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slowly pushed up from beneath at the rate of, we shall say, six inches in a hundred years, no one would be sensible of the movement, unless he had accurate sea-marks by which to measure it.
By the great mass of the population it would be unfelt and disregarded. If the rate of upheaval rose to as much as six feet in a century, the inhabitants of the inland districts would not observe it; and even along the coast-line it might only be discernible by comparing the former with the existing tide-marks on harbours, and on quays and prominent rocks. If the movement were, on the other hand, a slow, downward one, it would produce no sensible effect for a very long time in the interior of the country. Along the shore, its reality would be marked by the gradual rise of the tide-line, and the advance of the sea upon fields, roads, and harbours. In neither case is the importance of the movement to be measured by the effects produced during a single generation. A little reflection will shew us that a slow movement of this kind, quite inappreciable to ordinary observation, may yet, in the course of a few centuries, do more to alter the surface of the globe than all the earthquakes and volcanos which have been at work during the same interval.
The proofs that a country has been upraised or depressed must always be most visible along the sea-margin. Though there may be cosmical causes at work, which at wide intervals displace large quantities of sea-water, and thus alter the relative levels of sea and land, the level of the sea is assumed to remain constant at present ; and when we observe a difference between the height of the line to which the sea rises to-day, and that to which it rose a few generations or centuries ago, we say that it is the land which has been shifted upwards or downwards, and not that the sea has advanced or retired.
1. What, then, is the nature of the evidence that a country has been upraised? It consists, in brief, in the traces of the recent presence of the sea along a marginal strip of land which the waves no longer reach, and which at the same time is not merely due to the silting up of the shore. When proofs of the change of level can be drawn from human works, such as quays, piers, breakwaters, or bulwarks, they perhaps appeal most to our imagination. We realise vividly the nature and progress of the movement, when we are told that the piers to which, a few generations ago, our forefathers moored their vessels, are now high and dry above the tides. As a rule, however, it is not by the changed position of any piece of human workmanship that we mark the rise of the land ; other and not less convincing evidence is furnished by the shells of the sea and the sand, which have been laid down by the waves. Every one who has rambled along any of the rocky parts of our coast-line is familiar with the rough gray aspect given to the rocks by the crust of barnacles and limpets which fourish so abundantly between high and low water mark. The barnacles are more especially noticeable from their numbers, and the firm way they adhere to the stone. So tenaciously, indeed, do they cling to the spots to which they have taken hold, that we may now and then find their whitened shells still coating stones which were cast up by a stormn years ago. Besides these marine animals, the shore frequently abounds in molluscs, which, instead of clinging to the stone like the limpet or periwinkle, bore clean smooth holes in the stone, and live there. Now, when a country is undergoing an elevatory movement, the shore-rocks are carried upward beyond the reach of the waves. There the molluscs and barnacles die, and leave their hard parts on the places where they used to live. If the movement is exceedingly slow, and the destroying effects of waves, air, rain, and frosts happen there to be rapid, the traces of marine life may be effaced as fast as the land rises. Under favourable circumstances, however, these traces remain for a long time. The rocks acquire, perhaps, here and there a scanty herbage, shrubs or wild-flowers take root in their crevices, and, in strange contrast with their present position, we may still see the bleached barnacles firmly attached to the stone, and pick out the boring shells from the holes in which they lived and died. In Sweden such barnacle-crusted rocks occur at a height of more than one hundred feet above the present sea-margin.
More permanent memorials of the presence of the sea are afforded by the caves which are drilled by the waves even into the hardest rocks. When the land rises, these caves are borne upward beyond the limit of the breakers; the barnacles, anemones, and sea-weeds which once mottled their sides, give place to ferns and mosses. But though now more than half concealed among the copsewood, the long line of caves which has been cut about the same level on the slope of the steep bank or the face of the cliff, still remains to mark the old sea-margin, and to shew how much the land has risen.
