EFFECTS OF WEATHER ON THE EARTH'S HISTORY
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; 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.
[Illustration: One of the Colossal Natural Bridges of Utah
This is an instance in which water has hollowed out the lower strata,
leaving a harder upper stratum partially intact.]
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 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.
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, 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.
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 _hurricanes_,
in the south-east of Asia as _typhoons_, and in North America as
_tornadoes_. 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
_upwards_. Fortunately these tornadoes do not 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
_waterspout_. 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."
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.
[Illustration:
_Stereo Copyright, Underwood & U._ _London and New York_
The Garden of the Gods, Colorado
These peaks exhibit the gradual wearing away of hard rocks by the
action of rain and wind.]
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 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.
[Illustration:
_Stereo Copyright, Underwood & U._ _London and New York_
A Curious Rock greatly revered by the Natives
This is the Dance Rock of the Walpi Indians of Arizona. Its curious
shape is the result of weathering.]
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.
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 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.
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 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.