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Updated: May 22, 2025
Zone of Nulliporae. Conglomerate. Depth of lagoons. Sediment. Reefs submerged wholly or in part. Breaches in the reef. Ledge-formed shores round certain lagoons. Conversion of lagoons into land. I will here give a sketch of the general form and structure of the many atolls and atoll-formed reefs which occur in the Pacific and Indian Oceans, comparing them with Keeling atoll.
This durability is much the most curious fact in its history: it is due to a tough layer, a few inches thick, of calcareous matter, wholly formed by the successive growth and death of the small shells of Serpulae, together with some few barnacles and nulliporae.
If this has not taken place, the reef can in no part have increased outwards in breadth since its formation, or at least since the Nulliporae formed the convex mound on its margin; for the zone thus formed, and which stands between two and three feet above the other parts of the reef, is nowhere much above twenty yards in width.
Off Pernambuco, in Brazil, in about twelve fathoms, the bottom was covered with fragments dead and alive of a dull red Nullipora, and I infer from Roussin's chart, that a bottom of this kind extends over a wide area. On the beach, within the coral-reefs of Mauritius, vast quantities of fragments of Nulliporae were piled up.
These nulliporae, which are hard, very simply-organised sea-plants, play an analogous and important part in protecting the upper surfaces of coral-reefs, behind and within the breakers, where the true corals, during the outward growth of the mass, become killed by exposure to the sun and air.
If the whole, or a large part of the "flat," had been formed by the outward growth of the margin, each successive margin would naturally have been coated by the Nulliporae, and so much of the surface would have been of equal height with the existing zone of living Nulliporae: this is not the case, as may be seen in the woodcut.
It is, however, evident from the abraded state of the "flat," with its original inequalities filled up, that its surface has been much modified; and it is possible that the hinder portions of the zone of Nulliporae, perishing as the reef grows outwards, might be worn down by the surf.
This durability is much the most curious fact in its history: it is due to a tough layer, a few inches thick, of calcareous matter, wholly formed by the successive growth and death of the small shells of Serpulae, together with some few barnacles and nulliporae.
GAMBIER ISLANDS, in the southern part of the Low Archipelago; from the survey by Captain Beechey; height of highest island, 1,246 feet; the islands are surrounded by extensive and irregular reefs; the reef on the southern side is submerged. The annular reef on the north-west side is broken, and blends into a shoal sandbank, on which the sea breaks. Corals on the outer margin. Zone of Nulliporae.
Nothing can be more singular than the appearance at low tide of this "flat" of naked stone, especially where it is externally bounded by the smooth convex mound of Nulliporae, appearing like a breakwater built to resist the waves, which are constantly throwing over it sheets of foaming water. The characteristic appearance of this "flat" is shown in the foregoing woodcut of Whitsunday atoll.
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