known as to the precise nature of these curious storms. It
has been claimed, however, that they are related to the sun spots;
those periods when the solar spots are plenty, at intervals of about
eleven years, are the times of auroral discharges. Still further, it
seems probable that the magnetic currents of the earth, that circling
energy which encompasses the sphere, moving round in a general way
parallel to the equator, are intensified during these illuminations of
the circumpolar skies.
GEOLOGICAL WORK OF WATER.
We turn now to the geological work which is performed by falling
water. Where the rain or snow returns from the clouds to the sea, the
energy of position given to the water by its elevation above the earth
through the heat which it acquired from the sun is returned to the air
through which it falls or to the ocean surface on which it strikes. In
this case the circuit of the rain is short and without geological
consequence which it is worth while to consider, except to note that
the heat thus returned is likely to be delivered in another realm than
that in which the falling water acquired the store, thus in a small
way modifying the climate. When, however, the precipitation occurs on
the surface of the land, the drops of frozen or fluid water apply a
part of their energy in important geological work, the like of which
is not done where they return at once to the sea.
[Illustration: Fig. 10.--Showing the diverse action of rain on wooded
and cleared fields, _a_, wooded area; _b_, tilled ground.]
We shall first consider what takes place when the water in the form of
drops of rain comes to the surface of the land. Descending as they do
with a considerable speed, these raindrops apply a certain amount of
energy to the surface on which they fall. Although the beat of a
raindrop is proverbially light, the stroke is not ineffective.
Observing what happens where the action takes place on the surface of
bare rock, we may notice that the grains of sand or small pebbles
which generally abound on such surfaces, if they be not too steeply
inclined, dance about under the blows which they receive. If we could
cover hard plate glass, a much firmer material than ordinary stone,
with such bits, we should soon find that its surface would become
scratched all over by the friction. Moreover, the raindrops
perceptibly urge the small detached bits of stone down the slopes
toward the streams.
If all the earth's surface were bare rocks, the blow of the raindrops
would deserve to be reckoned among the important influences which lead
to the wearing of land. As it is, when a country is in a state of
Nature, only a small part of its surface is exposed to this kind of
wearing. Where there is rain enough to effect any damage, there is
sure to be sufficient vegetation to interpose a living and
self-renewed covering between the rocks and the rain. Even the lichens
which coat what at first sight often seems to be bare rock afford an
ample covering for this purpose. It is only where man bares the field
by stripping away and overturning this protecting vegetation that the
raindrops cut away the earth. The effect of their action can often be
noted by observing how on ploughed ground a flat stone or a potsherd
comes after a rain to cap a little column. The geologist sometimes
finds in soft sandstones that the same action is repeated in a larger
way where a thin fragment of hard rock has protected a column many
feet in height against the rain work which has shorn down the
surrounding rock.
When water strikes the moistened surface it at once loses the droplike
form which all fluids assume when they fall through the air.[4]
[Footnote 4: This principle of the spheroidal form in falling fluids is
used in making ordinary bird shot. The melted lead drops through
sievelike openings, the resulting spheres of the metal being allowed to
fall into water which chills them. Iron shot, used in cutting stone,
where they are placed between the saw and the surface of the rock, are
also made in the same manner. The descending fluid divides into drops
because it is drawn out by the ever-increasing speed of the falling
particles, which soon make the stream so thin that it can not hold
together.]
When the raindrops coalesce on the surface of the earth, the rôle of
what we may call land water begins. Thenceforward until the fluid
arrives at the surface of the sea it is continually at work in
effecting a great range of geological changes, only a few of which can
well be traced by the general student. The work of land water is due
to three classes of properties--to the energy with which it is endowed
by virtue of its height above the sea, a power due to the heat of the
sun; to the capacity it has for taking substances into solution; and
to its property of giving some part of its own substance to other
materials with which it comes in contact. The first of these groups of
properties may be called dynamical; the others, chemical.
The dynamic value of water when it falls upon the land is the amount
of energy it can apply in going down the slope which separates it from
the sea. A ton of the fluid, such as may gather in an ordinary rain on
a thousand square feet of ground in the highlands of a country--say at
an elevation of a thousand feet above the sea--expends before it comes
to rest in the great reservoir as much energy as would be required to
lift that weight from the ocean's surface to the same height. The ways
in which this energy may be expended we shall now proceed in a general
way to trace.
As soon as the water has been gathered, from its drop to its sheet
state--a process which takes place as soon as it falls--the fluid
begins its downward journey. On this way it is at once parted into two
distinct divisions, the surface water and the ground water: the former
courses more or less swiftly, generally at the rate of a mile or more
an hour, in the light of day; the latter enters the interstices of the
earth, slowly descends therein to a greater or less depth, and
finally, journeying perhaps at the rate of a mile a year, rejoins the
surface water, escaping through the springs. The proportion of these
two classes, the surface and the ground water, varies greatly, and an
intermixture of them is continually going on. Thus on the surface of
bare rock or frozen earth all the rain may go away without entering
the ground. On very sandy fields the heaviest rainfall may be taken
up by the porous earth, so that no streams are found. On such surfaces
the present writer has observed that a rainfall amounting to six
inches in depth in two hours produced no streams whatever. We shall
first follow the history of the surface water, afterward considering
the work which the underground movements effect.
If the student will observe what takes place on a level ploughed
field--which, after all, will not be perfectly level, for all fields
are more or less undulating--he will note that, though the surface may
have been smoothed by a roller until it appears like a floor, the
first rain, where the fall takes place rapidly enough to produce
surface streams, will create a series of little channels which grow
larger as they conjoin, the whole appearing to the eye like a very
detailed map, or rather model, of a river system; it is, indeed, such
a system in miniature. If he will watch the process by which these
streamlet beds are carved, he will obtain a tolerably clear idea as to
that most important work which the greater streams do in carving the
face of the lands. The water is no sooner gathered into a sheet than,
guided by the slightest irregularities which it encounters, it begins
to flow. At first the motion is so slow that it does not disturb its
bed, but at some points in the bottom of the sheet the movement soon
becomes swift enough to drag the grains of sand and clay from their
adhesions, bearing them onward. As soon as this beginning of a channel
is formed the water moves more swiftly in the clearer way; it
therefore cuts more rapidly, deepening and enlarging its channel, and
making its motion yet more free. The tiny rills join the greater, all
their channels sway to and fro as directed this way and that by chance
irregularities, until something like river basins are carved out,
those gentle slopes which form broad valleys where the carving has
been due to the wanderings of many streams. If the field be large,
considerable though temporary brooks may be created, which cut
channels perhaps a foot in depth. At the end of this miniature stream
system we always find some part of the waste which has been carved
out. If the streamlet discharges into a pool, we find the tiny
representative of deltas, which form such an important feature on the
coast line where large rivers enter seas or lakes. Along the lines of
the stream we may observe here and there little benches, which are the
equivalent in all save size of the terraces that are generally to be
observed along the greater streams. In fact, these accidents of an
acre help in a most effective way the student to understand the
greater and more complicated processes of continental erosion.
A normal river--in fact, all the greater streams of the
earth--originates in high country, generally in a region of mountains.
