Читать книгу The Romance of Modern Geology - Edwin Sharpe Grew - Страница 7

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Everybody who has ever been to the coast of these islands has become aware that changes in the outline of the land are continually taking place. In some parts of the east coast of England, such as that which lies between Harwich and Walton-on-the-Naze, the sea appears to be slowly encroaching on the land, so that places which were grazing-fields twenty or thirty years ago are now covered by the sea at high tide, and at low tide are mere sandy wastes threaded by rivulets of sea-water. On the south coast of the Isle of Wight, between Sandown and the Culver Cliff, which is the most easterly point, the same loss of land is going on in another way. Some years ago a fort stood rather near the edge of the cliff, and it would have been possible to climb round the seaward wall of the fort. It is not possible now, for the outer sea-wall of the fort has long ago slipped into the sea; so have some of the inner fortifications: and it has been necessary to dismantle the whole of this fort lest every part of even the inner landward wall should follow the outer parts and slip with the solid ground down the cliff. It is easy to see what is happening here. The wind and the waves are undermining and honeycombing the cliff. They are weakening its base and its body, and so the upper crust on which the fort was built, and into which its foundations were dug, is slipping away. If we imagine for a moment that nothing was done to save the fort or protect the cliff, but that all was left to nature to deal with, it would not be hard to picture what would happen. The cliff would gradually be eaten away: its gravel and clay would be drawn into the sea, and the Isle of Wight would become a little smaller. The same thing is going on at a good many places along the coast of the British Isles, as well as on the coast of Florida and in the Gulf of California in America.

The little islet of Heligoland in the North Sea, which once belonged to Great Britain, but was some years ago handed over to Germany, is so fiercely attacked by the sea in this way, that it almost has to be armour-plated in order to preserve its integrity. It is fenced in stone in order to protect it. What is happening on the coasts of islands like England and Heligoland is happening all over the world. It has always happened. If it had not happened in past ages there would be no British Isles at all, because once England and Scotland and Ireland were joined to Europe, and it would have been possible to walk across the North Sea from Harwich to the Hook of Holland. The North Sea was once dry land. But the sea encroached on it from the north, and the Atlantic Ocean battered a way through on the south, till the English Channel was bored through into the shallow waters of the newly-formed North Sea, and the lands that had once been part of Europe became these "sceptred isles set in the silver sea."

This is not all the story. What the sea takes away it gives again. Sir Thomas Holditch is our authority for saying that on some parts of the Pacific coast of America you may at some points see on the one hand dry land which by the shells found on it shows that the sea once flowed over it; while side by side with this raised land you may sail a boat over forests now sunk beneath the sea. The loss of bits and corners of England is serious—so serious that a Royal Commission on Sea Erosion, as the process is called, was appointed to inquire into the extent of the loss and the means by which it might be remedied. But in some parts of our coast the land is not losing, but gaining. If the sea takes away sand and gravel, chalk and shale and clay from the cliffs, these materials are not lost. Something is done with them. They must at some points, where the tides and currents of the sea deposit them, make the sea more shallow. Perhaps the sea lays them down as beds or sand-banks. Perhaps it carries them round the coast to some other point and there drops them. Can you not see that in this way the sea which at one point is dragging down the coast may at other points be building it up, or may be even constructing breakwaters made out of these stolen materials?

The sea is not the only carrier which is thus laying down beds of material. The rivers are doing the same thing. Every shower of rain washes some dirt—by which we mean sand or gravel or loam or chalk—from the land into the nearest rivulet. The rivulet hurries with it down to the neighbouring river, and the river carries it down to the sea. If the river is going very fast it carries most of its dirt along with it, and we generally find the river muddy after rain. But when the river slackens its pace, as it usually does when it nears the sea and meets the sea's tides, then it lets the dirt fall; and thus at the river's mouth we find mud-banks or sand-banks. If a river is left long enough to its own devices, these sand-banks will so increase in bulk that the mouth of the river will become shallower and shallower and will spread. It silts up, and when a river is needed for the navigation of ships large sums of money have to be spent, as in the Scheldt or at the mouth of the Thames, in dredging this mud so as to keep the channels clear.