But even where there are no rocks to retain the impress of the sea, the waves pile up on low sandy shores deposits which, as the country is upheaved, come to bear a not less clear and impressive testimony to the change of level. Layers of sand and gravel, often mixed with the common shore shells, are laid down between tidemarks to form what we call the beach. They give rise to a gently inclined platform or terrace which on the seaward side slopes into deeper water, and on the land side terminates against a low bank or cliff. When the land rises, this beach comes to form a narrow selvage of flat ground bordering the new sea-margin. It is then known as a raised beach, It comes to be covered with fields and gardens, roads are laid along its surface, and it bears many villages and towns. Along many leagues of the coast-line of the British Islands the raised beaches form one of the most characteristic features of the landscape. On both sides of Scotland most of the seaport towns are built upon an old beach, which now stands on an average about twenty-five feet above the present high-water mark. This terrace
winds along the coast, at times sweeping some way inland to mark the site of an ancient bay, and then curving back up to the margin of the present shore. Its inner edge often rests at the foot of a cliffline once the upper limit of the waves, but now green with mosses and wild-flowers, or fragrant with honeysuckle and thyme. Along the base of the cliff, too, we may see here and there the caves which were cut out when the sea dashed against this rocky shore. But the waves have for many centuries ceased to reach them, and this not because they have thrown up a barrier of sand and gravel against themselves, but because the whole body of the country has been insensibly lifted out of the waters.
Movements of this gentle kind sometimes affect wide regions. At the present moment the Scandinavian peninsula, except a part of its southern end, appears to be rising, for within the memory of man, sunken rocks have become visible, reefs have grown in size, shoals have been converted into land, and marks made on rocks to test the rate of rise are now found to be higher relatively to the sea-level than they were at first. The rate does not appear to be uniform over the whole country; in some districts it has been estimated at two or three feet in a century. The movement must have been in progress for a very long time, for we find beds of seashells of living species at heights of six hundred and seven hundred feet above the present sea-level. Among other regions which are either now rising or which have been but recently upraised, may be instanced the coast-line of Siberia for six hundred miles to the east of the Lena, the coast of Smith's Sound, different portions of the borders of the Mediterranean, and on the most marvellous scale, the western mountainous margin of South America.
2. When the slow movement of the land tends downward instead of upward, its results are seen in the gradual overflowing of objects which were formerly never touched by the waves. Buildings once a good way from the sea seem gradually to approach it, piers are at length submerged, and streets overflowed.
At the present moment, such is the case along the extreme southern margin of Sweden. Some of the streets in a number of the seaport towns are actually below the level of the Baltic. Other proofs of submergence are furnished by the trunks of trees still standing with their roots spreading through the soil in which they grew, but now covered by the tides. Such submerged forests, as they are called, occur in a good many places in the British Islands, particularly along the coasts of Devon and Cornwall.
It is impossible, indeed, to consider attentively the map of our islands without seeing in it evidence that they have undergone an extensive submergence at no very remote geological period. The numerous long inlets, lochs, or fjords, which allow the waters of the ocean to penetrate far into the land along our western shores, are evidently so many old submerged land valleys. The existing glen
at the head of each of these inlets is only an undepressed part of what was originally all one valley, when the British Islands had a much larger surface than they can boast now. In this respect, our western sea-board presents a strong resemblance to that of Norway. But the most extensive proofs of subsidence of the earth's crust are furnished to us by the coral islands of the Pacific and Indian Oceans. These white rings of rock, each with its lagoon of smooth water inside, rise out of the midst of water so deep that it has been sometimes called unfathomable. Many were the conjectures as to their origin, until Mr Darwin pointed out their true structure and history. He shewed that the first stage in the growth of a coral island was the formation of a reef of coral in front of the shore. This is known as a fringing reef. The coral polypes, by whose labours of secretion the reef is formed, can only live at a moderate depth of water. Let us suppose that one of these fringing reefs encircles some oceanic island, and that the sea-floor in that region is being slowly depressed. As the downward movement continues, the corals keep building up the reef to about the level of the waves. In this way the space of water between the island and the coral ring is greatly increased, while, of course, the area of the island itself is correspondingly lessened. The downward movement continues, the island grows less and less, until its last mountain-top sinks beneath the sea. Over the submerged island there now stretches a smooth sheet of green water known as a lagoon. Encircling it is the circular reef of coral, or atoll, along the outer margin of which the restless waves of the ocean are ever surging. Soil gradually forms on the reef, seeds borne to it by the waves or carried by birds take root, and the ring of coral reef becomes a habitable spot for
Such is the history of the growth of the coral islands. They have been built round the summits of a sinking continent, over whose mountains and valleys the great ocean now rolls.