Here, because of the elevation of the region, the streams have cut
deep gorges or extensive valleys, all of which have slopes leading
steeply downward to torrent beds. Down these inclined surfaces the
particles worn off from the hard rock by frost and by chemical decay
gradually work their way until they attain the bed of the stream. The
agents which assist gravitation in bearing this detritus downward are
many, but they all work together for the same end. The stroke of the
raindrop accomplishes something, though but little; the direct washing
action of the brooklets which form during times of heavy rain, but dry
out at the close of the storm, do a good deal of the work; thawing and
freezing of the water contained in the mass of detritus help the
movement, for, although the thrust is in both directions, it is most
effective downhill; the wedges of tree roots, which often penetrate
between and under the stones, and there expand in their process of
growth, likewise assist the downward motion. The result is that on
ordinary mountain slopes the layer of fragments constituting the rude
soil is often creeping at the rate of from some inches to some feet a
year toward the torrent bed. If there be cliffs at the top of the
slope, as is often the case, very extensive falls of rock may take
place from it, the masses descending with such speed that they
directly attain the stream. If the steeps be low and the rock divided
into vertical joints, especially where there is a soft layer at the
base of the steep, detached masses from the precipice may move slowly
and steadfastly down the slope, so little disturbed in their journey
that trees growing upon their summits may continue to develop for the
thousands of years before the mass enters the stream bed.
Although the fall of rocks from precipices does not often take place
in a conspicuously large way, all great mountain regions which have
long been inhabited by man abound in traditions and histories of such
accidents. Within a century or two there have been a dozen or more
catastrophes of this nature in the inhabited valleys of the Alps. As
these accidents are at once instructive and picturesque, it is well to
note certain of them in some detail. At Yvorgne, a little parish on
the north shore of the Rhône, just above the lake of Geneva, tradition
tells that an ancient village of the name was overwhelmed by the fall
of a great cliff. The vast _débris_ forming the steep slope which was
thus produced now bears famous vineyards, but the vintners fancy that
they from time to time hear deep in the earth the ringing of the bells
which belonged to the overwhelmed church. In 1806 the district of
Goldau, just north of Lake Lucerne, was buried beneath the ruins of a
peak which, resting upon a layer of clay, slipped away like a
launching ship on the surface of the soft material. The _débris_
overwhelmed a village and many detached houses, and partly filled a
considerable lake. The wind produced by this vast rush of falling rock
was so great that people were blown away by it; some, indeed, were
killed in this singular manner.
The most interesting field of these Swiss mountain falls is a high
mountain valley of amphitheatrical form, known as the Diablerets, or
the devil's own district. This great circus, which lies at the height
of about four thousand feet above the sea, is walled around on its
northern side by a precipice, above which rest, or rather once
rested, a number of mountain peaks of great bulk. The region has long
been valued for the excellent pasturage which the head of the valley
affords. Two costly roads, indeed, have been built into it to afford
footpaths for the flocks and herds and their keepers in the summer
season. Through this human experience with the valley, we have a
record of what has gone on in this part of the mountain wilderness.
Within the period of history and tradition, three very great mountain
falls have occurred in this field, each having made its memory good by
widespread disaster which it brought to the people of the _chalets_.
The last of these was brought about by the fall of a great peak which
spread itself out in a vast field of ruins in the valley below. The
belt of destruction was about half a mile wide and three miles long.
When the present writer last saw it, a quarter of a century ago, it
was still a wilderness of great rocks, but here and there the process
of their decay was giving a foothold for herbage, and in a few
centuries the field will doubtless be so verdure-clad that its story
will not be told on its face. It is likely, however, to be preserved
in the memory of the people, and this through a singular and pathetic
tradition which has grown up about the place, one which, if not true,
comes at least among the legends which we should like to believe.
As told the present writer by a native of the district, it happened
when, in the nighttime the mountain came down, the herdsmen and their
cows gathered in the _chalets_--stout buildings which are prepared to
resist avalanches of snow. In one of these, which was protected from
crushing by the position of the stones which covered it, a solitary
herdsman found himself alive in his unharmed dwelling. With him in the
darkness were the cows, a store of food and water, and his provisions
for the long summer season. With nothing but hope to animate him, he
set to work burrowing upward among the rocks, storing the _débris_ in
the room of the _chalet_. He toiled for some months, but finally
emerged to the light of day, blanched by his long imprisonment in the
darkness, but with the strength to bear him to his home. In place of
the expected warm welcome, the unhappy man found himself received as a
ghost. He was exorcised by the priest and driven away to the distance.
It was only when long afterward his path of escape was discovered that
his history became known.
Returning to the account of the _débris_ which descends at varied
speed into the torrents, we find that when the detritus encounters the
action of these vigorous streams it is rapidly ground to pieces while
it is pushed down the steep channels to the lower country. Where the
stones are of such size that the stream can urge them on, they move
rapidly; at least in times when the torrent is raging. They beat over
each other and against the firm-set rocks; the more they wear, the
smaller they become, and the more readily they are urged forward.
Where the masses are too large to be stirred by the violent current,
they lie unmoved until the pounding of the rolling stones reduces them
to the proportions where they may join the great procession.
Ordinarily those who visit mountains behold their torrents only in
their shrunken state, when the waters stir no stones, and fail even to
bear a charge of mud, all detachable materials having been swept away
when the streams course with more vigour. In storm seasons the
conditions are quite otherwise; then the swollen torrents, their
waters filled with clay and sand, bear with them great quantities of
boulders, the collisions of which are audible above the muffled roar
of the waters, attesting the very great energy of the action.
When the waste on a mountain slope lies at a steep angle, particularly
where the accumulation is due to the action of ancient glaciers, it
not infrequently happens that when the ground is softened with frost
great masses of the material rush down the slope in the manner of
landslides. The observer readily notes that in many mountain regions,
as, for instance, in the White Mountains of New Hampshire, the steep
slopes are often seamed by the paths of these great landslides. Their
movement, indeed, is often begun by sliding snow, which gives an
impulse to the rocks and earth which it encounters in its descent. At
a place known as the Wylie Notch, in the White Mountains, in the early
part of this century, a family of that name was buried beneath a mass
of glacial waste which had hung on the mountain slope from the ancient
days until a heavy rain, following on a period of thaw, impelled the
mass down the slope. Although there have been few such catastrophes
noted in this country, it is because our mountains have not been much
dwelt in. As they become thickly inhabited as the Alps are, men are
sure to suffer from these accidents.
As the volume of a mountain torrent increases through the junction of
many tributaries, the energy of its moving waters becomes sufficient
to sweep away the fragments which come to its bed. Before this stage
is attained the stream rarely touches the solid under rock of the
mountain, the base of the current resting upon the larger loose stones
which it was unable to stir. In this pebble-paved section, because the
stream could not attack the foundation rock, we find no gorges--in
fact, the whole of this upper section of the torrent system is
peculiarly conditioned by the fact that the streams are dealing not
with bed-rock, but with boulders or smaller loose fragments. If they
cut a little channel, the materials from either side slip the faster,
and soon repave the bed. But when the streams have by a junction
gained strength, and can keep their beds clear, they soon carve down a
gorge through which they descend from the upper mountain realm to the
larger valleys, where their conjoined waters take on a riverlike
aspect. It should be noted here that the cutting power of the water
moving in the torrent or in the wave, the capacity it has for abrading
rock, resides altogether in the bits of stone or cutting tools with
which it is armed. Pure water, because of its fluidity, may move over
or against firm-set stones for ages without wearing them; but in
proportion as it moves rocky particles of any size, the larger they
are, the more effective the work, it wears the rock over which it
flows. A capital instance of this may be found where a stream from a
hose is used in washing windows. If the water be pure, there is no
effect upon the glass; but if it be turbid, containing bits of sand,
in a little while the surface will appear cloudy from the multitude of
line scratches which the hard bits impelled by the water have
inflicted upon it. A somewhat similar case occurs where the wind bears
sand against window panes or a bottle which has long lain on the
shore. The glass will soon be deeply carved by the action, assuming
the appearance which we term "ground." This principle is made use of
in the arts. Glass vessels or sheets are prepared for carving by
pasting paper cut into figures on their surfaces. The material is then
exposed to a jet of air or steam-impelling sand grains; in a short
time all the surface which has not been protected by paper has its
polish destroyed and is no longer translucent.
