THE SEA AND THE SKY.
THE SEA—SALT WATER—WAVES AND THEIR EFFECTS—UNDER WATER—THE FLOOR OF THE
OCEAN.
From our childhood the sea has been the companion and playmate of
thousands, the seashore their playground. Men have selected it for
their professional training and livelihood. Authors write of it, poets
apostrophize, scientists lecture upon it, and fathom it, bringing
up from its depths many a new fact and illustration for those who
cannot study it for themselves. There is nothing like it, nothing more
majestic, more beautiful, more life-giving than the ocean—nothing so
changeable nor so true.
From the days when we could toddle along the beach, picking up the
shells, we have wondered at the ocean—What was beyond it? What did it
conceal?
“What hidest thou in thy treasure-caves and cells,
Thou hollow-sounding and mysterious main?”
Let us endeavour to find out.
The first thing that strikes us is the _saltness_ of the sea. Sea
water is salt. Why? One reason is because salts are carried into it by
rivers, and besides, it is more beneficial as salt water. But let us
look at the facts. We know that the earth contains many “salts,” as
we can see by the saline springs. We have already given the chemical
constitution of sea water, but it will be useful to repeat the
proportions.
Water 964·74372 grains.
Salt (Chloride of Sodium) 28·05948 ”
Chloride of Potassium 0·76552 ”
Chloride of Magnesium 3·66658 ”
Bromide of Magnesium 0·02929 ”
Sulphate of Magnesia 2·29578 ”
Sulphate of Lime 0·40662 ”
Carbonate of Lime (with traces of Iodine and Ammonia) 0·03301 ”
----------
1000·00000 ”
----------
Some portions of the sea are not so salt as others, or, in other words,
not so dense, and the saltness of the water prevents it being frozen so
quickly as fresh water, which freezes at 32°. Salt water requires to
be reduced to 28° before it freezes. Besides the various constituents
mentioned above, sea water has been found to contain boron, bromine,
strontia, etc., and even silver, for the copper of ships has been found
to be impregnated with that metal.
[Illustration: Fig. 697.—Going out.]
If there is so much salt in the sea, it may be asked, why does it not
continually become greatly saltier by additions. The reason is because
tons of fresh water are continually pouring in, and though we can
scarcely doubt that the sea is becoming gradually more salt as years
pass away, the increase is very slight. On the other hand, evaporation
is carrying water into the air and leaving the salt behind it. In seas
like the Red Sea, where there is a great deal of evaporation and very
little addition of fresh water in comparison, the water is extremely
salt and bitter. The Baltic has little salt relatively to some parts of
the Mediterranean.
Supposing that, as some allege, there are rocks of salt at the bottom
of the sea, we must remember that springs of fresh water frequently
bubble up to the surface of the ocean. This is a very curious
phenomenon, and has been attested by Humboldt. He states that near Cuba
these springs arise with considerable force, and the vessels trading on
that coast get supplies of fresh water from these ocean springs. There
is, or was, a similar uprising in the Gulf of Spezzia, and fresh water
crustacea inhabit these localities. These occurrences prevent the sea
from becoming too salt by evaporation. When salt water becomes tainted
it is very offensive—much more so than fresh water. If, therefore, the
ocean were not continually in movement, it would be very injurious.
So much for the water of the sea; let us now see what it does. We will
glance at the surface ere we plunge into the depths.
In childhood, and even in after years, we most of us delight in
watching the waves of the sea. What finer sight than that we can
obtain on the bold Cornish coast with a westerly wind, when the great
Atlantic waves come rolling in and dashing up to the tops of the
Tintagel cliffs, wearing and grinding them away; hissing up the sands
at New Quay, or thundering on the shores of “Bude and Boss”! Then the
wind abates, the sea goes down, the billows become waves, the waves
to wavelets grow, less and less, until there is a mere ripple on the
surface which is never still. The mighty heaving of the ocean breast is
the peculiarity of the sea.
[Illustration: Fig. 698.—Sea waves.]
