Every year since the world has gained its present form the streams and rivers have been pouring water, and carrying mud, stones, and gravel ceaselessly into the ocean. In addition to this, the surface water washes the stones away, animals (corals) build up islands from the depths, and take up space in the ocean. We know that if we put our hand in a basin full of water we displace a quantity of the fluid; so we might imagine that, the sea being already full, every island formed would tend to an overflow of the sea, and the land would be thereby buried. That the sea does encroach upon the earth we know, but it also recedes. Here is the balance of Nature.
Rivers pour in water and material. The sun absorbs the water and prevents overflow; tiny animals make shells from the material. All the causes we have mentioned tend to permit the encroachment of the waters, but volcanic action and even earthquakes act also to neutralize this tendency by upheaving hills and mountains, which prevent the invasion of the sea by its elevation or by land depression. We have seen in our chapters upon Geology how the ocean beds have been upheaved, and remains of marine animals are daily found upon our highest hills. Thus the forces which sometimes cause such destruction in the earth are the means whereby the waters are kept in their places. But for volcanic action the land might all disappear by denudation and continual wear and tear, and be deposited at the bottom of the sea!
If it were not for currents, of which many defined ones exist in the ocean, and the never-ceasing flow and ebb of the tides, the sea would soon lose its purity and clearness. Though the water is salt and becoming salter, animalculæ and all kinds of plant-animals would still increase and multiply; so the decay of animal and vegetable matter would quickly render the ocean a source of pestilence and death to mankind, and be most injurious to animal life generally. But the movement is so ceaseless, and the various fish and mammalia (whales, for instance), by preying upon each other, as other animals on earth do, keep up the balance of production, and the organic matter deposited in the sea is also cleared away.
That the constant currents of the sea prevent the formation and growth of seaweed is clearly shown by the great “Sargasso Sea,” or tract of weed (Fucus natans), called the Gulf-weed. This great tract embraces thousands of square miles, and is situated in the very middle of the Atlantic Ocean, where there are but few currents; but surrounding it is the Gulf-Stream, an enormous current of water running at a regular rate of four or five miles an hour. This Gulf-Stream is supposed to be caused by the same laws and influences which determine the trade-winds—namely, a constant rarefaction of the water at the tropical parts of the earth, and a corresponding condensation at the Arctic portions, for warm water is much lighter than cold, and when the waters of the tropical regions become lighter, the heavier waters of the cold regions pressing down more forcibly tend to raise them above their proper level; they therefore flow towards those very parts which have sunk down by their contraction, and a constant current takes place; this current is the Gulf-Stream. It runs from the Gulf of Mexico northwards towards Newfoundland, turning by Iceland towards the British Isles, by France and Spain, onwards to the coasts of Africa and South America, the West Indies, and again to the Gulf of Mexico, although the return current does not go by the name of Gulf-Stream. This great stream of water, warmed by the tropical sun, serves the same two purposes fulfilled by the trade-winds—namely, a circulation and distribution of the superfluous heat of the equatorial regions, warming the northern countries; and cooling, by the return of under-currents, those in the tropics. The fogs of Newfoundland are caused by the great current of warm water entering the cold region and carrying with them surface-currents of moist air, which the cold condenses into fog, just as the breath is visible in a cold atmosphere. England owes its moist and mild climate to the same cause. The depth of the Gulf-Stream itself is very little. It is a mere layer of warm water. (See Sir George Nares’ reports of the Challenger expedition.)
Fig. 705.—Atoll, or Coral Island.
In the foregoing pages you have now seen, and, we hope, gained, some information concerning the sea sufficient, at any rate, to induce you to enter more deeply into the subject than we can at present do. We have learnt how the sea water is composed, and what goes on on the surface. We have discussed waves, and referred to tides and currents, the wearing away and the renewal of land by the sea; we have dived beneath the surface, and found something to interest us at the bottom of the ocean. As we come up again we are surprised to find islands or reefs where none existed when we went down. What has caused this sudden appearance? They may have been slowly raised to the surface by coral insects, or suddenly by volcanic action. Let us consider the coral, which plays a very important part in our Physical Geography, before we proceed to the volcanic island.[32]
The low-lying islands are those formed by the skeletons of the coral insects, and the Coralline Islands are some of the most wonderful productions of nature. They are only found in warm climates, between the twenty-eighth degrees of north and south latitude, and limestone pure and simple is the chief component of the coral reef, as it is of the mountains erupted from the depths of the sea. “The detritus of corals, echinodermata shells, reticularia, and other living creatures,” says a writer on this subject, “deposit not only the salts of lime extracted from the ocean, but their own dead bodies to form the hard substance of the rock.”
Fig. 706.—Gorgonia guttata (natural size).