Читать книгу The Romance of Modern Geology - Edwin Sharpe Grew - Страница 14
ОглавлениеRECORDS LEFT BY THE SEA
We have already spoken of the story which the sea writes in the annals of geology. It is a story with two plots. In the first place, the sea is always wearing away the land. In the second place, it is arranging on its own bed the materials which it takes from the land, either directly or indirectly. As a sequel to both stories, the materials all neatly ranged, packed, and folded are revealed when the sea subsides from them, or when, in process of one of those great geological changes, the origin of which we have already attempted to account for, the sea bottom is raised to become the land of a continent. The first part of the sea's belligerent story is written so plainly for all eyes to see that one scarcely need dwell on it. Every strip of coast around these islands bears witness to it.
Off Shetland masses of rock twelve or thirteen tons in weight have been cut out from the cliff seventy feet above the smooth-water level. The sea's battering-rams are the masses of shingle, gravel, and loose blocks of stone which it carries with it; but it has subtler methods in the corrosive action of its salts, for just as it rusts or wears away iron, so its salts and acids must eat their way into many rocks.
But, after all, the coast-line of the world is a small fraction of the whole land surface of the globe; and a smaller fraction of the sea's own wide area. On that area are flung all the records and treasures which the sea has wrested from the land. The rivers, as we have already several times repeated, are the chief carriers of deposits to the sea. By their deltas they may be known. The deltas of the Ganges and Brahmaputra cover an area as large as that of England and Wales. The delta has been bored through to a depth of nearly five hundred feet, and has been found to consist of numerous alternations of fine clays, marls, and sands or sandstones, with occasional layers of gravel. In all this accumulation of sediment there are no traces of marine animals; but land plants and the plants and animals of the river and of the surrounding land have been discovered in quantity. The sea most often destroys land; but it sometimes deposits beaches; and, we might almost say, silts up the land. At Romney Marsh, for example, a tract of eighty square miles which was marsh in Julius Cæsar's time is now dry land, and has become so partly by the natural increase of shingle thrown up by the waves. The coarsest shingle usually accumulates towards the upper part of the beach, and the rest arranges itself generally according to size and weight, that which is finest being nearest to low-water mark.
It is often long before the stuff brought down by the rivers settles on the floor of the ocean. The finer particles may be carried out to sea for three hundred miles or more before they settle. Within this three-hundred-mile zone the land-derived materials are distributed over the floor in orderly succession. Nearer the land we shall find coarse gravel and sand. Beyond there will be tracts of finer sand and silt, with patches of gravel here and there. Still farther off will come fine blue and green muds, which are made of the tiny particles of such materials as form the ordinary rocks of the land. But when we are once past this zone of land material we come upon deposits which are the ocean's own freehold—materials which it does not derive from the continents, but which may be called oceanic in origin. First there are vast sheets of exceedingly fine red and brown clay. Whence comes it? It is by far the most common deposit in all the deeper parts of the ocean. It may either be the dust of volcanic fragments washed away from volcanic islands, or (which is much more likely) it may be supplied by eruptions under the sea. For it must be remembered that the sea floor is two to five miles nearer the hot rocks that are in the interior of the earth than the land surface is, and that consequently the water coming into contact with them may cause explosions arising from the action of steam. This is a question we shall have to consider later, and for the present we must ask the reader to accept the fact—and read on.
There is one very curious thing about this red clay, and it is that the accumulations of it appear to be built up very slowly. Where it occurs farthest from land great numbers of sharks' teeth with ear bones and other bones of whales have been dredged up from it. Some of these relics are quite fresh; others are coated with a crust of brown peroxide of manganese. Some are covered with this material and hidden in it. One haul of an ocean dredge will bring up the bones in all these states, so that they must be lying side by side. The bones are probably those of many generations of animals, and it must take a long time to cover them with the manganese deposit. But the clay is deposited even more slowly than the manganese, so that it must fall very slowly indeed.
But besides these things the bottom of the sea receives deposits of the remains of all kinds of shells, corals, and all sorts of marine creatures, great and small. As the countless myriads of the animals of the sea die, the shells with which they are covered, or the bones which form their framework, fall continually to the bottom of the oceanic gulfs in which they dwell. Then the ocean floor is covered with the remains of tiny animals incomparably more numerous than the stars of the sky; and this grey slimy ooze of organic matter hardens by pressure into sedimentary rock. In the course of ages, when the slow decline of the water lays it bare, it may become part of the land on which men dwell. But it is always forming, has always been forming, since life first appeared on the earth. It is on this ocean floor that man to-day lays his telegraph cables. Mr. Rudyard Kipling, in his verses "The Deep Sea Cables," has drawn a vivid picture of the bed of the deep ocean:—
The wrecks dissolve above us: their dust drops down from afar—
Down to the dark, to the utter dark where the blind white sea-snakes are.