Yet, again, as we stand to watch the waves, or run from them as they
sweep in foam upon the sloping sand, we shall find that they increase
or decrease in force, and the level of the water rises or sinks by
degrees. The _tide_ is flowing or ebbing as the case may be. So we know
the surface has another—a current motion—besides the undulation of the
water. The currents of the ocean are very valuable attributes, the Gulf
Stream in particular bringing us warmth and, indeed, rain. There are
three movements of the ocean—waves, currents, and tides.
The waves, perhaps, interest us most, as they come rolling in with
irregular force, but all mightily impelled by the wind. We have all
noticed the ripples on a puddle; the same action of the wind produces
the grandeur of the waves of the ocean. The wave comes rolling in
before the wind to break against the rocks or beach, and another
forms to break in its place; the higher the waves the more quickly
they appear to move. But when the wind has subsided the rolling, or
“swell,” remains,—a long, lazy, undulating motion—a rocking to sleep
of the billows of the sea. Without a ripple on the surface these huge
rollers will glide towards the shore and break upon the shingle with a
roaring sound which can be heard for miles, dragging the pebbles after
them as they recede with a rattling like bones and marbles. The pebble
ridge at Westward Ho! will illustrate this vividly at times, the sound
being heard far inland like continuous thunder, and on a calm night,
when there is no wind stirring, the roar of the ground swell is weird
and mysterious in the gloom.
[Illustration: Fig. 699.—The Piroroco on the Amazon.]
The height of waves is very varied. Observers say that forty-four
feet is about the highest-known wave from hollow to crest. Waves of
thirty-five feet have been often met with, and off the Irish coast and
in the Atlantic sailors tell of waves “as big as houses.” But houses
differ in size as do waves.
The rate which waves are estimated to travel varies with the
wind-propelling force. The average hurricane wave travels at about
forty-five miles an hour. But earthquake waves—those set in motion by
subaqueous disturbance—have been known to travel at the rate of six
hundred feet in a second for thousands of miles across the ocean. Such
a one occurred after the earthquake which destroyed the town of Arica
in August 1868, and the wave crossed the Pacific to Chetham Islands,
5,520 miles, in fifteen hours and twenty minutes. We have many of us
seen the great tidal waves, or “bores,” which at certain seasons rush
up our rivers—the Severn, for instance—with great violence, and at
times forty feet high.
These tidal waves are also experienced in the Ganges, the Amazon, and
at Bordeaux, as well as in China and elsewhere.
It may well be imagined that the tides also affect the land, and the
theory of these ocean movements is a very interesting study. We have
already referred to it under ASTRONOMY, for the Sun’s and Moon’s
attraction is the main cause of the phenomenon, which is so familiar
and yet so strange. But the consideration of the tides must be again
entered upon here ere we proceed to view the effects of the sea upon
the land, and how the physical geographical features alter.
Isaac Newton rightly attributed the cause of the tides to the
attraction of the moon and sun. Spring tides occur when both luminaries
are above the meridian, and the neap, or low tides, happen when the sun
and moon are farthest apart. The highest tides are perceived after a
new or full moon; the lowest, after she has passed the first or third
quarter. In January the spring tide is highest of all, because the
earth is nearest to the sun then, and his force of attraction, added
to that of the moon, causes a very high tide. With the assistance of
the accompanying diagrams we shall be able to make the tidal phenomena
clear.
[Illustration: Fig. 700.—Tidal Attraction.]
[Illustration: Fig. 701.—Tidal Attraction.]
Suppose the moon to be at M, the point J (the sea) will be nearest
to the moon and will be attracted, while the earth will exercise a
retarding power to a certain extent. This attraction of the water from
its usual level causes a kind of vacuum, into which the surrounding
water flows and causes a high tide at H. At the opposite side the
earth, not the water, is most attracted, and then the water rushes in
to a certain extent to fill the vacancy left by the earth’s movement
towards the moon. Another high tide is therefore caused at L, but not
so high as the tide upon the opposite side, as the Moon is so much
nearer the latter. The tide, then, is only the natural movement of the
sea water to fill up the space the earth and other portions of the
watery mass have vacated in obedience to lunar and solar attraction,
which is, to a certain extent, counterbalanced by the attraction and
resistance of the earth.