It appears, therefore, that the crust of the earth, instead of that solid immovable object which at first sight it appears to be, is in reality constantly pulsating in some part or other of its extent. Here an earthquake convulses the surface; there a volcano pours out rivers of lava, and darkens the air with clouds of ashes. In one wide region the ground is slowly heaved up; in another it is gradually depressed. But in some form the subterranean forces are ever manifesting their existence by effecting changes above ground.
There can be little doubt that earthquakes and volcanos are closely connected phenomena, and that these, as well as the more tranquil movements of the crust, are but different manifestations of one common force. If we look at a map of the world on which the regions visited by earthquakes are marked, we shall find that these regions are also those in which active volcanos occur. During violent volcanic eruptions, earthquakes take place; and, on the other hand,
during earthquake disturbances, volcanos have been observed to be more active. Though no invariable rule can be laid down on this subject, nevertheless there is such a frequent connection as to time and place of occurrence, between earthquakes and volcanos, that we are justified in regarding them as closely related phenomena. What may be the nature of the force which produces them, how it originated, and how it is kept up, are questions to which, in the present state of science, no very definite answer can be given. It is clear, indeed, that heat plays a large part in producing these changes. The temperature of the earth is found always to increase as we descend into the crust; and if the rate of increase which has been observed were to continue without any modification, we should reach the melting-point of even the most refractory substances at the depth of a few miles. It is certain, however, that the interior of the globe at that depth cannot be in a fluid state. From data of an astronomical kind, we know that if there is a fluid central mass, it must be covered with a solid crust at least eight hundred miles thick. Some writers on physical science, indeed, maintain that our planet possesses a rigidity equal to that of glass or even of steel. The old notion that there is still a central liquid part in an incandescent state, with a thin crust over it liable to be shaken and broken through by the commotions of the fluid interior, cannot now be held.
That the interior of the earth, however, whatever be its composition, is intensely hot, is indicated by all the evidence we can gather on the subject. Some portions must be liquid, as is shewn by the discharge of fluid lava at a white heat from volcanic vents. There seems, indeed, to be good reason to believe that though the main mass of the interior may now be solid, there nevertheless exist within it large lakes or reservoirs of melted rock, and that volcanos serve as the orifices of communication between these areas and the surface. When the water which is everywhere traversing the upper layers of the crust reaches these heated spaces, it is converted into steam, which exerts an enormous expansive force. The abundance with which steam is given off during volcanic eruptions has long been familiar, and serves to indicate that steam may be the agent more immediately employed in forcing melted lava to the surface. During the changes which are in progress underneath, a mass of water will sometimes be suddenly precipitated into an area of intensely heated rock, and its instant expansion will produce a sensible concussion or earthquake above ground.
The constant transference of materials from the interior to the surface, whether by the action of volcanos or by that of springs, must necessarily produce cavities within the crust. When, for example, we contemplate such a mountain as Etna, and reflect that all its vast piles of lava, scoriæ, and ashes have been abstracted from the interior of the earth, we see how real and important is this transference of material, and how easy it is to conceive of the