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If the data be thought insufficient, on which I have grounded my belief, respecting the depth at which the reef-building polypifers can exist, and it be assumed that they can flourish at a depth of even one hundred fathoms, yet the weight of the above argument is but little diminished, for it is almost equally improbable, that as many submarine mountains, as there are low islands in the several great and widely separated areas above specified, should all rise within six hundred feet of the surface of the sea and not one above it, as that they should be of the same height within the smaller limit of one or two hundred feet.
The common or reef-building coral has but little use except as a source of lime, and no intrinsic value except as an object of curiosity. Coral reefs may be arranged under three classes; namely, fringing reefs, barrier reefs, and atolls. The first class embraces the shallow-water reefs found close to land, either surrounding islands or skirting the shores of continents.
Through steps such as these, any thickness of rock, composed of a singular intermixture of various kinds of corals, shells, and calcareous sediment, might be formed; but without subsidence, the thickness would necessarily be determined by the depth at which the reef-building polypifers can exist.
From the limited depths at which reef-building polypifers can flourish, taken into consideration with certain other circumstances, we are compelled to conclude, as it will be seen, that both in atolls and barrier-reefs, the foundation on which the coral was primarily attached, has subsided; and that during this downward movement, the reefs have grown upwards.
The reef-building coral differs very materially in form and appearance from the precious or red coral; the former is confined to comparatively shallow water, while the latter is found most commonly at a depth of six hundred feet or more, and it occurs chiefly in the Mediterranean Sea.
Again, on what have the reef-building corals, which cannot live at great depths, based their encircling structures? This is a great apparent difficulty, analogous to that in the case of atolls, which has generally been overlooked.
Nearly all the smaller islands are of coral or of volcanic origin; in many instances both agencies have contributed to their formation. The coral and volcanic islands seem to be the tops of mountain ranges that, little by little, have sunk, until only their higher summits are now above sea level. The central part of the Pacific Ocean is pre-eminently the home of the reef-building coral.
A foundation must originally have been present beneath each atoll at that limited depth, which is indispensable for the first growth of the reef-building polypifers.
Again, on what have the reef-building corals, which cannot live at great depths, based their encircling structures? This is a great apparent difficulty, analogous to that in the case of atolls, which has generally been overlooked.
Such is the general distribution of the reef-building corals, but there are some very interesting and singular circumstances to be observed in the conformation of the reefs, when we consider them individually.
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