The _neap_ tides are caused by the opposing forces of attraction of the
sun and moon. The sun, as it were, pulls one way, the moon the other.
The latter (being nearer) having twice the power of the former, causes
a tide indeed, but it is a low one. The _spring_ tide occurs when sun
and moon together attract the water.
The effects of the rise of the tide are sometimes very disastrous,
and when the wind assists the sea, and heaps up the water, the sight
is grand in the extreme. On the coast of Schleswig, at Hallingen, the
sea has washed away a whole cluster of islands, and now the waves cause
tremendous inundations. About every six years, on the average, a great
flood happens for such trifles as a high tide are of no account. In
1362 and in 1834 terrible destruction was wrought; the coffins and
bodies were washed out of the graves. Piles of _débris_ are then washed
up, and sand and gravel accumulates for a time till again carried away.
Travellers to France will notice the “dunes,” or sandhills of Calais,
as the train winds its way to Boulogne. We find that whenever the
shore is flat the shingle and sand are blown inwards and form “dunes,”
and the sand is distributed far inland, checking all vegetation, and
altering the features of the country. The wearing away of rocks by the
water, the continual undermining of them by the waves, and sometimes
the disengagement of great blocks weighing many tons—all these effects
of the sea tend to alter the appearance of the land. We may observe the
denudation in many places along the coast—the caves, holes, and tunnels
eaten out by the water. In Norway the “Fiords” are very remarkable.
They were formed by the upheaval of the land, and tell us of the
glaciers which once filled them up. Thus by ice and water the solid
land is ground down and eaten away hourly, daily, and for countless
centuries, changing the place of the hard rock into a standing water,
and the flintstone into a springing well.
[Illustration: Fig. 702.—The Dunes.]
We must now plunge beneath the waves, never fearing the rough surface;
we shall find all smooth and quiet at the bottom of the sea.
THE BOTTOM OF THE SEA.
What can we tell about the bottom of the sea to which no man has ever
reached living, and from which we have no information? We can lay
our telegraph or telephone lines beneath the waves, and far from the
restless waves in those quiet depths where no billows can reach. What
treasures must lie hidden at the bottom of the sea! The treasures, the
gold and silver, the merchandize, the wealth of centuries. The sailor
lies sleeping there
... “Serene and safe
From tempest and from billow;
The storms that high above him chafe
Ne’er rock his peaceful pillow”!
What can we hope to find at the bottom of the sea we cannot reach? Yes,
but we can reach it. By sounding with Brooke’s lead (a cannon ball, as
shown in the illustration), we can arrive at a certain knowledge of
the composition of the ocean bed. The right-hand figure of the two is
the lead when being lowered, and while it is sinking the cord remains
tight. So soon as it touches the bottom the weight of the cannon ball
divides the line, and the tube is easily drawn up again. It has been
well greased, and so in the cavity of the rod some shells and sand are
found adhering. These fragments tell us the composition of the bottom
of the sea.
Here we find tiny shells, just as we find them in chalk, the same
formation as that which piled up the cliffs which have risen from, or
been discovered, by the sea. By other ingenious contrivances water can
be fetched up from the bottom of the ocean, and the temperature can be
gauged all along the sounding line. The expedition of the _Challenger_
brought many interesting facts to light. Far down in these solitudes
are marine animals,—crustacea, star-fish, seaweeds, and shells,—all
of which are carried up by the dredge worked by a steam engine; for
the resistance is very great, and the weight supported at the depth
of two miles must be considerable, and is equal to four atmospheres.
A thermometer has come up crushed even in its iron case, and so the
creatures which inhabit and find means to live at the bottom of the sea
must be specially fitted by Nature for the locality.
[Illustration: Fig. 703.—Brooke’s Lead.]