The passage from the torrent to the river, though not in a
geographical way distinct, is indicated to the observant eye by a
simple feature--namely, the appearance of alluvial terraces, those
more or less level heaps of water-borne _débris_ which accumulate
along the banks of rivers, which, indeed, constitute the difference
between those streams and torrents. Where the mountain waters move
swiftly, they manage to bear onward the waste which they receive. Even
where the blocks of stone cling in the bed, it is only a short time
before they are again set in motion or ground to pieces. If by chance
the detritus accumulates rapidly, the slope is steepened and the work
of the torrent made more efficient. As the torrent comes toward the
base of the mountains, where it neither finds nor can create steep
slopes over which to flow, its speed necessarily diminishes. With each
reduction in this feature its carrying power very rapidly diminishes.
Thus water flowing at the rate of ten miles an hour can urge stones
four times the mass that it can move when its speed is reduced to half
that rate. The result is that on the lowlands, with their relatively
gentle slopes, the combined torrents, despite the increase in the
volume of the stream arising from their confluence, have to lay down a
large part of their load of detritus.
If we watch where a torrent enters a mountain river, we observe that
the main stream in a way sorts over the waste contributed to it,
bearing on only those portions which its rate of flow will permit it
to carry, leaving the remainder to be built into the bank in the form
of a rude terrace. This accumulation may not extend far below the
point where the torrent which imported the _débris_ joins the main
stream; a little farther down, however, we are sure to find another
such junction and a second accumulation of terrace material. As these
contributions increase, the terrace accumulations soon become
continuous, lying on one side or the other of the river, sometimes
bordering both banks of the stream. In general, it can be said that so
long as the rate of fall of the torrent exceeds one hundred feet to
the mile it does not usually exhibit these shelves of detritus. Below
that rate of descent they are apt to be formed. Much, however, depends
upon the amount of detritus which the stream bears and the coarseness
of it; moreover, where the water goes through a gorge in the manner of
a flume with steep rocky sides, it can urge a larger amount before it
than when it traverses a wide valley, through which it passes, it may
be, in a winding way.
At first sight it may seem rather a fine distinction to separate
torrents from rivers by the presence or absence of terraces. As we
follow down the stream, however, and study its action in relation to
these terraces, and the peculiar history of the detritus of which they
are composed, we perceive that these latter accumulations are very
important features. Beginning at first with small and imperfect
alluvial plains, the river, as it descends toward the sea, gaining in
store of water and in the amount of _débris_ which comes with that
water from the hills, while the rate of fall and consequent speed of
the current are diminished, soon comes to a stage where it is engaged
in an endless struggle with the terrace materials. In times of flood,
the walls of the terraces compel the tide to flow over the tops of
these accumulations. Owing to the relative thinness of the water
beyond the bed, and to the growth of vegetation there, the current
moves more slowly, and therefore lays down a considerable deposit of
the silt and sand which it contains. This may result during a single
flood in lifting the level of the terrace by some inches in height,
still further serving to restrict the channel. Along the banks of the
Mississippi and other large rivers the most of this detritus falls
near the stream; a little of it penetrates to the farther side of the
plains, which often have a width of ten miles or more. The result is
that a broad elevation is constructed, a sort of natural mole or
levee, in a measure damming the flood waters, which can now only enter
the "back swamps" through the channels of the tributary streams. Each
of these back swamps normally discharges into the main stream through
a little river of its own, along the banks of which the natural levees
do not develop.
We have now to note a curious swinging movement of rivers which was
first well observed by the skilful engineers of British India. This
movement can best be illustrated by its effects. If on any river which
winds through alluvial plains a jetty is so constructed as to deflect
the stream at any point, the course which it follows will be altered
during its subsequent flow, it may be, for the distance of hundreds of
miles. It will be perceived that in its movements a river normally
strikes first against one shore and then against the other. Its water
in a general way moves as does a billiard ball when it flies from one
cushion to another. It is true that in a torrent we have the same
conditions of motion; but there the banks are either of hard rock or,
if of detritus, they are continually moving into the stream in the
manner before described. In the case of the river, however, its points
of collision are often on soft banks, which are readily undermined by
the washing action of the stream. In the ordinary course of events,
the river beginning, we may imagine, with a straight channel, had its
current deflected by some obstacle, it may be even by the slight
pressure of a tributary stream, is driven against one bank; thence it
rebounds and strikes the other. At each point of impinge it cuts the
alluvium away. It can bear on only a small portion of that which it
thus obtains; the greater part of the material is deposited on the
opposite side of the stream, but a little lower down, where it makes a
shallow. On these shallows water-loving plants and even certain trees,
such as the willows and poplars, find a foothold. When the stream
rises, the sediment settles in this tangle, and soon extends the
alluvial plain from the neighbouring bank, or in rarer cases the river
comes to flow on either side of an island of its own construction. The
natural result of this billiard-ball movement of the waters is that
the path of the stream is sinuous. The less its rate of fall and the
greater the amount of silt it obtains from its tributaries, the more
winding its course becomes. This gain in those parts of the river's
curvings where deposition tends to take place may be accelerated by
tree-planting. Thus a skilful owner of a tract of land on the south
bank of the Ohio River, by assiduously planting willow trees on the
front of his property, gained in the course of thirty years more than
an acre in the width of his arable land. When told by the present
writer that he was robbing his neighbours on the other side of the
stream, he claimed that their ignorance of the laws of river motion
was sufficient evidence that they did not deserve to own land.
In the primitive state of a country the water-loving plants,
particularly the trees which flourish in excessively humid conditions,
generally make a certain defence against these incursions of the
streams. But when a river has gained an opening in the bank it can,
during a flood, extend its width often to the distance of hundreds of
feet. During the inundations of the Mississippi the river may at times
be seen to eat away acres of land in a single day along one of the
outcurves of its banks. The undermined forests falling into the flood
join the great procession of drift timber, composed of trees which
have been similarly uprooted, which occupies the middle part of the
stream. This driftwood belt often has a width of three or four hundred
feet, the entangled stems and branches making it difficult for a boat
to pass from one side of the river to the other.
[Illustration: Fig. 11.--Oxbows and cut-off. Showing the changes in
the course of a river in its alluvial plain.]
When the curves of a river have been developed to a certain point (see
Fig. 11), when they have attained what is called the "oxbow" form, it
often happens that the stream breaks through the isthmus which
connects one of the peninsulas with the mainland. Where, as is not
infrequently the case, the bend has a length of ten miles or more, the
water just above and below the new-made opening is apt to differ in
height by some feet. Plunging down the declivity, the stream, flowing
with great velocity, soon enlarges the channel so that its whole tide
may take the easier way. When this result is accomplished, the old
curve is deserted, sand bars are formed across their mouths, which may
gradually grow to broad alluvial plains, so that the long-surviving,
crescent-shaped lake, the remnant of the river bed, may be seen far
from the present course of the ever-changing stream. Gradually the
accumulations of vegetable matter and the silt brought in by floods
efface this moat or oxbow cut-off, as it is so commonly termed.
