<h2 class="nobreak" id="CHAPTER_III">CHAPTER III</h2>
<h3>EFFECTS OF WEATHER ON THE EARTH'S HISTORY</h3></div>
<p>The same causes that produced the layers of peat
or sand, or limestone, or clay, which we find by
examination of the earth's surface, are acting
to-day. Coal is forming now; and so is limestone;
and so is sandstone; so even is granite. But these
layers or strata form very slowly, so that since man
has kept historical records the thickness of new strata
laid down could be measured in inches. Consequently
we are only able to see the beginnings of the processes.
After the materials were laid down by water
or the shifting winds, or by the decay of other materials
already in position, they underwent various changes. For
example, many layers, instead of consisting of loose materials
such as gravel, sand, or mud, are now hard stone.
Sometimes this consolidation has been the result of pressure.
As bed was piled over bed those at the bottom
would be more and more compressed by the increasing
weight of those laid down upon them; the water would
be squeezed out; the particles would stick closer together.
Mud, for example, might thus turn into clay;
<span class="pagenum" id="Page_40">-40-</span>
and clay, pressed harder and harder, might be converted
into mudstone or shale. But there is another agency at
work. We have all seen mortar hardening and binding
bricks together; or cement hardening into concrete.
Similarly sedimentary deposits are bound together by
cements, of which there are many which exist naturally.
For example, silica is a natural cement; and so is carbonate
of lime; and so is peroxide of iron. All these will
bind other particles together. But how do they arrive at
the layers of particles? By the same action which lays
down the particles themselves. They are rubbed off the
places where they exist by the wind or by water. Perhaps
they were laid down among the deposited particles
of mud or sand. Perhaps they were brought to them by
streams or rivers or lakes, and sank with the water into
them. In a red sandstone, for example, the quartz grains
of the rock may be often observed to be coated with
earthy iron peroxide, which serves to bind them together
into a rather hard stone. On the other hand, the process
is often being reversed. The weather frequently conspires
by frost and wind and rain to remove the binding
cement, and thereby to allow the stone to return to its
original condition of loose sediment.</p>
<div class="figcenter"> <ANTIMG class="w100" src="images/fpage40.png" width-obs="643" height-obs="379" alt="" /> <div class="figcaption"> <p class="tdc smcap">One of the Colossal Natural Bridges of Utah</p>
<p>This is an instance in which water has hollowed out the lower strata,
leaving a harder upper stratum partially intact.</p>
</div>
</div>
<p><span class="pagenum" id="Page_41">-41-</span></p>
<p>For millions of years the winds have blown over the
surface of the earth, the rain has fallen on it, the sun
heated it by day, the frost cracked it. Consider the
winds that have circled the earth. All movements of the
air are due in the first place to the sun which heats the
atmosphere and causes it to expand. The sun's rays
passing through the air do not heat it at once, or
directly, but heat the land and the sea, which absorb
some of the rays and reflect others and so warm the air
in contact with them. But, as will readily be understood,
the land and the sea do not absorb and reflect the heat
rays in the same way or to the same extent; nor do the
sun's rays fall equally or constantly on all portions of
the earth's surface. So that from various causes one
part of the earth is always being warmed in a different
way from other parts, and the air above the earth is
being warmed in an immeasurable number of different
ways. Even if the earth's surface were all water or all
land, we should expect therefore that there would be
movements of the air due to unequal heating. If, however,
the earth's surface were quite even and uniform,
we should expect that there would be a certain evenness
and uniformity about the movements of the air. These
movements would be due partly to the regular heating
and regular cooling of the surface, and partly due to the
fact that the earth is spinning round taking the air with
it—but not taking it quite evenly. The air does not fit
tightly on to the earth. It is rather like a loose, baggy
envelope with a tendency to slip as the earth moves
round. Furthermore, a point situated on the Equator
has much farther to travel in twenty-four hours as the
earth spins round than a point situated in the Arctic Circle,
where a tape measure placed along one of the parallels of
latitude (let us say the eighty-sixth parallel, where Nansen
turned back in his search for the Pole) would show the
earth's girth there to be, not twenty-four thousand miles,
but only so many hundreds. This also would make a
<span class="pagenum" id="Page_42">-42-</span>
difference in the way the air would be whirled round the
earth; but we could take this point into consideration,
and should be able, if, as we have said, our earth were
quite uniform, to say always and at all times of the year
in what direction the prevailing wind should blow.</p>
<p>Even with all the earth's irregularities we do know a
good deal with certainty about the earth's prevailing
winds: the trades; the anti-trades; the south-west
monsoon, which sets in so regularly in India that year by
year its advent hardly varies by more than a day; and,
in the descending scale of regularity, the east winds
that usually sweep England in March, and the prevailing
south-westerly to westerly winds which bend most of the
young trees of the country a little to the north-east.
