THE EARTH'S SHAPE
We have compared the earth to a golf ball, and as it spins through
space, impelled by a force millions of times greater than the
strongest driver ever imparted to the best-made "Haskell," its flight
and general appearance are not unlike those of the rubber-cored
ball. The earth, for one thing, is not smooth; it has roughnesses
and corrugations all over its surface, similar to those of a golf
ball, though much less regular, and it spins as it flies. But let us
now consider the differences. Suppose the golf ball had a spot of
water clinging to it as water clings to a greasy shot. Where would
the water lie? The first answer that occurs to one is that the water
would be shaken off the ball in the course of its flight; and that
is, indeed, very likely. But suppose the water were very sticky, or
were very much attracted by the golf ball (which is another way of
stating the same supposition), where would it lie then? To that we
can only say that there does not seem any very evident reason why
it should lie on one part of the flying golf ball more than on any
other--if the golf ball were perfectly round.
That is, on the whole, a reasonable answer. But apply the same
reasoning to the question of where the waters of the earth in the
shape of oceans ought to lie as they cling to the spinning globe.
They cling to the globe, not because they are sticky, but because
of the attraction which we say is due to gravity--the force which
makes everything in nature attract every other thing, and which makes
everything tend to fall to the earth (and to stay there). They do
so because the earth, being so very heavy and bulky in comparison
with anything in its neighbourhood, has such an enormous pull. How
great that pull is may be dimly gathered from the reflection that
though the earth is spinning at the rate of a thousand miles an hour,
nothing is ever shaken off. The oceans are not shaken off. They
cling. But why is it that they are not equally distributed all over
the face of the earth? If a map of the earth be examined, or still
better a globe with the oceans and continents correctly drawn on it,
it will be found that there is a great mass of land all lying grouped
together on one side of the earth, and a great basin of waters on
the other. Let the reader imagine himself a thousand miles above the
earth, looking down at a point in it about midway between Madeira and
the Bermudas. What would he see? He would see the Atlantic Ocean, but
all around it would be grouped great masses of land--Europe, Africa,
North America, Asia--and if it were his first sight of the earth and
he knew nothing of its geography, he would be likely to suppose that
the earth was nearly all land, with one comparatively small stretch
of unfrozen ocean. But now let the reader move round the earth to a
point exactly opposite that at which he took his first observations
and look down again. He will now see the Australian continent and the
land which covers the South Pole, but except for the pointed tail of
South America, and perhaps a glimpse of the blunter point of South
Africa, he will be looking down on a globe which seems to be largely
covered with water.
Why should this be? It must be due to the shape of the earth. The
fact is, the earth would make a very bad golf ball. It is by no
means of that perfection of symmetry which they say enables a golf
ball to fly well and to run true on the putting greens. The earth
is, in fact, not perfect as a sphere, either within or without.
Its centre is not in the same place as the centre of its weight,
and it is not round in shape. Everybody has heard that the earth
is slightly flattened at the poles; but its irregularity goes much
further than that. If we could strip it of its oceans, which fill up
a good many of its imperfections, we should find its shape not that
of a neat, round golf ball at all. The earth's actual shape without
its oceans, its "geoid," as it is called, is that of a pear. The
stalk of the pear is in the southern part of Australia, and contains
Australasia and the Antarctic continent. This is surrounded on all
sides but one (towards South America) by a sort of belt of depression
in which the waters lie. That is the waist of the pear. This again
is surrounded on all sides but one (towards the east of Japan) by
a belt of elevation. That is the protuberant part of the pear, and
here the great continental land areas rise. Finally, we find the
nose of the pear in the central Atlantic, between the Madeiras and
the Bermudas. Of course, the resemblance to a pear is not a very
marked one. Our observer a thousand miles above the earth would not
be able to perceive it, nor would the astronomers in the moon, if any
astronomers existed there. But the earth is pear-shaped to a small
extent nevertheless, and in the case of such an enormous mass a very
slight deviation from rotundity will produce very great effects.