The configuration of the ocean bed has given rise to many different
opinions. It has been maintained that there are mountains and valleys,
hills and dales underneath the water, all clothed with marine
vegetation, equal in height and depth to the terrestrial hills and
vales. Again it has been declared that the ocean bed is level; but we
find raised portions, which we call islands, which may be the tops of
mountains, or portions of the mainland separated from their parent
continent by an inroad of the sea, as are our islands of Great Britain.
The sea-bed, however, is very irregular. We find deep and steep
valleys, and high hills, but the picturesque peaks caused by the action
of air, frost, and water on earth are not, of course, represented
under water. Between the Irish coast and Newfoundland we are told the
bed is level for nearly four hundred miles. There is a deep declivity
before we reach this plain. The centre of the Atlantic is a plain,
and on it the most volcanic islands rise, such as Ascension and the
Azores. Between England and Greenland there was at one time a land
communication, as we have remarked under GEOLOGY, and there are
submarine terraces now. [An immense river once ran through Western
Europe somewhere about where our islands are.]
[Illustration: Fig. 704—The Drag Net.]
Under the Atlantic we have remains of _foraminifera_ and other tiny
animals, with red clay and volcanic remains which must have been of
submarine origin. The Pacific shows us tops of mountains as islands
(Hawaii Isles), and an enormous range must be hidden beneath the
waters. What a change in the physical geography of the earth a slight
sinking of the water of the ocean would make; England and the Continent
would be united, and many sea-mountains (islands) discovered. The
greatest ocean depth is four miles and a half, but in many places a few
hundred feet less depth than at present would reveal many changes in
the land.
Every year since the world has gained its present form the streams and
rivers have been pouring water, and carrying mud, stones, and gravel
ceaselessly into the ocean. In addition to this, the surface water
washes the stones away, animals (corals) build up islands from the
depths, and take up space in the ocean. We know that if we put our
hand in a basin full of water we displace a quantity of the fluid; so
we might imagine that, the sea being already full, every island formed
would tend to an overflow of the sea, and the land would be thereby
buried. That the sea does encroach upon the earth we know, but it also
recedes. Here is the balance of Nature.
Rivers pour in water and material. The sun absorbs the water and
prevents overflow; tiny animals make shells from the material. All the
causes we have mentioned tend to permit the encroachment of the waters,
but volcanic action and even earthquakes act also to neutralize this
tendency by upheaving hills and mountains, which prevent the invasion
of the sea by its elevation or by land depression. We have seen in
our chapters upon GEOLOGY how the ocean beds have been upheaved, and
remains of marine animals are daily found upon our highest hills. Thus
the forces which sometimes cause such destruction in the earth are the
means whereby the waters are kept in their places. But for volcanic
action the land might all disappear by denudation and continual wear
and tear, and be deposited at the bottom of the sea!
If it were not for currents, of which many defined ones exist in
the ocean, and the never-ceasing flow and ebb of the tides, the sea
would soon lose its purity and clearness. Though the water is salt
and becoming salter, animalculæ and all kinds of plant-animals would
still increase and multiply; so the decay of animal and vegetable
matter would quickly render the ocean a source of pestilence and death
to mankind, and be most injurious to animal life generally. But the
movement is so ceaseless, and the various fish and mammalia (whales,
for instance), by preying upon each other, as other animals on earth
do, keep up the balance of production, and the organic matter deposited
in the sea is also cleared away.
That the constant currents of the sea prevent the formation and
growth of seaweed is clearly shown by the great “Sargasso Sea,” or
tract of weed (_Fucus natans_), called the Gulf-weed. This great
tract embraces thousands of square miles, and is situated in the very
middle of the Atlantic Ocean, where there are but few currents; but
surrounding it is the Gulf-Stream, an enormous current of water running
at a regular rate of four or five miles an hour. This Gulf-Stream is
supposed to be caused by the same laws and influences which determine
the trade-winds—namely, a constant rarefaction of the water at the
tropical parts of the earth, and a corresponding condensation at the
Arctic portions, for warm water is much lighter than cold, and when
the waters of the tropical regions become lighter, the heavier waters
of the cold regions pressing down more forcibly tend to raise them
above their proper level; they therefore flow towards those very parts
which have sunk down by their contraction, and a constant current
takes place; this current is the Gulf-Stream. It runs from the Gulf of
Mexico northwards towards Newfoundland, turning by Iceland towards the
British Isles, by France and Spain, onwards to the coasts of Africa
and South America, the West Indies, and again to the Gulf of Mexico,
although the return current does not go by the name of Gulf-Stream.
