ABOUT THE LONG PAST
“The cold and silent past.”--WHITTIER.
“Thou coveredst it with the Deep as with a garment; the waters stood
above the mountains.”--Ps. civ. 6.
The very long past--how long no man can say. Some rash attempts have
been made to name the numbers of thousands of years which may have
elapsed since this or that particular stage in the world’s geological
history. Such reckonings can possess little or no true value. It is
better to confess frankly that we cannot fix dates. We only know that
certain changes did come about--very long ago.
Our Earth has not always been as she is now. She has altered greatly.
She is altering still. Development has gone on through the ages.
Development goes on still.
Continents and parts of Ocean have, so to speak, changed places.
Mountains have risen; and tablelands, high uplifted, have sunk low.
Sea-beds have become dry land; and dry lands have become sea-beds.
Rivers have carved new paths for themselves; and cataracts have worn
away vast masses of rock, carrying the débris to the sea. Ocean-waves
have battered down lines of cliffs, and new cliffs have emerged from
under water. All these things have come about, not in a few years, but
in hundreds of years, in thousands of years, some say in millions of
years.
Such time-possibilities are not, however, without limits. Sooner or
later the Astronomer steps in with a--“Hold! Enough! The Sun as a light
and heat giver could not have existed then.”
But we may leave such perplexing comparisons and calculations, and may
content ourselves with a general--“Very long since!”
There was a time when men believed our Earth to be at rest, in the
centre of a revolving Universe. That notion had to be given up as
knowledge grew. There was a time, far more recent, when we all felt
confident that this firm ground, on which we live and move, stand and
work, was solidly calm and immovable. That notion too has had to be
abandoned.
For the Earth-crust itself is in motion; certainly in parts; probably
as a whole. Here it is gently heaving upward; there it is slowly
sinking downward. True, we do not see or feel such movements. But
neither do we feel the whirl of our Earth upon her axis, or her rush
around the sun. Neither do we see from hour to hour the growing of a
boy into a man, or the change from a sapling into a tree.
Straight through from side to side, the Earth measures some eight
thousand miles; and of those eight thousand a very slight “skin” is all
that we can study, by anything approaching to direct observation. No
wells or mines on land can be sunk so deep as the sailor’s plumbline
in the ocean. If we add the six miles or more of the ocean’s greater
depths to the five miles or more of Earth’s higher mountains, we have
at most twelve miles, which, compared with the whole diameter of the
Earth, must be looked upon as a mere nothing.
By one means we are able to learn something of what lies deeper in the
earth-crust.
Half a dozen deep wells may be sunk in different places, the same
boring apparatus being used, and the same methods being followed. But
the same results would not be obtained. Earth’s crust is not one solid
continuous substance, like a shell of iron. It is made up of different
substances, lying one upon another, or mixed confusedly together. When
men work their way downwards, in well-sinking or in mine-sinking, they
come across a great variety of layers, each unlike the rest.
Here perhaps is a stratum of stiff clay, and there is a deposit of
sandstone. Here hard granite bars the path, and there an easy road
is found through crumbling chalk. Here a great thickness of pebbles
appears, bound together into a rock-like conglomerate; and there seams
of coal alternate with limestone.
Each whispers a tale of Earth’s past history; and not the same tale.
They were not all made in the same workshop, or after the same mode.
Each has its own separate biography.
And the various layers do not lie flat and even, one upon another. Once
upon a time they may so have lain, but since those days they have been
moved and shoved about, tilted and lowered, heaved up and dragged down,
so that some of the upper layers have in places disappeared below the
pile, and in other places some of the undermost layers have been pushed
to the top. It is by studying the latter, where they happen to crop
up within reach, that the geologist can learn a little about deeper
portions of the earth-crust and about the rocks of which it is made.
By that word “rock” must be understood, not only such hard kinds as
granite and marble, but softer materials, such as chalks, clays, sands,
and even muds.
Rocks generally have been roughly divided into two classes--the
Stratified and the Unstratified.
Stratified Rocks, known also as Sedimentary, have been put together in
the past, grain by grain, layer by layer, under water, built gradually
through ages, and slowly welded into firmness.
Unstratified Rocks, known also as Igneous, from the Latin word for
“fire,” have been melted down by great heat into a liquid state, and
have then cooled into solid crystallized masses.
When these fire-made rocks have been heaved up from lower depths and
are exposed to the wearing effects of rain and wind, rivers and waves,
they too have to part with much of their material, like stratified
rocks, only more slowly, owing to their harder make. These materials
are carried seaward, to be used in the building of fresh stratified
rocks. Then again, some kinds of stratified rocks, when exposed
to intense heat, will crystallize into igneous rocks. So the one
kind may be the child of the other; and many rocks partake of the
characteristics of both.
With regard to the wearing away of rocks, the destruction of land by
water, it was said in the close of the last chapter that, but for
certain “counteracting forces,” the whole of the Continents might in
the course of ages be slowly carried off by the Ocean, and be buried in
the mighty deep.
This sounds like what, in modern parlance, may be termed “a very large
order.” Who could imagine a feeble substance like water having any
effect upon massive granite cliffs?
Few people grasp the tremendous battering force of ocean-waves upon
a rocky coast, and fewer still realise the wasting power of running
rivers, or of endless successions of raindrops.
