[7] In the West Indies, as I am informed by Captain Bird Allen, R.N., it is the common belief of those, who are best acquainted with the reefs, that the coral flourishes most, where freely exposed to the swell of the open sea.
[8] “Annales des Sciences Naturelles,” tome vi, pp. 276, 278.—“Là où les ondes sont agitées, les Lytophytés ne peuvent travailler, parce qu’elles détruiraient leurs fragiles édifices,” etc.
From these facts, it is certain, that the strongest and most massive corals flourish, where most exposed. The less perfect state of the reef of most atolls on the leeward and less exposed side, compared with its state to windward; and the analogous case of the greater number of breaches on the near sides of those atolls in the Maldiva Archipelago, which afford some protection to each other, are obviously explained by this circumstance. If the question had been, under what conditions the greater number of species of coral, not regarding their bulk and strength, were developed, I should answer,—probably in the situations described by MM. Quoy and Gaimard, where the water is tranquil and the heat intense. The total number of species of coral in the circumtropical seas must be very great: in the Red Sea alone, 120 kinds, according to Ehrenberg,[[9]] have been observed.
[9] Ehrenberg, “Über die Natur,” etc., p. 46.
The same author has observed that the recoil of the sea from a steep shore is injurious to the growth of coral, although waves breaking over a bank are not so. Ehrenberg also states, that where there is much sediment, placed so as to be liable to be moved by the waves there is little or no coral; and a collection of living specimens placed by him on a sandy shore died in the course of a few days.[[10]] An experiment, however, will presently be related in which some large masses of living coral increased rapidly in size, after having been secured by stakes on a sandbank. That loose sediment should be injurious to the living polypifers, appears, at first sight, probable; and accordingly, in sounding off Keeling atoll, and (as will hereafter be shown) off Mauritius, the arming of the lead invariably came up clean, where the coral was growing vigorously. This same circumstance has probably given rise to a strange belief, which, according to Captain Owen,[[11]] is general amongst the inhabitants of the Maldiva atolls, namely that corals have roots, and therefore that if merely broken down to the surface, they grow up again; but, if rooted out, they are permanently destroyed. By this means the inhabitants keep their harbours clear; and thus the French Governor of St. Mary’s in Madagascar, “cleared out and made a beautiful little port at that place.” For it is probable that sand would accumulate in the hollows formed by tearing out the corals, but not on the broken and projecting stumps, and therefore, in the former case, the fresh growth of the coral might be thus prevented.
[10] Ibid., p. 49.
[11] Captain Owen on the Geography of the Maldiva Islands, Geograph. Journal, vol. ii, p. 88.
In the last chapter I remarked that fringing-reefs are almost universally breached, where streams enter the sea.[[12]] Most authors have attributed this fact to the injurious effects of the fresh water, even where it enters the sea only in small quantity, and during a part of the year. No doubt brackish water would prevent or retard the growth of coral; but I believe that the mud and sand which is deposited, even by rivulets when flooded, is a much more efficient check. The reef on each side of the channel leading into Port Louis at Mauritius, ends abruptly in a wall, at the foot of which I sounded and found a bed of thick mud. This steepness of the sides appears to be a general character in such breaches. Cook,[[13]] speaking of one at Raiatea, says, “like all the rest, it is very steep on both sides.” Now, if it were the fresh water mingling with the salt which prevented the growth of coral, the reef certainly would not terminate abruptly, but as the polypifers nearest the impure stream would grow less vigorously than those farther off, so would the reef gradually thin away. On the other hand, the sediment brought down from the land would only prevent the growth of the coral in the line of its deposition, but would not check it on the side, so that the reefs might increase till they overhung the bed of the channel. The breaches are much fewer in number, and front only the larger valleys in reefs of the encircling barrier class. They probably are kept open in the same manner as those into the lagoon of an atoll, namely, by the force of the currents and the drifting outwards of fine sediment. Their position in front of valleys, although often separated from the land by deep water lagoon-channels, which it might be thought would entirely remove the injurious effects both of the fresh water and the sediment, will receive a simple explanation when we discuss the origin of barrier-reefs.
[12] Lieutenant Wellstead and others have remarked that this is the case in the Red Sea; Dr. Rüppell (“Reise in Abyss.” Band. i, p. 142) says that there are pear-shaped harbours in the upraised coral-coast, into which periodical streams enter. From this circumstance, I presume, we must infer that before the upheaval of the strata now forming the coast-land, fresh water and sediment entered the sea at these points; and the coral being thus prevented growing, the pear-shaped harbours were produced.
[13] Cook’s “First Voyage,” vol. ii, p. 271 (Hawkesworth’s edit.)