As soon as the river breaks through the neck of a peninsula in the
manner above described, the current of the stream becomes much swifter
for many miles below and above the opening. Slowly, however, the
slopes are rearranged throughout its whole course, yet for a time the
stream near the seat of the change becomes straighter than before, and
this for the reason that its swifter current is better able to dispose
of the _débris_ which is supplied to it. The effect of a change in the
current produced by such new channels as we have described as forming
across the isthmuses of bends is to perturb the course of the stream
in all its subsequent downward length. Thus an oxbow cut-off formed
near the junction of the Ohio and Mississippi may tend more or less to
alter the swings of the Mississippi all the way to the Gulf of Mexico.
Although the swayings of the streams to and fro in their alluvial
plains will give the reader some idea as to the struggle which the
greater rivers have with the _débris_ which is committed to them, the
full measure of the work and its consequences can only be appreciated
by those who have studied the phenomena on the ground. A river such
as the Mississippi is endlessly endeavouring to bear its burden to the
sea. If its slope were a uniform inclined plane, the task might
readily be accomplished; but in this, as in almost all other large
water ways, the slope of the bed is ever diminishing with its onward
course. The same water which in the mountain torrent of the
Appalachians or Cordilleras rolled along stones several feet in
diameter down slopes of a hundred feet or more to the mile can in the
lower reaches of the stream move no pebbles which are more than one
fourth of an inch in diameter over slopes which descend on the average
about half a foot in a mile. Thus at every stage from the torrent to
the sea the detritus has from time to time to rest within the alluvial
banks, there awaiting the decay which slowly comes, and which may
bring it to the state where it may be dissolved in the water, or
divided into fragments so small that the stream may bear them on. A
computation which the present writer has made shows that, on the
average, it requires about forty thousand years for a particle of
stone to make its way down the Mississippi to the sea after it has
been detached from its original bed. Of course, some bits may make the
journey straightforwardly; others may require a far greater time to
accomplish the course which the water itself makes at most in a few
weeks. This long delay in the journey of the detritus--a delay caused
by its frequent rests in the alluvial plain--brings about important
consequences which we will now consider.
As an alluvial plain is constructed, we generally find at the base
pebbly material which fell to the bottom in the current of the main
stream as the shores grew outward. Above this level we find the
deposits laid down by the flood waters containing no pebbles, and this
for the reason that those weightier bits remained in the stream bed
when the tide flowed over the plain. As the alluvial deposit is laid
down, a good deal of vegetable matter was built into it. Generally
this has decayed and disappeared. On the surface of the plain there
has always been growing abundant vegetation, the remains of which
decayed on the surface in the manner which we may observe at the
present day. This decomposing vegetable matter within and upon the
porous alluvial material produces large quantities of carbonic acid, a
gas which readily enters the rain water, and gives it a peculiar power
of breaking up rock matter. Acting on the _débris_, this gas-charged
water rapidly brings about a decay of the fragments. Much of the
material passes at once into solution in this water, and drains away
through the multitudinous springs which border the river. As this
matter is completely dissolved, as is sugar in water, it goes straight
away to the sea without ever again entering the alluvium. In many, if
not most, cases this dissolving work which is going on in alluvial
terraces is sufficient to render a large part of the materials which
they contain into the state where it disappears in an unseen manner;
thus while the annual floods are constantly laying down accumulations
on the surface of these plains, the springs are bearing it away from
below.
In this way, through the decomposition which takes place in them, all
those river terraces where much vegetable matter is mingled with the
mineral substances, become laboratories in which substances are
brought into solution and committed to the seas. We find in the water
of the ocean a great array of dissolved mineral substances; it,
indeed, seems probable that the sea water contains some share, though
usually small, of all the materials which rivers encounter in their
journey over and under the lands. As the waters of the sea obtain but
little of this dissolved matter along the coast, it seems likely that
the greater share of it is brought into the state of solution in the
natural laboratories of the alluvial plains.
Here and there along the sides of the valleys in which the rivers flow
we commonly find the remains of ancient plains lying at more or less
considerable heights above the level of the streams. Generally these
deposits, which from their form are called terraces, represent the
stages of down-wearing by which the stream has carved out its way
through the rocks. The greater part of these ancient alluvial plains
has been removed through the ceaseless swinging of the stream to and
fro in the valley which it has excavated.
In all the states of alluvial plains, whether they be the fertile
deposits near the level of the streams which built them, or the poorer
and ruder surfaced higher terraces, they have a great value to
mankind. Men early learned that these lands were of singularly uniform
goodness for agricultural use. They are so light that they were easily
delved with the ancient pointed sticks or stone hoes, or turned by the
olden, wooden plough. They not only give a rich return when first
subjugated, but, owing to the depth of the soil and the frequency with
which they are visited by fertilizing inundations, they yield rich
harvests without fertilizing for thousands of years. It is therefore
not surprising that we find the peoples who depended upon tillage for
subsistence first developed on the great river plains. There, indeed,
were laid the foundations of our higher civilization; there alone
could the state which demands of its citizens fixed abodes and
continuous labour take rise. In the conditions which these fields of
abundance afforded, dense populations were possible, and all the arts
which lead toward culture were greatly favoured. Thus it is that the
civilization of China, India, Persia, and Egypt, the beginnings of
man's higher development, began near the mouths of the great river
valleys. These fields were, moreover, most favourably placed for the
institution of commerce, in that the arts of navigation, originating
in the sheltered reaches of the streams, readily found its way through
the estuaries to the open sea.
Passing down the reaches of a great river as it approaches the sea, we
find that the alluvial plains usually widen and become lower. At
length we attain a point where the flood waters cover the surface for
so large a part of the year that the ground is swampy and untillable
unless it is artificially and at great expense of labour won to
agriculture in the manner in which this task has been effected in the
lower portion of the Rhine Valley. Still farther toward the sea, the
plain gradually dips downward until it passes below the level of the
waters. Through this mud-flat section the stream continues to cut
channels, but with the ever-progressive slowing of its motion the
burden of fine mud which it carries drops to the bottom, and
constantly closes the paths through which the water escapes. Every few
years they tend to break a new way on one side or the other of their
former path. Some of the greatest engineering work done in modern
times has been accomplished by the engineers engaged in controlling
the exits of large rivers to the sea. The outbreak of the Yellow River
in 1887, in which the stream, hindered by its own accumulations,
forced a new path across its alluvial plains, destroyed a vast deal of
life and property, and made the new exit seventy miles from the path
which it abandoned.
Below the surface of the open water the alluvial deposits spread out
into a broad fan, which slopes gradually to a point where, in the
manner of the continental shelf, the bottom descends steeply into deep
water.
