Coral Reefs; Volcanic Islands; South American Geology — Complete

by Charles Darwin

LOUISIADE.—The dangerous reefs which front and surround the western, southern, and northern coasts of this so-called peninsula and archipelago, seem evidently to belong to the barrier class. The land is lofty, with a low fringe on the coast; the reefs are distant, and the sea outside them profoundly deep. Nearly all that is known of this group is derived from the labours of D’Entrecasteaux and Bougainville: the latter has represented one continuous reef ninety miles long, parallel to the shore, and in places as much as ten miles from it; coloured pale blue. A little distance northward we have the Laughlan Islands, the reefs round which are engraved in the “Atlas of the Voyage of the Astrolabe,” in the same manner as in the encircled islands of the Caroline Archipelago, the reef is, in parts, a mile and a half from the shore, to which it does not appear to be attached; coloured blue. At some little distance from the extremity of the Louisiade lies the Wells reef, described in G. Hamilton’s “Voyage in H.M.S. Pandora” (p. 100): it is said, “We found we had got embayed in a double reef, which will soon be an island.” As this statement is only intelligible on the supposition of the reef being crescent or horse-shoe formed, like so many other submerged annular reefs, I have ventured to colour it blue.

SOLOMON ARCHIPELAGO.—The chart in Krusenstern’s “Atlas” shows that these islands are not encircled, and as coral appears from the works of Surville, Bougainville, and Labillardiere, to grow on their shores, this circumstance, as in the case of the New Hebrides, is a presumption that they are fringed. I cannot find out anything from D’Entrecasteaux’s “Voyage,” regarding the southern islands of the group, so have left them uncoloured.—Malayta Island in a rough MS. chart in the Admiralty has its northern shore fringed.—Ysabel Island, the N.E. part of this island, in the same chart, is also fringed: Mendana, speaking (Burney, vol. i, p. 280) of an islet adjoining the northern coast, says it is surrounded by reefs; the shores, also of Port Praslin appear regularly fringed.—Choiseul Island. In Bougainville’s “Chart of Choiseul Bay,” parts of the shores are fringed by coral-reefs.—Bougainville Island. According to D’Entrecasteaux the western shore abounds with coral-reefs, and the smaller islands are said to be attached to the larger ones by reefs; all the before-mentioned islands have been coloured red.—Bouka Islands. Captain Duperrey has kindly informed me in a letter that he passed close round the northern side of this island (of which a plan is given in his “Atlas of the Coquille’s Voyage”), and that it was “garnie d’une bande de récifs à fleur d’eau adherentes au rivage;” and he infers, from the abundance of coral on the islands north and south of Bouka, that the reef probably is of coral; coloured red.

Off the north coast of the Solomon Archipelago there are several small groups which are little known; they appear to be low, and of coral-formation; and some of them probably have an atoll-like structure; the Chevallier Dillon, however, informs me that this is not the case with the B. de Candelaria.—Outong Java, according to the Spanish navigator, Maurelle, is thus characterised; but this is the only one which I have ventured to colour blue.

NEW IRELAND.—The shores of the S.W. point of this island and some adjoining islets, are fringed by reefs, as may be seen in the “Atlases of the Voyages of the Coquille and Astrolabe.” M. Lesson observes that the reefs are open in front of each streamlet. The Duke of York’s Island is also fringed; but with regard to the other parts of New Ireland, New Hanover, and the small islands lying northward, I have been unable to obtain any information. I will only add that no part of New Ireland appears to be fronted by distant reefs. I have coloured red only the above specified portions.

NEW BRITAIN AND THE NORTHERN SHORE OF NEW GUINEA.—From the charts in the “Voyage of the Astrolabe,” and from the “Hydrog. Memoir,” it appears that these coasts are entirely without reefs, as are the Schouten Islands, lying close to the northern shore of New Guinea. The western and south-western parts of New Guinea, will be treated of when we come to the islands of the East Indian Archipelago.

ADMIRALTY GROUP.—From the accounts by Bougainville, Maurelle, D’Entrecasteaux, and the scattered notices collected by Horsburgh, it appears, that some of the many islands composing it, are high, with a bold outline; and others are very low, small and interlaced with reefs. All the high islands appear to be fronted by distant reefs rising abruptly from the sea, and within some of which there is reason to believe that the water is deep. I have therefore little doubt they are of the barrier class.—In the southern part of the group we have Elizabeth Island, which is surrounded by a reef at the distance of a mile; and two miles eastward of it (Krusenstern, “Append.” 1835, p. 42) there is a little island containing a lagoon.—Near here, also lies Circular-reef (Horsburgh, “Direct.,” vol. i, p. 691, 4th ed.), “three or four miles in diameter having deep water inside with an opening at the N.N.W. part, and on the outside steep to.” I have from these data, coloured the group pale blue, and Circular-reef dark blue.—The Anachorites, Echequier, and Hermites, consist of innumerable low islands of coral-formation, which probably have atoll-like forms; but not being able to ascertain this, I have not coloured them, nor Durour Island, which is described by Carteret as low.