Here is a fact as to the strength of ocean-waves. Some of us may have
watched the majestic ground-swell which beats upon the western coast
of Scotland. It is said that, upon a rough average, taking smooth and
tempestuous weather together, each summer wave that breaks upon that
shore from the Atlantic does so with a force of over six hundred pounds
upon the square foot. For winter months alone, when gales have sway,
the average blow rises to about one ton; and some mighty billows are
known to batter with a weight of three tons.
Such a fierce assault as this, continued through thousands of years,
might well in time wear away the hardest rocks, demolish the loftiest
cliffs.
A counteracting force, however, exists, and has already been named.
Our earth-crust is in motion. Parts of it are sinking, and in those
regions the ocean has the best of the contest; for fight as man may,
raise walls and bulwarks and breakwaters as he will, he can but retard
the inevitable. If the sinking continue, no matter how sluggishly, he
must in the end be beaten, and the persistent sea will encroach upon
the land’s domain. But many coasts are slowly rising, slowly lifting
themselves out of the sea, slowly shaking themselves free from Ocean’s
dominion; and in those regions, no less inevitably, the action of the
sea is thwarted.
As to the why and the wherefore of such Crust-movements, science
suggests explanations.
Once upon a time, long ago, our Earth was a glowing molten mass. Since
that period it has been cooling, and it is cooling still. Many facts
point to the great probability of a still molten centre.
Not only do we find volcanoes scattered over the Earth, on land and
under the ocean; but numberless hot-water springs exist, some of them
a thousand miles away from any known volcano. Then again, when deep
mining or boring operations take place, it is noted that the degree of
warmth for a while corresponds to the state of the weather above, but
that beyond the limits of seasonal change the temperature rises with
increase of depth. This looks like greater heat below.
Many scientists have held strongly that, as above suggested, the whole
inside of our Earth is so far heated as to be in a molten condition,
contained within a cool hard crust, which may be from twenty to fifty
miles in thickness.
Others have maintained that the said crust cannot be less than two
thousand miles thick, with a small molten core.
Others believe that the entire Earth is actually solid throughout; the
outer parts from coolness; the inner parts from great pressure.
Again, a theory has been started of a solid centre and a solid crust,
with an intervening “fire-sea” of molten rocks; and another of a
gaseous inner globe, surrounded first by molten layers, then by a firm
inclosing crust.
All these different explanations rest upon one foundation--that of a
steadily cooling Earth--that of a more or less heated interior, and of
a hardened crust.
The cooling of the inside goes on slowly, and not uniformly, since
conditions differ in different places. As the heated materials become
cooler they lessen in size. Thus the nucleus is constantly getting a
little too small for the inclosing crust.
If the said crust were a solid shell, compact and strong enough in
all its parts to resist the strain of its enormous weight, it might
be expected to keep its shape and position unchanged, holding loosely
within itself the shrinking centre. But it is formed of a great number
of materials, some hard, some soft and yielding. Therefore, as the
central parts lessen in size, the crust sinks downwards, and in so
doing it wrinkles into huge folds, like the rind of a shrivelling
orange or the skin of a very old man’s face. A ridge is pushed up here;
a furrow extends there. Here a long range of mountains is found; there
we see a succession of valleys.
By these movements the Ocean is both helped and hindered in its
“building.” As new land is formed under water, the “crust-creep”
perhaps raises it gently, and a strip of sea becomes dry land.
But another portion of the work, that of pulling land to pieces,
is checked; for let the waves strive as they may to wear away a
coast-line, a very slight upheaval is enough to counterbalance their
utmost exertions.
Such changes are usually gradual--so quiet, so deliberate, as only to
become apparent in the course of centuries.
Not that crust-movements are always sluggish. The ceaseless sinking
and crinkling of vast extents of country, the crowding together into a
smaller space of immense masses of material, cannot but cause enormous
pressure; and here or there a portion suddenly _gives_ under the
strain. Then the ground shakes and heaves, or some terrible landslip
takes place.
Earthquakes are now looked upon generally as signs of abrupt yielding
to intense pressure. Sometimes they indicate that a huge weight of
material deep down has slid to a lower level, packing more closely
or pushing aside other masses, and causing what miners call “a
fault”--that is a break or dislocation in the regular lines of the
stratified rocks. Or slip after slip may take place, one mass being
disturbed by the movements of a neighbouring mass; and so the ground
above may tremble again and again with recurring shocks.
Such shocks are fearfully common in some parts. In Japan, for instance,
it was noted that nine thousand had been felt in the course of only
eight years; and in one great earthquake alone--that of October 26th,
1891--almost ten thousand human beings perished.
Movements of the crust, whether slow or rapid, are not due always and
only to the shrinkage of the central portions of our globe, and to the
sinking and re-adjustment of the strata.
There are mighty fire-forces below; molten materials pent up, waiting
for the slightest yielding of the solid crust to fight their way
out; heated and imprisoned gases struggling for freedom. Sometimes
these captive giants escape through what may be termed the orthodox
safety-valves, old volcanic vents. Sometimes they break open new
craters for themselves. Sometimes they fail to get out, and only shake
the ground, or force it gently upward. Though earthquakes are more
usually ascribed to the effects of a steady “crust-creep,” they are
sometimes caused by heated gases seeking liberty.
These heavings and twistings, these crumplings and swayings, of the
earth-crust do not belong only to land, but go on in the same manner
under the ocean. Many an earthquake felt on land has its origin under
the sea; and others are so far out as to be felt only by passing ships.
In the Pacific alone it is said that about two thousand five hundred
shocks occur in the course of a year. Of the eight or ten thousand
shocks, believed to be about the average number throughout the world
annually, fully half have their origin beneath ocean-waters.