[34] I am indebted to Mr. Stokes for having kindly communicated this fact to me, together with much other valuable information.
[35] I will record in the form of a note all the facts that I have been able to collect on the depths, both within and without the tropics, at which those corals and corallines can live, which there is no reason to suppose ever materially aid in the construction of a reef.
Ellis (“Nat. Hist. of Coralline,” p. 96) states that Ombellularia was procured in latitude 79° N. sticking to a line from the depth of 236 fathoms; hence this coral either must have been floating loose, or was entangled in stray line at the bottom. Off Keeling atoll a compound Ascidia (Sigillina) was brought up from 39 fathoms, and a piece of sponge, apparently living, from 70, and a fragment of Nullipora also apparently living from 92 fathoms. At a greater depth than 90 fathoms off this coral island, the bottom was thickly strewed with joints of Halimeda and small fragments of other Nulliporæ, but all dead. Captain B. Allen, R.N., informs me that in the survey of the West Indies it was noticed that between the depth of 10 and 200 fathoms, the sounding lead very generally came up coated with the dead joints of a Halimeda, of which he showed me specimens. 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 Nulliporæ were piled up. From these facts it appears, that these simply organized bodies are amongst the most abundant productions of the sea.
[36] Fleming’s “British Animals,” genus Caryophyllia.
| Name of Zoophyte | Depth in Fathoms | Country and S. Latitude | Authority |
| Sertularia | 40 | Cape Horn 66° | (Where none is given, the observation is my own.) |
| Cellaria | Ditto | Ditto | |
| Cellaria. A minute scarlet encrusted species, found living | 190 | Keeling Atoll 12° | |
| Cellaria. An allied, small stony sub-generic form | 48 | S. Cruz River 50° | |
| A coral allied to Vincularia, with eight rows of cells | 40 | Cape Horn | |
| Tubulipora, near to T. patima | Ditto | Ditto | |
| Ditto | 94 | East Chiloe 43° | |
| Cellepora, several species, and allied sub-generic forms | 40 | Cape Horn | |
| Ditto | 40 and 57 | Chonos Arch. 45° | |
| Ditto | 48 | S. Cruz 50° | |
| Eschara | 30 | Tierra del Fuego 53° | |
| Ditto | 48 | S. Cruz R. 50° | |
| Retepora | 40 | Cape Horn | |
| Ditto | 100 | C. Good Hope 34° | Quoy and Gaimard, Ann. Scien. Nat., t. vi, p. 284. |
| Millepora, a strong coral with cylindrical branches, of a pink colour, abut two inches high, resembling in the form of its orifices M. aspera of Lamarck | 94 and 30 | E. Chiloe 43° Tierra del Fuego 53° | |
| Coralium | 120 | Barbary 33° N. | Peyssonel in paper read to Royal society May 1752. |
| Antipathes | 16 | Chonos 45° | |
| Gorgonia (or an allied form) | 160 | Abrolhos on the coast of Brazil 18° | Capt. Beechey informed me of this fact in a letter. |
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.
It has been argued[[37]] that reefs may possibly rise from very great depths through the means of small corals, first making a platform for the growth of the stronger kinds. This, however, is an arbitrary supposition: it is not always remembered, that in such cases there is an antagonist power in action, namely, the decay of organic bodies, when not protected by a covering of sediment, or by their own rapid growth. We have, moreover, no right to calculate on unlimited time for the accumulation of small organic bodies into great masses. Every fact in geology proclaims that neither the land, nor the bed of the sea retain for indefinite periods the same level. As well might it be imagined that the British Seas would in time become choked up with beds of oysters, or that the numerous small corallines off the inhospitable shores of Tierra del Fuego would in time form a solid and extensive coral-reef.
[37] Journal of the Royal Geographical Society, 1831, p. 218.
Chapter V
THEORY OF THE FORMATION OF THE DIFFERENT CLASSES OF CORAL-REEFS
The atolls of the larger archipelagoes are not formed on submerged craters, or on banks of sediment.—Immense areas interspersed with atolls.—Their subsidence.—The effects of storms and earthquakes on atolls.—Recent changes in their state.—The origin of barrier-reefs and of atolls.—Their relative forms.—The step-formed ledges and walls round the shores of some lagoons.—The ring-formed reefs of the Maldiva atolls.—The submerged condition of parts or of the whole of some annular reefs.—The disseverment of large atolls.—The union of atolls by linear reefs.—The Great Chagos Bank.—Objections from the area and amount of subsidence required by the theory, considered.—The probable composition of the lower parts of atolls.
The naturalists who have visited the Pacific, seem to have had their attention riveted by the lagoon-islands, or atolls,—those singular rings of coral-land which rise abruptly out of the unfathomable ocean—and have passed over, almost unnoticed, the scarcely less wonderful encircling barrier-reefs. The theory most generally received on the formation of atolls, is that they are based on submarine craters; but where can we find a crater of the shape of Bow atoll, which is five times as long as it is broad ([Plate I], Fig. 4); or like that of Menchikoff Island ([Plate II], Fig. 3), with its three loops, together sixty miles in length; or like Rimsky Korsacoff, narrow, crooked, and fifty-four miles long; or like the northern Maldiva atolls, made up of numerous ring-formed reefs, placed on the margin of a disc,—one of which discs is eighty-eight miles in length, and only from ten to twenty in breadth? It is, also, not a little improbable, that there should have existed as many craters of immense size crowded together beneath the sea, as there are now in some parts atolls. But this theory lies under a greater difficulty, as will be evident, when we consider on what foundations the atolls of the larger archipelagoes rest: nevertheless, if the rim of a crater afforded a basis at the proper depth, I am far from denying that a reef like a perfectly characterised atoll might not be formed; some such, perhaps, now exist; but I cannot believe in the possibility of the greater number having thus originated.