Most of us have played at such ball games as bowls or billiards;
and I have assumed that everybody knows something about golf. What
happens in a game at bowls to the bowl which is not evenly weighted
all through? It will not run straight. It has a bias. What happens
to a billiard ball which is not perfectly round, or has lost its
symmetry through age? It wobbles. And what happens to a badly made
golf ball? That performs all sorts of exasperating antics. It ducks,
it soars, it curls, it takes a slice. It also wobbles. Now that is
exactly what the spinning, unevenly shaped globe which we call the
earth has been doing for millions of years. It has been wobbling;
and as we showed in the last chapter, it has always been trying to
right itself. Thus the two poles have not always been in the same
position; the oceans have not always been where they are. The waters
have sometimes crawled up the land towards the poles and sometimes
receded. Regions that have sometimes been frozen and cold have
become warmer, and have covered themselves now with oceans, and now
with forests, and now with deserts. There is no corner of the whole
world which has not undergone changes of climate. These changes
are very slow. There is no reason for supposing, in spite of the
laments we sometimes hear about the loss of old-fashioned winters
and old-fashioned summers, that the climate of England, for example,
has changed in the least since Cæsar's legions landed on its shores.
The Roman settlers in Britain doubtless experienced sloppy winters
and wet summers now and again, just as we do; and King Arthur's
knights, no doubt, had their saddening experiences of November
fogs. Yet slowly and surely changes of climate do take place, and
nothing except the winds influence them more than does the presence
of a neighbouring sea or ocean. Most of us reckon the warmth of a
locality's climate by the distance it is from the pole. That is,
however, a very rough and ready method. Vladivostok is roughly the
same distance from the North Pole as Venice; but there is a good deal
of difference in the temperature of the two places. In Manchuria when
the Russians and Japanese were entrenched before Mukden men died of
cold and were frozen at their posts at a time when other people in
Mentone and Monte Carlo, at the same distance from the Arctic Circle,
were complaining of the heat. So that we see that it must not be
assumed that a place like England (where for two thousand years we
occasionally have had winters that would kill trees like eucalyptus
or fig trees, and where oranges could never ripen in the open air)
was always equally cold. It may have been, in fact we know it must
have been, warm enough once to encourage and support what resembled
a tropical vegetation. It must also have been at one time as cold as
Siberia in the winter.
Therefore we should expect to find, if we digged down in the
earth, or in any portion of the earth which had undergone these
changes, some traces of them. For example, if at one time the sea
covered England for thousands or hundreds of thousands of years,
depositing the remains of millions of animals on the sea's bottom
during that period, we should expect to find some traces of these
remains--perhaps in the form of chalk, seeing that the bones and
shells of fishes dwelling in the sea contain a good deal of lime.
Or again, if a forest covered England and grew and decayed there,
not merely for a period like that which has elapsed since the Romans
first set foot in Britain, but for a hundred times as long, we should
expect to find some sort of vegetable deposit, hardened most probably
by other layers above it. Do we? Well, coal is a vegetable deposit.
If there was a time when ice covered the land we should expect to
find traces of that; if a time when the land was desert; or when it
was a lake--each and every one of these periods ought to leave some
remains, some epitaph of itself. So they do.
Let us for a moment consider with Sir Archibald Geikie[1] the
subsoil beneath cities that have been inhabited for many centuries.
In London, for example, when excavations are made for drainage,
building, and other purposes, there are sometimes found, many
feet below the level of the present streets, mosaic pavements and
foundations, together with earthern vessels, bronze implements,
ornaments, coins, and other relics of Roman time. Now if we knew
nothing from actual authentic history of the existence of such a
people as the Romans these discoveries deep beneath the surface
of modern London would prove that long before the present streets
were built the site of the city was occupied by a civilised race
which employed bronze and iron for the useful purposes of life,
had a metal coinage, and showed not a little artistic skill in its
pottery, glass, and sculpture. But down beneath the rubbish wherein
the Roman remains are embedded lie gravels and sands from which
rudely fashioned human implements of flint, arrow-heads, hammers,
and the like have been obtained. From that we learn that before the
Romans came an earlier race had been there which employed weapons and
instruments of roughly chipped flint.