This great stream of water, warmed by the tropical sun, serves the same
two purposes fulfilled by the trade-winds—namely, a circulation and
distribution of the superfluous heat of the equatorial regions, warming
the northern countries; and cooling, by the return of under-currents,
those in the tropics. The fogs of Newfoundland are caused by the great
current of warm water entering the cold region and carrying with them
surface-currents of moist air, which the cold condenses into fog, just
as the breath is visible in a cold atmosphere. England owes its moist
and mild climate to the same cause. The depth of the Gulf-Stream itself
is very little. It is a mere layer of warm water. (_See_ Sir George
Nares’ reports of the _Challenger_ expedition.)
[Illustration: Fig. 705.—Atoll, or Coral Island.]
In the foregoing pages you have now seen, and, we hope, gained, some
information concerning the sea sufficient, at any rate, to induce you
to enter more deeply into the subject than we can at present do. We
have learnt how the sea water is composed, and what goes on on the
surface. We have discussed waves, and referred to tides and currents,
the wearing away and the renewal of land by the sea; we have dived
beneath the surface, and found something to interest us at the bottom
of the ocean. As we come up again we are surprised to find islands or
reefs where none existed when we went down. What has caused this sudden
appearance? They may have been slowly raised to the surface by coral
insects, or suddenly by volcanic action. Let us consider the coral,
which plays a very important part in our Physical Geography, before we
proceed to the volcanic island.[32]
The low-lying islands are those formed by the skeletons of the coral
insects, and the Coralline Islands are some of the most wonderful
productions of nature. They are only found in warm climates, between
the twenty-eighth degrees of north and south latitude, and limestone
pure and simple is the chief component of the coral reef, as it is of
the mountains erupted from the depths of the sea. “The detritus of
corals, echinodermata shells, reticularia, and other living creatures,”
says a writer on this subject, “deposit not only the salts of lime
extracted from the ocean, but their own dead bodies to form the hard
substance of the rock.”
[Illustration: Fig. 706.—Gorgonia guttata (natural size).]
[Illustration: Fig. 707.—Coral (_Madrepora brachiata_).]
[Illustration: Fig. 708.—Spicules of Gorgonia (magnified).]
The coral insect is a zoophyte (Anthozoa), which, as may be seen from
the illustrations, assumes curious and elegant forms, and the coral it
produces is a limy or calcareous deposit, which is fixed upon a rocky
base. As years go on these accretions become greater and greater, and
at length rise above the water. When a little distance below it, the
reefs form dangerous and frequently unsuspected barriers, upon which
ships are wrecked. The red coral is dredged up from the Mediterranean,
where there are extensive coral fisheries. This coral is found deep in
the water, and never rises to the surface. Formerly there were coral
reefs in the European seas, but the changes of temperature stopped
their production. The “atolls,” or circular coral reefs with an opening
at one side, have been described by Professor Darwin. “Who,” says the
great naturalist, “would not be struck with wonder and admiration on
catching sight for the first time of this vast ring of coral rock,
often many miles in diameter? Sometimes a low green island is seen
beyond it, with a shore of dazzling whiteness; outside is the foaming
surf of the ocean, and within it a broad expanse of tranquil water,
of pale green colour and exquisite purity.” These “atolls” mark the
situation of sunken islands, and the extension of them and the barrier
reefs would seem to indicate a slow but decided sinking of the bottom
of the Indian and other oceans; but the “reefs” tell us that the land
to which they are attached has not become depressed, and may have
become elevated. We may then conclude that a continual rising and
depression of the land is taking place in various oceans, indicating
a sinking of the ocean bed in one locality and the result of volcanic
activity in another, for no active volcanoes are found in the regions
of depression.