It is the custom of naturalists to divide the lower section of river
deposits--that part of the accumulation which is near the sea--from
the other alluvial plains, terming the lower portion the delta. The
word originally came into use to describe that part of the alluvium
accumulated by the Nile near its mouth, which forms a fertile
territory shaped somewhat like the fourth letter of the Greek
alphabet. Although the definition is good in the Egyptian instance,
and has a certain use elsewhere, we best regard all the detritus in a
river valley which is in the state of repose along the stream to its
utmost branches as forming one great whole. It is, indeed, one of the
most united of the large features which the earth exhibits. The
student should consider it as a continuous inclined plane of
diminishing slope, extending from the base of the torrents to the
sea, and of course ramifying into the several branches of the river
system. He should further bear in mind the fact that it is a vast
laboratory where rock material is brought into the soluble state for
delivery to the seas.
The diversity in the form of river valleys is exceedingly great.
Almost all the variety of the landscape is due to this impress of
water action which has operated on the surface in past ages. When
first elevated above the sea, the surface of the land is but little
varied; at this stage in the development the rivers have but shallow
valleys, which generally cut rather straight away over the plain
toward the sea. It is when the surface has been uplifted to a
considerable height, and especially when, as is usually the case, this
uplifting action has been associated with mountain-building, that
valleys take on their accented and picturesque form. The reason for
this is easily perceived: it lies in the fact that the rocks over
which the stream flows are guided in the cutting which they effect by
the diversities of hardness in the strata that they encounter. The
work which it does is performed by the hard substances that are
impelled by the current, principally by the sand and pebbles. These
materials, driven along by the stream, become eroding tools of very
considerable energy. As will be seen when we shortly come to describe
waterfalls, the potholes formed at those points afford excellent
evidence as to the capacity of stream-impelled bits of stone to cut
away the firmest bed rocks. Naturally the ease with which this carving
work is done is proportionate to the energy of the currents, and also
to the relative hardness of the moving bits and the rocks over which
they are driven.
So long as the rocks lie horizontally in their natural construction
attitude the course of the stream is not much influenced by the
variations in hardness which the bed exhibits. Where the strata are
very firm there is likely to be a narrow gorge, the steeps of which
rise on either side with but slight alluvial plains; where the beds
are soft the valley widens, perhaps again to contract where in the
course of its descent it encounters another hard layer. Where,
however, the beds have been subjected to mountain-building, and have
been thrown into very varied attitudes by folding and faulting, the
stream now here and now there encounters beds which either restrain
its flow or give it freedom. The stream is then forced to cut its way
according to the positions of the various underlying strata. This
effect upon its course is not only due to the peculiarities of
uplifted rocks, but to manifold accidents of other nature: veins and
dikes, which often interlace the beds with harder or softer partitions
than the country rock; local hardenings in the materials, due to
crystallization and other chemical processes, often create
indescribable variations which are more or less completely expressed
in the path of the stream.
When a land has been newly elevated above the sea there is often--we
may say, indeed, generally--a very great difference between the height
of its head waters and the ocean level. In this condition of a country
the rivers have what we may call a new aspect; their valleys are
commonly narrow and rather steep, waterfalls are apt to abound, and
the alluvial terraces are relatively small in extent. Stage by stage
the torrents cut deeper; the waste which they make embarrasses the
course of the lower waters, where no great amount of down-cutting is
possible for the reason that the bed of the stream is near sea level.
At the same time the alluvial materials, building out to sea, thus
diminish the slope of the stream. In the extreme old age of the river
system the mountains are eaten down so that the torrent section
disappears, and the stream becomes of something like a uniform slope;
the higher alluvial plains gradually waste away, until in the end the
valley has no salient features. At this stage in the process, or even
before it is attained, the valley is likely to be submerged beneath
the sea, where it is buried beneath the deposits formed on the floor;
or a further uplift of the land may occur with the result that the
stream is rejuvenated; or once more endowed with the power to create
torrents, build alluvial plains, and do the other interesting work of
a normal river.
It rarely, if ever, happens that a river valley attains old age before
it has sunk beneath the sea or been refreshed by further upliftings.
In the unstable conditions of the continents, one or the other of
these processes, sometimes in different places both together, is apt
to be going on. Thus if we take the case of the Mississippi and its
principal tributaries, the Ohio and Missouri, we find that for many
geological ages the mountains about their sources have frequently, if
not constantly, grown upward, so that their torrent sections, though
they have worn down tens of thousands of feet, are still high above
the sea level, perhaps on the average as high as they have ever been.
At the same time the slight up-and-down swayings of the shore lands,
amounting in general to less than five hundred feet, have greatly
affected the channels of the main river and its tributaries in their
lower parts. Not long ago the Mississippi between Cairo and the Gulf
flowed in a rather steep-sided valley probably some hundreds of feet
in depth, which had a width of many miles. Then at the close of the
last Glacial period the region sank down so that the sea flooded the
valley to a point above the present junction of the Ohio River with
the main stream. Since then alluvial plains have filled this estuary
to even beyond the original mouth. In many other of our Southern
rivers, as along the shore from the Mississippi to the Hudson, the
streams have not brought in enough detritus to fill their drowned
valleys, which have now the name of bays, of which the Delaware and
Chesapeake on the Atlantic coast, and Mobile Bay on the Gulf of
Mexico, are good examples. The failure of Chesapeake and Delaware Bays
to fill with _débris_ in the measure exhibited by the more southern
valleys is due to the fact that the streams which flow into them to a
great extent drain from a region thickly covered with glacial waste, a
mass which holds the flood waters, yielding the supply but slowly to
the torrents, which there have but a slight cutting power.
In our sketch of river valleys no attention has been given to the
phenomena of waterfalls, those accidents of the flow which, as we have
noted, are particularly apt to characterize rivers which have not yet
cut down to near the sea level. Where the normal uniform descent which
is characteristic of a river's bed is interrupted by a sudden steep,
the fact always indicates the occurrence of one of a number of
geological actions. The commonest cause of waterfalls is due to a
sudden change in the character of horizontal or at least nearly level
beds over which the stream may flow. Where after coursing for a
distance over a hard layer the stream comes to its edge and drops on a
soft or easily eroded stratum, it will cut this latter bed away, and
create a more or less characteristic waterfall. Tumbling down the face
of the hard layer, the stream acquires velocity; the _débris_ which it
conveys is hurled against the bottom, and therefore cuts powerfully,
while before, being only rubbed over the stone as it moved along, it
cut but slightly. Masses of ice have the same effect as stones. Bits
dropping from the ledge are often swept round and round by the eddies,
so that they excavate an opening which prevents their chance escape.
In these confined spaces they work like augers, boring a deep,
well-like cavity. As the bits of stone wear out they are replaced by
others, which fall in from above. Working in this way, the fragments
often develop regular well-like depressions, the cavities of which
work back under the cliffs, and by the undermining process deprive the
face of the wall of its support, so that it tumbles in ruin to the
base, there to supply more material for the potholing action.
Waterfalls of the type above described are by far the commonest of
those which occur out of the torrent districts of a great river
system. That of Niagara is an excellent specimen of the type, which,
though rarely manifested in anything like the dignity of the great
fall, is plentifully shown throughout the Mississippi Valley and the
basin of the Great Lakes. Within a hundred miles of Niagara there are
at least a hundred small waterfalls of the same type. Probably three
quarters of all the larger accidents of this nature are due to the
conditions of a hard bed overlying softer strata.
Falls are also produced in very many instances by dikes which cross
the stream. So, too, though rarely, only one striking instance being
known, an ancient coral reef which has become buried in strata may
afford rock of such hardness that when the river comes to cross it it
forms a cascade, as at the Falls of the Ohio, at Louisville, Ky. It is
a characteristic of all other falls, except those first mentioned,
that they rarely plunge with a clean downward leap over the face of a
precipice which recedes at its base, but move downward over an
irregular sloping surface.