Besides these regularly or irregularly defined winds,
there are certain paths along the earth's surface where
the winds always move like a trout stream with eddies
in it. These eddies of the air we call cyclones, and they
are continually travelling in one direction. No doubt
they arise from the air in one place becoming hotter or
moister than in the surrounding regions. As the air
grows hotter it becomes lighter and ascends, while the
heavier air round it pours in. These eddies always travel
eastwards and incline in the northern hemisphere towards
the north. They usually originate somewhere on the
North American continent, and move across the Atlantic
about the pace of a slow railway train, winds whirling
round them all the time at a much greater pace. Usually
the centres of these eddies bear northward past the north
coast of Scotland to the north-west of Norway. Sometimes,
<span class="pagenum" id="Page_43">-43-</span>
however, they take a more southerly course,
keeping to the south of the British Isles and passing
over Central Europe on to Siberia, where they appear
to die away.</p>
<p>Such are the cyclones which are in the main part responsible
for British weather; and the winds that accompany
them vary a great deal in strength. They depend
on the size of the eddy. If the eddy is a very big one
(and sometimes the eddies are thousands of miles across)
the winds will not be so strong as in the smaller ones. It
is, therefore, the smaller ones which cause the violent
storms. In the tropical regions whirling eddies of a
rather different character occur. To quote Mr. J. H. N.
Stephenson: "Instead of being measured by some hundreds
or even thousands of miles, they are usually only
some hundreds of yards across; and as we found that the
smaller the cyclone the more violent the wind, we shall
not be surprised that the wind in these is more violent
than anything we ever experience in this part of the
world. They are called by many different names; in the
West Indies they are known as <i>hurricanes</i>, in the south-east
of Asia as <i>typhoons</i>, and in North America as
<i>tornadoes</i>. These hurricanes or tornadoes travel much
faster than the larger cyclones, and the winds blowing
into them are so violent that everything—trees, houses,
bridges—are swept before them, and so strong is the in-draught
of air in the centre that strong walls are sucked
in just as a piece of paper is in front of a grate when the
fire begins to blaze up; and even heavy metal objects are
carried <i>upwards</i>. Fortunately these tornadoes do not
<span class="pagenum" id="Page_44">-44-</span>
travel continuously along the ground but bump along it,
so to speak, sometimes passing harmlessly overhead, then
striking the earth again and causing more havoc. Where
they pass over the surface of the sea the water is sometimes
sucked in just in the same way, causing what is
known as a <i>waterspout</i>. These may do even more damage
than a tornado on land, for the water is sometimes carried
bodily on to the land, sweeping everything away in a
deluge. This happened many years ago in the delta of
the Ganges, when thousands of people perished."</p>
<p>Now let us see how these winds might leave traces in
the geological record. When soil is exposed to the sun
its surface becomes dust, and the wind carries it off.
Even where turf protects the surface, bare places may
always be found whence this covering has been removed.