There are many striking examples of this land-building by rivers; and the deltas of rivers, so called from their resemblance to the Greek letter Δ, form in some instances great areas. The Mississippi, the Nile, and the Ganges, for example, are surrounded by great tracts of land at their mouths, which are formed entirely from matter brought down by the rivers and deposited at lower levels than those at which the rivers originated. The Mississippi, which drains a river basin of 1,147,000 square miles, has an annual discharge of sediment of no less than 7,459,267,200 cubic feet. The Italian River Po, draining an area of 30,000 square miles, discharges 1,510,137,000 cubic feet of sediment annually. This is equivalent to a lowering of its whole drainage area by ¹/₇₂₉th of a foot per annum, so that in a thousand years the whole area over which it flows has been lowered by the river by more than a foot. The Thames alone carries down 5,000,000 tons of material each year. All this must be redeposited somewhere. Where the redeposition takes place we find new land forming, new beds, new strata, in which in ages to come the future tenants of the globe may find relics of the people and animals living to-day.

Thus there are several evident ways in which the coast-line of a country might be altered, either in the direction of enlarging its boundaries by additions to it made by the sea or by rivers; or in the direction of losing parts of its territory by wear and tear. But there are other changes going on which are not so easy to perceive, and which are not so easy to account for. The thing hardest to explain is why what is now dry land should have risen out of the sea, as certainly it did. The white cliffs of Dover are made of chalk, and chalk is made of innumerable shells of tiny animals which once lived in the sea and which at their death sank to the sea's bottom. They steadily accumulated there for ages in a grey ooze, and in course of time this grey ooze rose above the waves. It dried and became land. But chalk is not found in cliffs by the sea only. It is found far inland. It is found, for example, in the North Downs, which run from Guildford to Reigate and from Reigate to Limpsfield and Westerham—a great ridge of chalk, at some points 600 to 800 feet high. That ridge must at one time have been at the sea bottom. And if we were to examine the whole of England and sink borings in it, we should at one point or another come to some remains of rocks, or some "strata," as they are called, which are of such make and material that we can only believe them to have been laid down at the sea bottom. The only conclusion we can come to, therefore, is that by some means or other, and at some time or other, the islands of England were slowly lifted above the sea, and that at some other time the sea was slowly lifted above them. What is true of England is true of nearly all the regions of the world that have been closely examined by geologists. Everywhere there is the evidence of different stages of existence in the land's history—stages when it was covered by the sea; stages when it was dry land again; perhaps stages when it was covered by lakes, by vast forests; stages when it may have been covered by ice; stages when it was desert. Some of these stages show far vaster upheavals than others, and the changes wrought were of far greater extent. Everybody has heard that the great Saharan desert was perhaps once the bed of an ocean. That is an assertion to which, perhaps, we may be a little chary of committing ourselves; but there is excellent reason for believing that once some of the great African lakes were connected with the sea; and we are quite certain that once Africa was an island. So that in the case of that vast continent we know that it must have seen periods of great depression and elevation; ages when it was much lower than it is now, and ages when it was higher.

We will not at this moment stop to give further examples. We will only try to see whether there is any explanation which would make it possible to understand why there should be these slow upheavals and subsidences of the earth's surface. The chief and most important reason is that the earth is not so solid as it looks, and not so solid as it feels. It would be easier to realise this if, instead of living in a part of the earth like Great Britain, where there are very few earthquakes, we lived in Japan, or Central America, or in the archipelago of islands which runs from Java to Borneo and further south. In these places, where never a year passes but that the earth can be felt to quiver beneath one's feet, and where earthquakes which wreck houses are at least as common as eclipses of the moon, it is easier to believe that the earth is a rather shaky body; or, as scientific men would call it, a rather unstable body. But if, like those scientific men who take up the study of earthquakes, or "seismology," we equipped ourselves with instruments to measure or record earthquakes, we should perceive even in England that the earth is nearly always quivering. Something is always snapping or giving way in its interior, and producing trembling fits that sometimes can be felt hundreds of miles away, and sometimes can be felt all over the earth. There are on the average at least twenty earthquakes a year which make the whole of this round globe tremble.

It would seem, therefore, that either these shocks or breakages in the earth's crust, or the earth's interior, must be very great indeed, or else that the earth must be composed of rather shaky materials. Well, perhaps both these suppositions are true. We spoke just now of the instruments which seismologists use to record earthquakes. They are known as "seismometers," and a great many of them are used in Japan and on the Californian or Pacific coast of America. Now it is perhaps scarcely necessary to say here (when we recollect how many cyclones and anticyclones England receives from the Atlantic) that a storm or rainy weather is usually heralded or accompanied by a fall in the barometer, or a depression. Now when there is a depression in the barometer that means that the weight of air above the barometer is less than it was before, though it is not so great a difference that human beings could tell it, unless it were accompanied by other signs. But the earth can tell it, and the mere fall of the barometer, owing to changes of the air, will make the earth tremble or quiver slightly, as if it were a jelly. We cannot perceive it; but the delicate seismometers can; and when a storm is coming to Japan or to California from the Pacific, the instruments show that the earth feels the passage of it. The comparison of the earth to a jelly—a very stiff jelly—is on the whole a useful one.

