Harper's Young People, November 14, 1882 / An Illustrated Weekly

by Various

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.