Rabbits and moles bring up the earth to the surface;
the earthworms sometimes bring as much as ten
tons of earth to the surface of a single acre of turf in
the course of a year. The earthworms bring up only the
finest particles of mould; and these, of course, are the
very particles readily converted into dust and borne
away by the wind if they are not washed away by rain.
In tropical countries the white ant conveys a prodigious
amount of fine earth up into the open air, building walls
sometimes sixty feet high. Although, therefore, the
layer of vegetable soil which covers the land appears to be
a permanent protection, it does not really prevent a large
amount of material from being removed even from grassy
ground. The wind carries this fine dust far and wide
over the land, and over the sea as well. After the eruption
of the island of Krakatoa in 1883, the dust which
was the product of that mighty explosion was carried
round the world, and even in England we saw the dust
particles furnishing extraordinary colours in sunset skies.</p>
<div class="figcenter"> <ANTIMG class="w100" src="images/fpage44.png" width-obs="448" height-obs="644" alt="" /> <div class="txtlf"><i>Stereo Copyright, Underwood & U.</i></div>
<div class="txtrt"><i>London and New York</i></div>
<div class="figcaption" style="clear: both; padding-top:1em;">
<p class="tdc"><span class="smcap">The Garden of the Gods, Colorado</span></p>
<p>These peaks exhibit the gradual wearing away of hard rocks by the action
of rain and wind.</p>
</div>
</div>
<p><span class="pagenum" id="Page_45">-45-</span></p>
<p>In dry countries, especially in the large tracts of
Central Asia and of Africa, the air is often so thick
with a fine yellow dust that the sun's light struggles
through it as through a London fog. The dust settles
on everything, and after many centuries a deposit, which
may be hundreds of feet deep, is thus accumulated on the
surface of the land. Some of the ancient cities of the old
world, Nineveh and Babylon for example, after being long
abandoned by man, have gradually been buried under the
fine soil which the wind blew over them. Even in
England the Roman town of Silchester, not far from
Reading, after falling into decay when its inhabitants
left it, has been buried under the accumulations of two
thousand years, and its walls and floors now lie underground
and have to be carefully unearthed in order to
lay them bare. But we need not seek these exceptional
cases in order to perceive what the wind is doing with
sand and the fine dust of the earth's uppermost layers.
At many places round the coast are sand-dunes. On
sandy shores, exposed to the winds that blow off the
sea, the sand is dried and carried away from the beach,
gathering into long mounds or ridges which run parallel
to the coast-line. These ridges are often fifty or sixty
feet, sometimes even more than 250 feet high, with deep
troughs and irregular hollows between them, and they
sometimes form a strip several miles broad bordering the
<span class="pagenum" id="Page_46">-46-</span>
sea. These sand-hills creep farther inland, till their
progress is stopped by the fields or woods they encounter,
or till, by seeds finding a root, vegetation springs up on
them and they harden and consolidate under the influence
of their own vegetation and move inland no farther. But
in many parts of Western Europe and Eastern America
the dunes are marching inland at the rate of twenty feet
a year. Off the coast of Friesland and North Germany
the danger has grown so threatening that scientific
attention has been given to the problem; and the
German scientific men have employed ingenious devices
of planting wind-stakes—something like the wooden
breakwaters that are to be found along every seaside
beach, but arranged at different angles,—of forcing the
sand-dune to heap itself up so as to form an obstruction
to further arrivals; or of sowing those plants in the sand
that will bind its particles together, in order to preserve
the land from further invasion.</p>
<div class="figcenter"> <ANTIMG class="w100" src="images/fpage46.png" width-obs="420" height-obs="640" alt="" /> <div class="txtlf"><i>Stereo Copyright, Underwood & U.</i></div>
<div class="txtrt"><i>London and New York</i></div>
<div class="figcaption" style="clear: both; padding-top:1em;">
<p class="tdc"><span class="smcap">A Curious Rock greatly revered by the Natives</span></p>
<p>This is the Dance Rock of the Walpi Indians of Arizona. Its curious shape
is the result of weathering.</p>
</div>
</div>
<p><span class="pagenum" id="Page_47">-47-</span></p>