We must now leave the sea and come to land again, to consider volcanoes
and volcanic action there.
VOLCANOES AND EARTHQUAKES.
The various phenomena of volcanoes form a subject very difficult to be
explained, as it is impossible to ascertain positively the cause of
volcanic action. Whether the earth is interiorly a mass of molten rock
and fire, or whether the heat is created by the intense contractile
force and movement of rocks, and their motion thus developed into heat
aided by chemical combination, we cannot absolutely determine. The
theory restricting volcanic phenomena to the upper crust of the earth,
by supposing the local accumulations of hot liquid masses of rock,
which are forcibly emptied by the expansion of vapours, may perhaps be
found the true one.
The majority of the volcanoes are found near, or at no very great
distance from, the sea. We may therefore expect to find that water
has something to do with the eruptions as it has in the case of the
Geysers. But this hypothesis will scarcely hold good in every case,
though volcanoes of later ages are limited to regions very different
from those in which volcanic action used to be. For instance, in
America we have only volcanoes on the Pacific side, and the Andes
furnish several. Mexico, Central America, and California possess many
volcanoes, and as far north as Alaska we find Mount Elias. There are
plenty of extinct volcanoes in Europe, but the Mediterranean produces
the active vents; and about the Red Sea and the Caspian, and even in
the central chain of Asia, there are volcanoes far from water. The
Hawaii isles, on the other hand, are all volcanic, and Australasia
furnishes us with remarkable specimens; so altogether the testimony
tends to prove that where volcanic remains are apparent the sea had at
one time been, or now is, near at hand.
Burning mountains have been familiar to us from our childhood in
pictures, and by stirring narrative of destruction wrought by them.
The volcano is generally a mountain rising to a cone, but Vesuvius
presented quite the appearance of a hollow basin at the top, before it
suddenly broke forth and buried Herculaneum in ashes. Von Buck visited
it in 1799, and declares it had at one time risen, like an island,
from the sea. There are about two hundred and seventy volcanoes at
present in activity; four in Europe; eleven in Iceland and Jan Meyen’s
land; in Asia, ninety-three; in Africa, twenty-six; forty-six in North
America and the Aleutian Isles; twenty-seven in Central America and the
Antilles; in South America, thirty-one; and twenty-four islands with
volcanic tendencies largely developed. There may be many more “resting.”
Volcanoes, then, are openings or vents which communicate with the
melted rock within the earth, and the conical form of volcanoes is
owing to the deposits of volcanic matter as it falls from the opening
called the _crater_. If we let a small spade full of mould run through
our hands, or from the spade, it will form a small cone, the heavier
particles sliding to the base at a certain slope. Thus the volcano
builds its own hill, and inside the crater we find cones from which
smoke and steam issue. These cones within the cone are the points of
issue of vapour and smoke, miniature volcanoes making up a whole.
[Illustration: Fig. 709.—Eruption of Vesuvius, August 26th, 1872.]
The signs of eruptions are much the same, and usually occur a couple
of days before the actual outbreak takes place. First smoke is
perceived, perhaps, and the escape of various noxious gases accompanied
by earthquakes occur. Now the eruption may commence and blow away the
summit of the mountain, as in the case of the commencement of the
catastrophe of A.D. 79, when the whole side of Vesuvius was torn away,
and continuous showers of ashes fell for days and nights, burying
everything, while the hot lava poured down the sides. Stones and ashes
with vapours are hurled into the air. Clouds of steam are formed,
and vivid electrical discharges take place in these clouds, while
water dashes down, carrying stones (“volcanic bombs”), and reflected
lurid flames from within are cast on the steaming clouds, which look
like fiery columns. Then the lava issues in a white, hot, steady,
irresistible stream, covering everything, and burning up all vegetation.
[Illustration: Fig. 710.—Birth of a volcano.]
New volcanoes are continually in process of formation, and at Santorin
for hundreds of years volcanic action has been busy in forming islands.