There is no sound, no echo of sound, in the deserts of the deep,
On the great grey level plains of ooze, where the shell-burred cables creep.
It is in these silent depths that for uncounted and innumerable years the crust of the earth has been forming and has been growing outwards, while it has been slowly hardening inwards above the fires of its unplumbed interior.
It has been calculated that in a square mile of the ocean down to a depth of one hundred fathoms there exist more than sixteen tons of carbonate of lime in the form of the bones or shells of living animals. A continual fine "snow-storm" of dead chalky animals is therefore falling on to the bottom. Here and there, especially among volcanic islands, portions of the sea-bed have been raised up into land and masses of modern limestone. Though these rocks are full of the same kinds of shells as are still living in the neighbouring sea, they have been cemented into hard rock. This cementing is due to the water which has penetrated and permeated the stone, dissolving chalky matter from the outside shells, and depositing it once more lower down and farther in, like a fine mortar, so as to bind the mass together.
Every one has heard of coral reefs. They are one of the best and most familiar examples of the way in which great masses of solid rock can be built up by the dead bodies of animals. In the warmer seas of the earth, and notably in the track of the great ocean currents, various kinds of coral polyps, as they are called, take root on the edges and summits of submerged rocks and peaks, as well as on the shelving shores of islands. The coral polyp is a jelly-like creature, but it has a hard chalky skeleton inside its transparent body. It is a great colonist, with no liking for a solitary life, but with, on the contrary, a great fancy for its neighbours; in fact, the polyps grow and thrive in clumps, and the clumps unite to form communities, and the communities increase to colonies and nations, till they unite to form what is called a reef. The coral polyps are rather exigent in the choice of their residential neighbourhoods. They cannot live at a greater depth than fifteen or twenty fathoms, and in defiance of the inclinations which rule human beings, they have the strongest distaste for sun and air; in fact, they die when exposed to it.
Now when the polyp dies its skeleton remains behind it, and millions upon millions of these coral skeletons make a layer of coral. These layers of coral gradually lift the generations of polyps upwards to the surface of the water. But as we have seen, the living polyps die when they get so far, and consequently the reef then spreads outwards. On the outer edges of the reef the coral polyps flourish in the most vigorous way. There they are as completely provided for as in a County Council Utopia. The breakers bring them the food on which they live; the water and the climate suit them exactly. The only blot on their lives are the occasional storms which break off fragments of the coral foundation on which they live. But even this, while it is disastrous to the individual polyp, is for the good of the community, because these blocks as they roll down form a new foundation on which new generations of polyps can grow and feed. Moreover, it is better for the polyp to take the risks of these evictions than to vegetate inside the reef, for there in the calmer water he will not have enough to eat, and will dwindle and die. Thus the tendency of all reefs must be to grow seawards, and to increase in breadth. Perhaps their breadth may tell us roughly how old they are. But there is another possibility to be taken into consideration, which is that while the polyps are building the sea bottom or island foundation may be slowly sinking. In that case it is quite likely that the coral builders might just keep pace with the subsiding foundations of their home, and build up a great thickness of coral rock during the countless years of change.
Sir Archibald Geikie has called attention to the swiftness with which the structure of the coral polyp's skeleton is effaced from the foundation and a compact mass of rock put in its place. The sea-water's chemical and dissolving action, and the vast amount of mud and sand produced by the breakers are chiefly responsible for this. As the rock is being formed it is always being cemented. On the portion of a reef laid dry at low water, the coral rock looks in many places as solid and old as some of the ancient white limestones and marbles of the land. In pools where a current of water keeps the grains of coral sand in motion, each grain may be seen to be rounded. This is because on each particle of coral the dissolved carbonate of lime in the water is always being deposited (like the sediment in the bottom of a kettle). A mass of these rounded or egg-like grains all gathered together in a lump is called oolite, from the Greek word "oon" (Latin "ovum"), an egg. In many limestones, now forming parts of agricultural land, this oolitic structure is strikingly shown, and there can be no doubt that in such cases it was produced just as now coral reefs are being formed before our eyes. In the coral tracts of the Pacific Ocean there are nearly three hundred coral islands, besides extensive reefs round volcanic islands. Others occur in the Indian Ocean. Coral reefs abound in the West Indian seas, where in many of the islands they have been upraised into dry land—in Cuba to a height of 1100 feet above the sea-level. The Great Barrier Reef that fronts the north-eastern coast of Australia is 1250 miles long and from ten to ninety miles broad.
It will thus be seen that, apart from any other consideration, the animals of past ages leave permanent records of their existence merely by the accumulation of their dead bodies. Nevertheless, alike on land and on sea, the proportions of organic remains thus sealed and preserved is only a small part of the total population of plants and animals living at any given time.