If a very tall jelly is allowed to stand for some time, or if the table on which it stands is shaken a good deal, then, as we know, rifts will sometimes appear in the jelly. The reason for these breakdowns in the jelly's composition is that owing to the distribution of its weight it is always in what we call a state of strain; and it is sometimes not strong enough to support this strain, and, almost without apparent cause, will sometimes give way. Much more solid bodies than jelly act in the same way. The great bridge near Quebec which collapsed in 1907 was to all appearance quite sound and strong; but there were strains in the iron girders, and without warning these strains suddenly produced rifts in the iron and steel framework and it broke down. Similarly the towers of churches and cathedrals, which are built on arches, will give away quite suddenly after standing to all appearance quite firm for hundreds of years. There is an architect's maxim which runs, "The arch never sleeps." That means that the arches on which the great weight of a church or cathedral tower rests are always in a state of strain; they are always, as it were, imperceptibly quivering; and they are always liable, if the strain on them should be increased in the slightest degree, to give way, or to resettle the weight on their shoulders in some way.

The whole of the great globe which we call the earth is in this state of strain; and it is always liable to rifts within itself and to readjustments of the weights of its own parts. It is not so easy to understand how a great globe spinning through space can be in a state of strain, or can attempt to readjust the weight of its parts, as in the instances we have just given of the quivering jelly or the solid cathedral tower. Perhaps another illustration may help us. We will presume that nearly everybody is acquainted with the modern rubber-cored golf ball. The modern golf ball, as those who are aware who either intentionally or unintentionally have cut through its outer cover, consists first of a small hard core. Round this is wound very tightly some two hundred yards of elastic. The tighter this is wound the better, or at any rate the more "bouncing" will be the resulting ball of india-rubber elastic. But consider what is the usual condition of this rubber-wound ball. Like our jelly it is always in a state of stretch or strain. Even when covered with the outer shell which completes the golf ball, the whole ball is still, we might say, in a state of strain or tension. That is one of the reasons why it bounces, and why it flies better than the old solid ball off the face of a golf club. But if you were to keep a golf ball for a hundred years these strains in its interior would alter and adjust themselves. One result would certainly be that the golf ball might lose its elasticity. Another result would be that its shape would slightly alter.

Now a golf ball, however carefully it is made, is not always evenly made. It weighs a little more on one side than another; and the best golf balls, those which fly truest and farthest, are those which are most evenly made: so that we might say of them that the centre of their weight was exactly the same as the centre of the ball. If it is not, then the strains in the ball are always pulling it a little more out of shape; and the ball, as golfers say, flies badly. Now the earth is like a badly made golf ball. The centre of its weight, or, as we call it, the centre of gravity, is not quite at the centre of the earth. Moreover, owing to the enormous pressures which exist right through the earth, and which are by no means the same at every place inside the earth, but are, in fact, continually changing, owing to hundreds of causes, the whole of the earth's interior is in a state of unequal strain. What is the consequence that you would expect? Is it not that the earth should always be making efforts to adjust its weight, and, as it were, to distribute it evenly? It has been doing this for millions of years. It has not yet finished.

Lastly, the cover of a golf ball is comparatively a stiff and unyielding substance which does not betray on its surface, if it is allowed to lie at rest, the tensions and strains of the rubber core inside. But the crust of the earth, which we have compared to the golf ball's cover, is not unyielding or rigid. It is practically a part of the case of the earth; and it does show and reflect the strains and tensions of the movements and rifts of the core. So that as in the course of ages the straining core changes, and gives way, alters itself and adjusts itself—so the crust of the earth alters with it. Some of these changes are sudden and violent. Some of them take place very slowly, occupying thousands or hundreds of thousands of years in the gradual process of change; and then perhaps for ages the earth's crust will be slowly sinking in one place and slowly rising in another. Thus, what was once a depression in the earth's surface may be now an elevation; what was once below the level of the sea may be now a continent of land; and what was once land may now have sunk beneath the incoming sea. Thus, what was sandstone rock of the earth's surface may become covered with forest, and the forest may sink below the sea, only to be pushed up again and become dry land a million years later. Each of these changes will leave its mark, each will be accompanied by deposits. The deposits may be vegetable matter, trees and mosses, and the growth of swamps, such as coal was first made of; or they may be the ocean sludge, which at last became chalk or limestone.