These violent efforts of Nature frequently give rise to earthquakes,
which are the most destructive of natural convulsions. The records of
late occurrences are fresh in the minds of all readers, and need not
be specified. The slow subsidence and gradual upheaval of the land is
still going on, but we are frequently startled by the account of a
rupture of the ground or the destruction of a portion of a city.
The motion of the earthquake is generally in a direct line, and
undulating. Sometimes what are termed vertical shocks arise and destroy
solidly-built edifices. Mountains have been overturned by earthquake
shocks, and trees have been twisted round. Sometimes the ground
yawns into enormous fissures. The sea is tossed into great waves and
encroaches upon the land, and when the sea recedes the recession of the
water is followed by a more terrible invading wave sweeping all before
it. Earth tremblings often occur far away from volcanoes, and without
any visible connection with volcanic action. There are many aspects of
land and water which the student of geography will remember, but which
need not be separately treated of. We must, however, refer to plains,
plateaus, and lakes. The mountains also play a most important part in
Physical Geography and in “Climatology,” as they collect the vapours
for rain, and make the valleys fertile, and thick with vegetation. We
have spoken of the mountains under GEOLOGY, and the various formations
and strata will be found enumerated there, but now we have to do with
the mountain chains in their physical aspect as regards their shape and
appearance on the globe.
[Illustration: Fig. 711.—Earthquake fissures.]
Any elevation rising from a base more than 1,000 feet may fairly be
termed a mountain, and solitary mountains are usually volcanic, because
eruptive rock does not produce chains of mountains. The origin of
mountains is probably due to the contraction and compression of the
crust of the earth—not merely the surface, but the whole thickness
between us and the supposed molten interior. Mountains did not exist
from everlasting, for the very good reason that they are (in most
cases) composed of stratified rocks. Stratified rocks are sedimentary
rocks, and must have been deposited below water, and hardened long
before they were thrust up by pressure. Moreover, we find (as has
already been explained) shells and remains of marine animals on the
higher summits, which prove to demonstration that these mountains are
composed of rocks which were laid down under the sea.
Professor Dana was one of the first geologists to advance the theory
that contraction and lateral displacement are the causes of the
elevation of mountains. A very good illustration of this theory was
made by Chamontier, who covered an india-rubber balloon with a thick
layer of wax, and when it had hardened sufficiently he pricked a hole
in the bladder, which immediately contracted, and the wax at once rose
up into tiny similitudes of mountains, showing in a sufficiently clear
manner that such protuberances may be produced by the pressure of the
earth’s contraction, and in such a mass as our earth the elevations
would naturally be very great.
Professor Geikie has shown how, by a very simple experiment, the
contortion of mountain strata is effected by pressure. A number of
cloths or towels placed flat on a table represent the sedimentary
rocks. Place a board with a weight on the top, and the towels will
remain flattened. But by holding two boards at the sides and pressing
them together (the weighted board still remaining), we shall find
the towels crumpled and upheaved like the Jurassic strata shown in
the illustration (fig. 712). Professor Heim calls the central masses
wrinkles of the earth’s crust. So the Alps were pressed up or heaved
into the air, the weather—rain, frost, snow, and sunshine—imparting
the infinite variety of “Horn,” “Needle,” and “Peak,” so expressively
applied in Alpine nomenclature;—the Matterhorn, Wetterhorn, Weisshorn;
the Pic du Midi, Aiguille de Dru, Aiguille Verte, and many other
mountains in well-trodden Switzerland will occur to the reader at once.
[Illustration: Fig. 712.—Anticlinal and synclinal curves of the Jura
Mountains.]
