The mighty deep

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.