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Updated: May 3, 2025
But there are still some cases where the little we know of a life, whose workings are invisible to the naked eye, suggests the possibility of advantageously directing the efforts of troops of artisans that we cannot see. Upon coasts occupied by the corallines, the reef-building animalcule does not work near the mouth of rivers.
But towards the lower part of the reef, at a depth of about 120 feet, these creatures are less active, and fewer of them at work; and at greater depths than that you find no living coral polype at all; and it may be laid down as a rule, derived from very extensive observation, that these reef-building corals cannot live in a greater depth of water than about 120 to 150 feet.
As the reef-building corals require food, are preyed upon by other animals, are killed by sediment, cannot adhere to a loose bottom, and may be easily carried down to a depth whence they cannot spring up again, we need feel no surprise at the reefs both of atolls and barriers becoming in parts imperfect.
Keeling Island. Singular appearance. Scanty Flora. Transport of seeds. Birds and insects. Ebbing and flowing springs. Fields of dead coral. Stones transported in the roots of trees. Great crab. Stinging corals. Coral-eating fish. Coral formations. Lagoon islands or atolls. Depth at which reef-building corals can live. Vast areas interspersed with low coral islands. Subsidence of their foundations.
Although the limit of depth, at which each particular kind of coral ceases to exist, is far from being accurately known; yet when we bear in mind the manner in which the clumps of coral gradually became infrequent at about the same depth, and wholly disappeared at a greater depth than twenty fathoms, on the slope round Keeling atoll, on the leeward side of the Mauritius, and at rather less depth, both without and within the atolls of the Maldiva and Chagos Archipelagoes; and when we know that the reefs round these islands do not differ from other coral formations in their form and structure, we may, I think, conclude that in ordinary cases, reef-building polypifers do not flourish at greater depths than between twenty and thirty fathoms.
As we see that in those parts of the ocean, where coral-reefs are most abundant, one island is fringed and another neighbouring one is not fringed; as we see in the same archipelago, that all the reefs are more perfect in one part of it than in another, for instance, in the southern half compared with the northern half of the Maldiva Archipelago, and likewise on the outer coasts compared with the inner coasts of the atolls in this same group, which are placed in a double row; as we know that the existence of the innumerable polypifers forming a reef, depends on their sustenance, and that they are preyed on by other organic beings; and, lastly, as we know that some inorganic causes are highly injurious to the growth of coral, it cannot be expected that during the round of change to which earth, air, and water are exposed, the reef-building polypifers should keep alive for perpetuity in any one place; and still less can this be expected, during the progressive subsidences, perhaps at some periods more rapid than at others, to which by our theory these reefs and islands have been subjected and are liable.
But we shall immediately see, that in this, as well as in the crater-theory, a most important consideration has been overlooked, namely, on what have the reef-building corals, which cannot live at a great depth, based their massive structures?
The scale is .57 of an inch to a mile; it is taken from the "Atlas of the Voyage of the 'Coquille'," by Duperrey. In the succeeding chapter it will be shown that reef-building polypifers cannot flourish at great depths, for instance, it is highly improbable that they could exist at a quarter of the depth represented by the plummet on the right hand of the woodcut.
Now, bearing in mind that reef-building coral cannot live at a greater depth than from 20 to 30 fathoms, and that the scale is so small that the plummets on the right hand show a depth of 200 fathoms, on what are these barrier-reefs based? Are we to suppose that each island is surrounded by a collar-like submarine ledge of rock, or by a great bank of sediment, ending abruptly where the reef ends?
For a long time it was supposed that the Coral animals inhabited very deep waters, for they were sometimes brought up on sounding-lines from a depth of many hundreds or even thousands of feet, and it was taken for granted that they must have had their home where they were found; but the facts recently ascertained respecting the subsidence of ocean-bottoms have shown that the foundation of a Coral wall may have sunk far below the place where it was laid, and it is now proved beyond a doubt that no Reef-Building Coral can thrive at a depth of more than fifteen fathoms, though Corals of other kinds occur far lower, and that the dead Reef-Corals sometimes brought to the surface from much greater depths are only broken fragments of some Reef that has subsided with the bottom on which it was growing.
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