the attention of seamen and navigators has long been attracted by the number of circular islands in the warm parts of the pacific and indian oceans. generally each one of these circular islands contains a lake of quiet water extending almost to its outer shores, so that the island looks like a fairy ring of land floating in the ocean, and adorned with tropical trees and plants.
happily for the boys and girls of the present day, this subject, with other equally fascinating branches of science, has now been studied by naturalists, who give us the rich results of their labors. it seems scarcely possible that the dainty beautiful corals which we examined not long ago in young people can have anything to do with the making of islands, but so it is. coral reefs are vast masses of coral which have grown in warm oceans. their formation must have been slow, yet they sometimes extend hundreds of miles. florida and many other parts of our solid continents are known to have been formed from coral reefs.
let us now try to picture to ourselves the beginning of one of these reefs, and by following its growth step by step we may at least understand how it has been formed. there are hills and valleys on the bottom of the ocean as well as on the land. we will fancy that some young coral polyps which have been swimming about in the sea settle on the sides of one of these hills, and begin to grow and spread all around it. they will increase also by the deposit of eggs until they form a circular wall.
as the coral wall grows, the lower polyps and the inner ones die, their skeletons forming a solid foundation for all that grow above them. there may be only about an inch of living coral on the outside of the reef.
fig. 1.—a, a, surface of the water; b, natural red of the ocean; c, c, coral formation; d, lagoon.
these walls rise nearly straight, and you will see that in doing so they inclose a circular basin of quiet water, and now you can understand why it is that a coral island mostly has a lake in the centre, as is shown in fig. 1. the lakes are called lagoons.
the bottom of the wall is formed of brain-coral and other solid kinds which live only in deep water, and they die when a certain height is reached. the formation of the new island does not stop with their death, however. the wall having now reached the proper height to suit branching corals, which require shallower water, their young polyps will settle upon it, and finish the structure. we might suppose a reef formed of branching corals would be open and unsubstantial, but in their growth the branches are thickly interlaced. the spaces between them become filled with substances floating in the ocean, and with pieces of coral which are broken from the reef by the fierce dashing of the waves. the whole forms a solid mass, stronger, perhaps, than any stone masonry. the fragments of coral suffer no serious injury by breaking, but if lodged in some favorable spot they continue to grow.
the outer edge of the wall is steep and abrupt. soundings taken just outside show very deep water. in this portion of the wall the corals live and thrive, always supplied with clear water. the breakers dash against it with such fury that apparently the hardest rock must in time yield to the tremendous force of the waves. but, strange as it may appear, the soft jelly-like bodies of the polyps give to the reef the power of resisting the billows.
the inner surface of the wall slopes gently to the land, and being washed by quiet waters often containing sand and mud, it is not favorable to the growth of polyps. still, there are certain kinds of coral which thrive within the lagoons; some of them are exceedingly brilliant and beautiful.
the coral polyps die before they reach the surface of the ocean, as no corals can live out of water. the remainder of the island is built up by shells, pieces of broken coral, sea-weed, and other floating materials which are washed upon it, raising the wall higher and higher. the never-ceasing action of the waves grinds up these shells and broken coral, until at last they form a soil of sand and mud which is now ready to receive any seeds that may float on the water or be brought by the winds and the birds. the seeds take root in the new soil, and young plants begin to appear on the glistening white surface. floating cocoa-nuts often lodge on the shores, and cocoa-nut-trees are among the first to grow upon them. as the plants drop their leaves and decay, the soil is enriched little by little, and fitted for the home of various animals and birds, which in some mysterious manner find their way to these lonely spots far out at sea. in time our coral reef may become a beautiful tropical island fringed with waving trees and plants, and inhabited by man.
fig. 2.—an atoll.
circular islands seldom form complete rings. there is generally an opening into the lake on the side most sheltered from the wind. a safe harbor in mid-ocean is thus made, in which vessels may take shelter, but it requires an expert navigator to pass the perils at its entrance. to anchor on the outer shore would be impossible. in fig. 2 is a pretty little coral island with ships in its lagoon. if a lake is entirely inclosed by the coral wall, it may in time be changed to fresh water by the rains that fall into it.
coral reefs often extend to a depth of three hundred feet below the surface of the ocean, and formerly persons were puzzled to know how they could have grown in such deep water, as no coral polyps can live at a greater depth than twenty or thirty fathoms. this puzzling question was settled by the late charles darwin, who first showed that coral islands occur where there has been a gradual sinking of the bottom of the ocean. as the reef rises in height, the sinking of the foundation partly counteracts the upward growth of the coral; consequently the proper[pg 21] depth of water is secured, and the reef appears to be stationary, whereas it is really growing upward.
whenever a coral reef rises above the surface of the ocean, we may know that the coral, which grew under water, has been lifted above the level of the sea by a rising of the ocean-bed.
these circular reefs are called "atolls." they are quite different from the "fringing reefs," which extend along the shores of continents and islands. there are usually openings or breaks in fringing reefs directly opposite the mouths of rivers and fresh-water streams, as the corals can not endure currents which carry mud or sediment. perhaps the grandest reef to be found in any part of the world is the one extending along the northeast coast of australia. it is nearly one thousand miles in length, and proves to us that the helpless coral polyps have played no trifling part in the formation of our earth. all they have accomplished has been done merely by their living and growing.