[Footnote 1: Sir Archibald Geikie's _Introduction to Geology_.]
We have no doubt that this was the order of the successive peoples
occupying the site of London. It is obvious. Why is it? We see that
there are, broadly, three layers or deposits. The upper layer is that
which encloses the foundations and rubbish of our own era and times.
Next below is that which encloses the relics of Roman occupation. At
the bottom lies that which encloses the scanty traces of the early
flint-folk. The uppermost deposit is necessarily the newest, for it
could not be laid down until after the accumulation of those below
it; and those below it must be progressively older, as they are
traced deeper from the surface. By the mere fact that the layers lie
one above the other we are furnished with a simple clue which enables
us to determine the order of their formation. We may know nothing
whatever as to how old they are, measured by years or centuries. But
we can be absolutely certain that the bottom layer came first, and
the top layer came last. This kind of observation will enable us to
find proofs everywhere that the surface of the land has not always
been what it is to-day. In some districts, for example, when the dark
layer of soil in which vegetables grow is turned up, there may be
found beneath it sand and gravel full of smooth, well-rounded stones.
Such materials are to be seen in course of formation where water
keeps them moving to and fro, as on the beds of rivers, the margins
of lakes, or the shallow shores of the sea. Wherever smooth-rolled
pebbles occur they point to the influence of moving water, so that
we conclude, even though the site is now dry, that water once moved
above it. Again, below the soil in other regions lie layers of
oysters and other sea shells.
Pits, quarries, and mines that cut down still deeper into the earth
and lay it bare bring before our eyes most impressive testimony
regarding the ancient changes of the land. Suppose, by way of further
illustration, that underneath a bed of sand full of oyster shells
there lies a dark brown band of peat. This substance, composed of
mosses and other water-loving plants, is formed in boggy places by
the growth of marshy vegetation. Below the peat there might occur
a layer of soft white marl full of lake shells, such as may be
observed on the bottom of many lakes at the present time. These
three layers--oyster beds, peat, and marl--would be like a family
pedigree showing the history of the place. The bottom layer of white
marl would show that there was once a lake. The next layer of peat
would show that by the growth of marshy vegetation the lake became
choked up and was gradually changed into a swamp and then a morass.
The other layer of oyster shells would show that the ground was
afterwards submerged by the sea. The present condition of the ground
would show that the sea at last retired, and the place passed into
dry land as it is to-day.
By such a method of examination we may frame for ourselves pictures
of the earth's surface long before history began, or before man
roamed the earth. It is for this reason that geology has been
called the science that investigates the history of the earth. The
records in which this history is chronicled are the soils and rocks
underneath our feet. It is the task of the geologist so to arrange
and interpret these records as to show through what successive
changes the globe has passed, and how the dry land came to wear the
aspect which it presents at the present time.
To do this efficiently the geologist has to learn many things. He has
to observe very closely the changes which are going on about him on
the world's surface. Only in so far as he makes himself acquainted
with these sudden changes can he hope to follow intelligently and
successfully the story of earlier phases in the earth's progress. Nor
is it sufficient to observe, however closely, inanimate things. If
he did not know the peculiarities of fresh-water shells, how would
he be able to say the shells in the marl deposit were fresh-water
animals (and that therefore a lake once lay there) and not sea
shells. If the labour of the geologist were concerned merely with
the former changes of the earth's surface--how sea and land have
changed places, how rivers have altered their courses, how valleys
have been dug out, and how mountains have been carved, how plains
have been spread out, and how all these things have been written on
the framework of the earth--he would still feel one very great want,
the want of living interest. But that also his science gives him,
for in these past eras living things dwelt and moved and had their
being. And it is one of the most entrancing pursuits of the geologist
to trace their lives, their descent and ascent, and the relics of
themselves that they left.