In the torrent district of rivers waterfalls are commonly very
numerous, and are generally due to the varying hardness in the rocks
which the streams encounter. Here, where the cutting action is going
on with great rapidity, slight differences in the resistance which the
rocks make to the work will lead to great variations in the form of
the bed over which they flow, while on the more gently sloping bottoms
of the rivers, where the _débris_ moves slowly, such variations would
be unimportant in their effect. When the torrents escape into the main
river valleys, in regions where the great streams have cut deep
gorges, they often descend from a great vertical height, forming
wonderful waterfalls, such as those which occur in the famous
Lauterbrunnen Valley of Switzerland or in that of the Yosemite in
California. This group of cascades is peculiar in that the steep of
the fall is made not by the stream itself, but by the action of a
greater river or of a glacier which may have some time taken its
place.
Waterfalls have an economic as well as a picturesque interest in that
they afford sources of power which may be a very great advantage to
manufacturers. Thus along the Atlantic coast the streams which come
from the Appalachian highlands, and which have hardly escaped from
their torrent section before they attain the sea, afford numerous
cataracts which have been developed so that they afford a vast amount
of power. Between the James on the south and the Ste. Croix on the
north more than a hundred of these Appalachian rivers have been turned
to economic use. The industrial arts of this part of the country
depend much upon them for the power which drives their machinery. The
whole of the United States, because of the considerable size of its
rivers and their relatively rapid fall, is richly endowed with this
source of energy, which, originating in the sun's heat and conveyed
through the rain, may be made to serve the needs of man. In view of
the fact that recent inventions have made it possible to convert this
energy of falling water into the form of electricity, which may be
conveyed to great distances, it seems likely that our rivers will in
the future be a great source of national wealth.
We must turn again to river valleys, there to trace certain actions
less evident than those already noted, but of great importance in
determining these features of the land. First, we have to note that in
the valley or region drained by a river there is another degrading or
down-wearing action than that which is accomplished by the direct work
of the visible stream. All over such a valley the underground waters,
soaking through the soil and penetrating through the underlying rock,
are constantly removing a portion of the mineral matter which they
take into solution and bear away to the sea. In this way, deprived of
a part of their substance, the rocks are continually settling down by
underwear throughout the whole basin, while they are locally being cut
down by the action of the stream. Hence in part it comes about that in
a river basin we find two contrasted features--the general and often
slight slope of a country toward the main stream and its greater
tributaries, and the sharp indentation of the gorge in which the
streams flow, these latter caused by the immediate and recent action
of the streams.
If now the reader will conceive himself standing at any point in a
river basin, preferably beyond the realms of the torrents, he may with
the guidance of the facts previously noted, with a little use of the
imagination, behold the vast perceptive which the history of the river
valley may unfold to him. He stands on the surface of the soil, that
_débris_ of the rocks which is just entering on its way to the ocean.
In the same region ten thousand years ago he would have stood upon a
surface from one to ten feet higher than the present soil covering. A
million years ago his station would have been perhaps five hundred
feet higher than the surface. Ten million years in the past, a period
less than the lifetime of certain rivers, such as the French Broad
River in North Carolina, the soil was probably five thousand feet or
more above its present plane. There are, indeed, cases where river
valleys appear to have worked down without interruption from the
subsidence of the land beneath the sea to the depth of at least two
miles. Looking upward through the space which the rocks once occupied,
we can conceive the action of the forces in their harmonious
co-operation which have brought the surface slowly downward. We can
imagine the ceaseless corrosion due to the ground water, bringing
about a constant though slow descent of the whole surface. Again and
again the streams, swinging to and fro under the guidance of the
underlying rock, or from the obstacles which the _débris_ they carried
imposed upon them, have crossed the surface. Now and then perhaps the
wearing was intensified by glacial action, for an ice sheet often cuts
with a speed many times as great as that which fluid water can
accomplish. On the whole, this exercise of the constructive
imagination in conceiving the history of a river valley is one of the
most enlarging tasks which the geologist can undertake.
Where in a river valley there are many lateral streams, and especially
where the process of solution carried on by the underground waters is
most effective, as compared with erosive work done in the bed of the
main river, we commonly find the valley sloping gently toward its
centre, the rivers having but slight steeps near their banks. On the
other hand, where, as occasionally happens, a considerable stream fed
by the rain and snow fall in its torrent section courses for a great
distance over high, arid plains, on which the ground water and the
tributaries do but little work, the basin may slope with very slight
declivity to the river margins, and there descend to great depths,
forming very deep gorges, of which the Colorado Cañon is the most
perfect type. As instances of these contrasted conditions, we may
take, on the one hand, the upper Mississippi, where the grades toward
the main stream are gentle and the valley gorge but slightly
exhibited; on the other, the above-mentioned Colorado, which bears a
great tide of waters drawn from the high and relatively rainy region
of the Rocky Mountains across the vast plateau lying in an almost
rainless country. In this section nearly all the down-wearing has been
brought about in the direct path of the stream, which has worn the
elevated plain into a deep gorge during the slow uprising of the
table-land to its present height. In this way a defile nearly a mile
in depth has been created in a prevailingly rather flat country. This
gorge has embranchments where the few great tributaries have done like
work, but, on the whole, this river flows in an almost unbroken
channel, the excavation of which has been due to its swift,
pebble-bearing waters.
The tendency of a newly formed river is to cut a more or less distinct
cañon. As the basin becomes ancient, this element of the gorge tends
to disappear, the reason for this being that, while the river bed is
high above the sea, the current is swift and the down-cutting rapid,
while the slow subsidence of the country on either side--a process
which goes on at a uniform rate--causes the surface of that region to
be left behind in the race for the sea level. As the stream bed comes
nearer the sea level its rate of descent is diminished, and so the
outlying country gradually overtakes it.
In regions where the winters are very cold the effect of ice on the
development of the stream beds both in the torrent and river sections
of the valley is important. This work is accomplished in several
diverse ways. In the first place, where the stream is clear and the
current does not flow too swiftly, the stones on the bottom radiate
their heat through the water, and thus form ice on their surfaces,
which may attain considerable thickness. As ice is considerably
lighter than water, the effect is often to lift up the stones of the
bed if they be not too large; when thus detached from the bottom, they
are easily floated down stream until the ice melts away. The ice which
forms on the surface of the water likewise imprisons the pebbles along
the banks, and during the subsequent thaw may carry them hundreds of
miles toward the sea. It seems likely, from certain observations made
by the writer, that considerable stones may thus be carried from the
Alleghany River to the main Mississippi.
Perhaps the most important effect of ice on river channels is
accomplished when in a time of flood the ice field which covered the
stream, perhaps to the depth of some feet, is broken up into vast
floes, which drift downward with the current. When, as on the Ohio,
these fields sometimes have the area of several hundred acres, they
often collide with the shores, especially where the stream makes a
sharp bend. Urged by their momentum, these ice floes pack into the
semblance of a dam, which may have a thickness of twenty, thirty, or
even fifty feet. Beginning on the shore, where the collision takes
place, the dam may swiftly develop clear across the stream, so that in
a few minutes the way of the waters is completely blocked. The
on-coming ice shoots up upon the accumulation, increases its height,
and extends it up stream, so that in an hour the mass completely bars
the current. The waters then heap up until they break their way over
the obstacle, washing its top away, until the whole is light enough
to be forced down the stream, where, by the friction it encounters on
the bottom and sides of the channel, it is broken to pieces. It is
easy to see that such moving dams of ice may sweep the bed of a river
as with a great broom.