<p>What goes on along the coast finds a parallel in the
interior of continents where, as in Arizona, in America,
or by the desert of Gobi, in Asia, or in the Karroo of
South Africa, or in Central Australia and Africa, there
is great dryness of climate and a continual disintegration
of the surface rocks. Sometimes the dust or sand
remains and gradually consolidates or hardens. More
often it is only a temporary visitor. Wind and rain are
continually removing it, sometimes in vast quantities,
into the sea; and in the course of time the most astounding
changes are wrought in the surface and appearance
of the land. The softer rocks are worn down; the
harder ones are left sticking out. Gradually the surface
is carved out into heights and hollows. The harder
rocks become the hills and ridges; the softer rocks are
worn into valleys and plains. If there were no water
left on the earth's surface a great deal of this process
would still go on. In some respects it might become
more violent, for owing to the absence of moisture the
winds of the earth would always be laden with fine
particles; and every one who has seen a "sand-blast"
at work, or even the modified sand-blast which is sometimes
used for cleaning the stonework of some of our
cities, will appreciate what a tornado laden with sand
grains might do in the way of destroying the surface
of any rock on which it was playing. But, as a matter
of fact, the action of water in carving the surface of the
earth is the most important of all the factors we have at
present to consider.</p>
<p>As rain falls from the clouds it absorbs the gases of the
air, including oxygen and carbonic acid. Now both these
are what we call corroding agents. If water is allowed to
fall on a steel knife the knife rusts; but it has been shown
by Dr. Gerald Moody, during the last few years, that if
there were no acid gas present, the rusting would not
take place. Oxygen and carbonic acid will rust other
things beside metal; they will rust stone. Moreover,
when the rain reaches the earth it absorbs any other
acids of the soil which rotting vegetation may afford,
and reinforced by these it goes on to attack the stones
over which it flows. When it rolls along as a brook or
a river it is no doubt attacking in this way the rocks
<span class="pagenum" id="Page_48">-48-</span>
and stones of its channel, though this action is not very
strikingly shown. But sometimes the rusting or dissolving
action of water is very evident. When it issues
from a peat bog, for example, and is consequently highly
charged with acid, it will make a very great impression
on any limestones it may encounter; for as any schoolboy
knows who has ever put a piece of chalk in vinegar,
or in any of the stronger acids of the school laboratory,
all the limestones are peculiarly susceptible to this form
of chemical attack. Peat-water eats into limestone
rapidly, while the limestone above the stream escapes,
though it is a little (and much more slowly) dissolved by
rain. Hence arise some curious features in the scenery
of limestone districts. The walls of limestone above the
water are not eaten away so fast as their base over which
the water flows. Consequently they are undermined and
are sometimes cut into tunnels and caverns and caves.</p>
<p>The rivers carry away the dissolved material. The
carbonate of lime is taken to the sea; and this substance,
of which sea shells, for example, are principally formed, is
constantly supplied to the sea by the rivers that transport
it from the land. The rivers of Western Europe
have been known to convey one part of dissolved mineral
matter in every 5000 parts of water, and of this mineral
matter one half is carbonate of lime. The Rhine alone
bears enough carbonate of lime to the sea every year to
make 332,000,000,000 oyster shells of the usual size. The
Thames conveys 180,000 tons of sulphate of lime past
London every year. It has been computed that more
than 8,000,000 tons of dissolved mineral matter are removed
<span class="pagenum" id="Page_49">-49-</span>
from the rocks of England and Wales in one year.
That is equivalent to a general lowering of the surface
of the country, by chemical solution alone, at the rate
of one foot in 13,000 years. That is not much, it may
seem; but in a million years, which is not a long period
in geological time, half the present towns of England
would be sunk under water by this cause alone.</p>
<hr class="chap x-ebookmaker-drop" />
<p><span class="pagenum" id="Page_50">-50-</span></p>
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