The slopes of mountains—though to the casual observer they may appear
very much the same—are very different. We sometimes find a long,
easy ascent, —more usually a steepish inclination, perhaps 20°;—in
other places, such as on the Matterhorn, an almost perpendicular
face. Forty-five degrees rise is very steep, and 53° is the limit
of any great mountain’s slope. Cliffs and precipices there are, of
course; witness the terrible fall from the Matterhorn to the glacier
below—thousands of feet with one tremendous leap from the rock to the
ice underneath; but mountain _slopes_ are not precipices. As a rule,
we find that one side of a mountain chain is steeper than the opposite
one. It is harder to climb up from Italy to Switzerland than to ascend
in the opposite direction. The Pyrenees are also steeper on the south
side. The Scandinavian mountains likewise are steeper in the west.
The Himalaya are steepest towards the sea, so are the Ghauts. We here
find a difference between the slopes of the NEW and OLD WORLDS. In the
former we have the less precipitous mountain slopes towards the east;
in the old world they are towards the north, and an inspection of a
physical map of the world leads us to the conclusion that the Atlantic
and Pacific Oceans are the boundaries of entirely different degrees of
slopes. The Pacific and Indian Oceans would appear to border the more
precipitous mountain sides; the Atlantic and its connections those less
steep.
As a rule, we have the most elevated portions of the earth, mountains,
and high tablelands, in equatorial regions; and within the torrid
zone every terrestrial climate is to be found, owing to the snows of
the high mountains and the heat of the valleys, which are naturally
closely connected with the upheaval of mountain ranges. We have already
spoken of the never-ceasing influences of the air and water upon the
rocks, and we need say little about valleys. There are valleys of
dislocation, denudation, and undulation. The great valley of Western
Asia, wherein lie the Caspian and Aral seas, seems to have been caused
by the upheaval of the Caucasus and the Persian plateau.
PLAINS are very varied. We have European _Heaths_ and _Landes_;
American _Savannahs_, _Prairies_, and _Pampas_; Asian _Steppes_, and
African _Deserts_. All of these possess certain features in common,
more or less vegetation, and sometimes absolute sterility.
PLATEAUS, or _Tablelands_, are elevated plains frequently undulating
in character. The Plateau of Bolivia is 13,000 feet high, and extends
along by the Andes. The tableland of Quito is nearly 10,000 feet high,
and borders on the giants Cotopaxi and Chimbarazo.
[Illustration: Fig. 713.—The Staubbach (Lauterbrunnen).]
Rivers and lakes add not only to the wealth of nations by their
usefulness, but, by the additional picturesqueness of their appearance,
to the beauty of the landscape. The velocity of rivers would be very
much increased if it were not for the strong resistance offered by the
banks and the stones to the current, and by friction. The Rhine and the
Rhone, if thus unimpeded, would flow at a rate considerably over one
hundred miles an hour; and our own little stream (the Thames), instead
of eddying peacefully and twirling gracefully by Medmenham or Cookham,
would rush along at the speed of the train which so often crosses it on
its way to the sea.
The slopes of river-beds, like the slopes of mountains, vary very
considerably, and the inclination of a river varies at different
places; in a distance of seven hundred miles the Amazon only falls
twelve feet, and the current flows chiefly by impetus already acquired.
A slope of one foot in two hundred precludes all navigation, and at
still greater inclines rapids and cataracts are formed—the great
falls wearing away the river-bed by degrees; so it is calculated that
hundreds of years ago Niagara Fall was much farther down the river, and
the cataract is slowly moving up stream. In time, as the rock wears
away, the height will disappear as the celebrated “Falls,” and will
become a rapid within a few miles of the lake.
Lakes are derived from river-drainage and springs. Some are very salt,
owing to evaporation carrying away so much water, and leaving the
accumulated mineral salts. These very salt lakes are likely to dry up,
as the supply of water is not equal to the demands of evaporation.
Floating islands appear and disappear on many lakes. Derwentwater is
one instance. On the uses of lakes and rivers it would be superfluous
to dwell. We are more concerned to examine their influence on climate,
and in this sense we must also consider mountains. But we will now
group all the phenomena of the air and water, and their effect upon
climate, under “Meteorology” in the chapters next following.
FOOTNOTES:
[32] Those who wish to study the subject fully should read “Corals and
Coral Reefs,” by the late Charles Darwin, and Dana’s “Coral Islands.”