Sometimes where the gorges do not form a stationary dam large cakes of
ice become turned on edge and pack together so that they roll down the
stream like great wheels, grinding the bed rock as they go.
In high northern countries, as in Siberia, the rivers, even the
deepest, often become so far frozen that their channels are entirely
obstructed. Where, as in the case of these Siberian rivers, the flow
is from south to north, it often happens that the spring thaw sets in
before the more northern beds of the main stream are released from
their bondage of frost. In this case the inundations have to find new
paths on either side of the obstructed way. The result is a type of
valleys characterized by very irregular and changeable stream beds,
the rivers having no chance to organize themselves into the shapely
curves which they ordinarily follow.
The supply which finds its way to a river is composed, as has been
already incidentally noted, in part of the water which courses
underground for a greater or less distance before it emerges to the
surface, and in part of that which moves directly over the ground.
These two shares of water have somewhat different histories. On the
share of these two depends the stability of the flow. Where, as in New
England and other glaciated countries, the surface of the earth is
covered with a thick layer of sand and gravel, which, except when
frozen, readily admits the water; the rainfall is to a very great
extent absorbed by the earth, and only yielded slowly to the streams.
In these cases floods are rare and of no great destructive power.
Again, where also the river basin is covered by a dense mantle of
forests, the ground beneath which is coated, as is the case in
primeval woods, with a layer of decomposing vegetation a foot or more
in depth, this spongy mass retains the water even more effectively
than the open-textured glacial deposits above referred to. When the
woods, however, are removed from such an area, the rain may descend to
the streams almost as speedily as it finds its way to the gutters from
the house roofs. It thus comes about that all regions, when reduced to
tillage, and where the rainfall is enough to maintain a good
agriculture, are, except when they have a coating of glacial waste,
exceedingly liable to destructive inundations.
Unhappily, the risk of river floods is peculiarly great in all the
regions of the United States lying much to the east of the Rocky
Mountains, except in the basin of the Great Lakes and in the district
of New England, where the prevalence of glacial sands and gravels
affords the protection which we have noted. Throughout this region the
rainfall is heavy, and the larger part of it is apt to come after the
ground has become deeply snow-covered. The result is a succession of
devastating floods which already are very damaging to the works of
man, and promise to become more destructive as time goes on. More than
in any other country, we need the protection which forests can give us
against these disastrous outgoings of our streams.
LAKES.
In considering the journey of water from the hilltops to the sea, we
should take some account of those pauses which it makes on its way
when for a time it falls into the basin of a lake. These arrests in
the downward motion of water, which we term lakes, are exceedingly
numerous; their proper discussion would, indeed, require a
considerable volume. We shall here note only the more important of
their features, those which are of interest to the general student.
The first and most noteworthy difference in lakes is that which
separates the group of dead seas from the living basins of fresh
water. When a stream attains a place where its waters have to expand
into the lakelike form, the current moves in a slow manner, and the
broad surface exposed to the air permits a large amount of
evaporation. If the basin be large in proportion to the amount of the
incurrent water, this evaporation may exceed the supply, and produce a
sea with no outlet, such as we find in the Dead Sea of Judea, in that
at Salt Lake, Utah, and in a host of other less important basins. If
the rate of evaporation be yet greater in proportion to the flow, the
lake may altogether dry away, and the river be evaporated before it
attains the basin where it might accumulate. In that case the river is
said to sink, but, in place of sinking into the earth, its waters
really rise into the air. Many such sinks occur in the central portion
of the Rocky Mountain district. It is important to note that the
process of evaporation we are describing takes place in the case of
all lakes, though only here and there is the air so dry that the
evaporation prevents the basin from overflowing at the lowest point on
its rim, forming a river which goes thence to the sea. Even in the
case of the Great Lakes of North America a considerable part of the
water which flows into them does not go to the St. Lawrence and thence
to the sea. As long as the lake finds an outlet to the sea its waters
contain but little more dissolved mineral matter than that we find in
the rivers. But because all water which has been in contact with the
earth has some dissolved mineral substances, while that which goes
away by evaporation is pure water, a lake without an outlet gradually
becomes so charged with these materials that it can hold no more in
solution, but proceeds to lay them down in deposits of that compound
substance which from its principal ingredient we name salt. The water
of dead seas, because of the additional weight of the substances which
it holds, is extraordinarily buoyant. The swimmer notes a difference
in this regard in the waters of rivers and fresh-water lakes and those
of the sea, due to this same cause. But in those of dead seas,
saturated with saline materials, the human body can not sink as it
does in the ordinary conditions of immersion. It is easy to understand
how the salt deposits which are mined in many parts of the world have
generally, if not in all cases, been formed in such dead seas.[5]
[Footnote 5: In some relatively rare cases salt deposits are formed in
lagoons along the shores of arid lands, where the sea occasionally
breaks over the beach into the basin, affording waters which are
evaporated, leaving their salt behind them.]
It is an interesting fact that almost all the known dead seas have in
recent geological times been living lakes--that is, they poured over
their brims. In the Cordilleras from the line between Canada and the
United States to central Mexico there are several of these basins. All
of those which have been studied show by their old shore lines that
they were once brimful, and have only shrunk away in modern times.
These conditions point to the conclusion that the rainfall in
different regions varies greatly in the course of the geologic ages.
Further confirmation of this is found in the fact that very great salt
deposits exist on the coast of Louisiana and in northern
Europe--regions in which the rainfall is now so great in proportion to
the evaporation that dead seas are impossible.
Turning now to the question of how lake basins are formed, we note a
great variety in the conditions which may bring about their
construction. The greatest agent, or at least that which operates in
the construction of the largest basins, are the irregular movements of
the earth, due to the mountain-building forces. Where this work goes
on on a large scale, basin-shaped depressions are inevitably formed.
If all those which have existed remained, the large part of the lands
would be covered by them. In most cases, however, the cutting action
of the streams has been sufficient to bring the drainage channels down
to the bottom of the trough, while the influx of sediments has served
to further the work by filling up the cavities. Thus at the close of
the Cretaceous period there was a chain of lakes extending along the
eastern base of the Rocky Mountains, constituting fresh-water seas
probably as large as the so-called Great Lakes of North America. But
the rivers, by cutting down and tilling up, have long since
obliterated these water areas. In other cases the tiltings of the
continent, which sometimes oppose the flow of the streams, may for a
time convert the upper part of a river basin which originally sloped
gently toward the sea into a cavity. Several cases of this description
occurred in New England in the closing stages of the Glacial period,
when the ground rose up to the northward.
We have already noted the fact that the basin of a dead sea becomes in
course of time the seat of extensive salt deposits. These may, indeed,
attain a thickness of many hundred feet. If now in the later history
of the country the tract of land with the salt beneath it were
traversed by a stream, its underground waters may dissolve out the
salt and in a way restore the basin to its original unfilled
condition, though in the second state that of a living lake. It seems
very probable that a portion at least of the areas of Lakes Ontario,
Erie, and Huron may be due to this removal of ancient salt deposits,
remains of which lie buried in the earth in the region bordering these
basins.
By far the commonest cause of lake basins is found in the
irregularities of the surface which are produced by the occupation of
the country by glaciers. When these great sheets of ice lie over a
land, they are in motion down the slopes on which they rest; they wear
the bed rocks in a vigorous manner, cutting them down in proportion to
their hardness. As these rocks generally vary in the resistance which
they oppose to the ice, the result is that when the glacier passes
away the surface no longer exhibits the continued down slope which the
rivers develop, but is warped in a very complicated way. These
depressions afford natural basins in which lakes gather; they may vary
in extent from a few square feet to many square miles. When a glacier
occupies a country, the melting ice deposits on the surface of the
earth a vast quantity of rocky _débris_, which was contained in its
mass. This detritus is irregularly accumulated; in part it is disposed
in the form of moraines or rude mounds made at the margin of the
glacier, in part as an irregular sheet, now thick, now thin, which
covers the whole of the field over which the ice lay. The result of
this action is the formation of innumerable pools, which continue to
exist until the streams have cut channels through which their waters
may drain away, or the basins have become filled with detritus
imported from the surrounding country or by peat accumulations which
the plants form in such places.
Doubtless more than nine tenths of all the lake basins, especially
those of small size, which exist in the world are due to
irregularities of the land surface which are brought about by glacial
action. Although the greater part of these small basins have been
obliterated since the ice left this country, the number still
remaining of sufficient size to be marked on a good map is
inconceivably great. In North America alone there are probably over a
hundred and fifty thousand of these glacial lakes, although by far the
greater part of those which existed when the glacial sheet disappeared
have been obliterated.
Yet another interesting group of fresh-water lakes, or rather we
should call them lakelets from their small size, owes its origin to
the curious underground excavations or caverns which are formed in
limestone countries. The water enters these caverns through what are
termed "sink holes"--basins in the surface which slope gently toward a
central opening through which the water flows into the depths below.
The cups of the sink holes rarely exceed half a mile in diameter, and
are usually much smaller. Their basins have been excavated by the
solvent and cutting actions of the rain water which gathers in them to
be discharged into the cavern below. It often happens that after a
sink hole is formed some slight accident closes the downward-leading
shaft, so that the basin holds water; thus in parts of the United
States there are thousands of these nearly circular pools, which in
certain districts, as in southern Kentucky, serve to vary the
landscape in much the same manner as the glacial lakes of more
northern countries.
Some of the most beautiful lakes in the world, though none more than a
few miles in diameter, occupy the craters of extinct volcanoes. When
for a time, or permanently, a volcano ceases to do its appointed work
of pouring forth steam and molten rock from the depths of the earth,
the pit in the centre of the cone gathers the rain water, forming a
deep circular lake, which is walled round by the precipitous faces of
the crater. If the volcano reawakens, the water which blocks its
passage may be blown out in a moment, the discharge spreading in some
cases to a great distance from the cone, to be accumulated again when
the vent ceases to be open. The most beautiful of these volcanic lakes
are to be found in the region to the north and south of Rome. The
original seat of the Latin state was on the shores of one of these
crater pools, south of the Eternal City. Lago Bolsena, which lies to
the northward, and is one of the largest known basins of this nature,
having a diameter of about eight miles, is a crater lake. The volcanic
cone to which it belongs, though low, is of great size, showing that
in its time of activity, which did not endure very long, this crater
was the seat of mighty ejections. The noblest specimen of this group
of basins is found in Crater Lake, Oregon, now contained in one of the
national parks of the United States.
Inclosed bodies of water are formed in other ways than those
described; the list above given includes all the important classes of
action which produce these interesting features. We should now note
the fact that, unlike the seas, the lakes are to be regarded as
temporary features in the physiography of the land. One and all, they
endure for but brief geologic time, for the reason that the streams
work to destroy them by filling them with sediment and by carving out
channels through which their waters drain away. The nature of this
action can well be conceived by considering what will take place in
the course of time in the Great Lakes of North America. As Niagara
Falls cut back at the average rate of several feet a year, it will be
but a brief geologic period before they begin to lower the waters of
Lake Erie. It is very probable, indeed, that in twenty thousand years
the waters of that basin will be to a great extent drained away. When
this occurs, another fall or rapid will be produced in the channel
which leads from Lake Huron to Lake Erie. This in turn will go through
its process of retreat until the former expanse of waters disappears.
The action will then be continued at the outlets of Lakes Michigan and
Superior, and in time, but for the interposition of some actions which
recreate these basins, their floors will be converted into dry land.
It is interesting to note that lakes owe in a manner the preservation
of their basins to an action which they bring about on the waters that
flow into them. These rivers or torrents commonly convey great
quantities of sediment, which serve to rasp their beds and thus to
lower their channels. In all but the smaller lakelets these turbid
waters lay down all their sediment before they attain the outlet of
the basin. Thus they flow away over the rim rock in a perfectly pure
state--a state in which, as we have noted before, water has no
capacity for abrading firm rock. Thus where the Niagara River passes
from Lake Erie its clean water hardly affects the stone over which it
flows. It only begins to do cutting work where it plunges down the
precipice of the Falls and sets in motion the fragments which are
constantly falling from that rocky face. These Falls could not have
begun as they did on the margin of Lake Ontario except for the fact
that when the Niagara River began to flow, as in relatively modern
times, it found an old precipice on the margin of Lake Ontario, formed
by the waves of the lake, down which the waters fell, and where they
obtained cutting tools with which to undermine the steep which forms
the Falls.
Many great lakes, particularly those which we have just been
considering, have repeatedly changed their outlets, according as the
surface of the land on which they lie has swayed up and down in
various directions, or as glacial sheets have barred or unbarred the
original outlets of the basins. Thus in the Laurentian Lakes above
Ontario the geologist finds evidence that the drainage lines have
again and again been changed. For a time during the Glacial period,
when Lake Ontario and the valley of the St. Lawrence was possessed by
the ice, the discharge was southward into the upper Mississippi or the
Ohio. At a later stage channels were formed leading from Georgian Bay
to the eastern part of Ontario. Yet later, when the last-named lake
was bared, an ice dam appears to have remained in the St. Lawrence,
which held back the waters to such a height that they discharged
through the valley of the Mohawk into the Hudson. Furthermore, at some
time before the Glacial period, we do not know just when, there
appears to have been an old Niagara River, now filled with drift,
which ran from Lake Erie to Ontario, a different channel from that
occupied by the present stream.
The effects of lakes on the river systems with which they are
connected is in many ways most important. Where they are of
considerable extent, or where even small they are very numerous, they
serve to retain the flood waters, delivering them slowly to the
excurrent streams. In rising one foot a lake may store away more water
than the river by its consequent rise at the point of outflow will
carry away in many months, and this for the simple reason that the
lake may be many hundred or even thousand times as wide as the stream.
Moreover, as before noted, the sediment gathered by the stream above
the level of the lake is deposited in its basin, and does not affect
the lower reaches of the river. The result is that great rivers, such
as drain from the Laurentian Lakes, flow clear water, are exempt from
floods, are essentially without alluvial plains or terraces, and form
no delta deposits. In all these features the St. Lawrence River
affords a wonderful contrast to the Mississippi. Moreover, owing to
the clear waters, though it has flowed for a long time, it has never
been able to cut away the slight obstructions which form its rapids,
barriers which probably would have been removed if its waters had been
charged with sediment.
[Illustration: _Muir Glacier, Alaska, showing crevasses and dust
layer on surface of ice._]