The CAROLINE ARCHIPELAGO is now well-known, chiefly from the hydrographical labours of Lutké; it contains about forty groups of atolls, and three encircled islands, two of which are engraved in Figs 2 and 7, [Plate I.] Commencing with the eastern part; the encircling reef round Ualen appears to be only about half a mile from the shore; but as the land is low and covered with mangroves (“Voyage autour du Monde,” par F. Lutké, vol. i, p. 339), the real margin has not probably been ascertained. The extreme depth in one of the harbours within the reef is thirty-three fathoms (see charts in “Atlas of Coquille’s Voyage”), and outside at half a mile distant from the reef, no bottom was obtained with two hundred and fifty fathoms. The reef is surmounted by many islets, and the lagoon-like channel within is mostly shallow, and appears to have been much encroached on by the low land surrounding the central mountains; these facts show that time has allowed much detritus to accumulate; coloured pale blue.—Pouynipète, or Seniavine. In the greater part of the circumference of this island, the reef is about one mile and three quarters distant; on the north side it is five miles off the included high islets. The reef is broken in several places; and just within it, the depth in one place is thirty fathoms, and in another, twenty-eight, beyond which, to all appearance, there was “un porte vaste et sur” (Lutké, vol. ii, p. 4); coloured pale blue.—Hogoleu or Roug. This wonderful group contains at least sixty-two islands, and its reef is one hundred and thirty-five miles in circuit. Of the islands, only a few, about six or eight (see “Hydrog. Descrip.” p. 428, of the “Voyage of the Astrolabe,” and the large accompanying chart taken chiefly from that given by Duperrey) are high, and the rest are all small, low, and formed on the reef. The depth of the great interior lake has not been ascertained; but Captain D’Urville appears to have entertained no doubt about the possibility of taking in a frigate. The reef lies no less than fourteen miles distant from the northern coasts of the interior high islands, seven from their western sides, and twenty from the southern; the sea is deep outside. This island is a likeness on a grand scale to the Gambier group in the Low Archipelago. Of the groups of low[^[1]] islands forming the chief part of the Caroline Archipelago, all those of larger size, have the true atoll-structure (as may be seen in the “Atlas” by Captain Lutké), and some even of the very small ones, as Macaskill and Duperrey, of which plans are given in the “Atlas of the Coquille’s Voyage.” There are, however, some low small islands of coral-formation, namely Ollap, Tamatam, Bigali, Satahoual, which do not contain lagoons; but it is probable that lagoons originally existed, but have since filled up: Lutké (vol. ii, p. 304) seems to have thought that all the low islands, with only one exception, contained lagoons. From the sketches, and from the manner in which the margins of these islands are engraved in the “Atlas of the Voyage of the Coquille,” it might have been thought that they were not low; but by a comparison with the remarks of Lutké (vol. ii, p. 107, regarding Bigali) and of Freycinet (“Hydrog. Memoir L’Uranie Voyage,” p. 188, regarding Tamatam, Ollap, etc.), it will be seen that the artist must have represented the land incorrectly. The most southern island in the group, namely Piguiram, is not coloured, because I have found no account of it. Nougouor, or Monte Verdison, which was not visited by Lutké, is described and figured by Mr. Bennett (United Service Journal, January 1832) as an atoll. All the above-mentioned islands have been coloured blue.

[1] In D’Urville and Lottin’s chart, Peserare is written with capital letters; but this evidently is an error, for it is one of the low islets on the reef of Namonouyto (see Lutké’s charts)—a regular atoll.

WESTERN PART OF THE CAROLINE ARCHIPELAGO.—Fais Island is ninety feet high, and is surrounded, as I have been informed by Admiral Lutké, by a narrow reef of living coral, of which the broadest part, as represented in the charts, is only 150 yards; coloured red.—Philip Island., I believe, is low; but Hunter, in his “Historical Journal,” gives no clear account of it; uncoloured.—Elivi; from the manner in which the islets on the reefs are engraved, in the “Atlas of the Astrolabe’s Voyage,” I should have thought they were above the ordinary height, but Admiral Lutké assures me this is not the case: they form a regular atoll; coloured blue.—Gouap (Eap of Chamisso), is a high island with a reef (see chart in “Voyage of the Astrolabe”), more than a mile distant in most parts from the shore, and two miles in one part. Captain D’Urville thinks that there would be anchorage (“Hydrog. Descript. Astrolabe Voyage,” p. 436) for ships within the reef, if a passage could be found; coloured pale blue.—Goulou, from the chart in the “Astrolabe’s Atlas,” appears to be an atoll. D’Urville (“Hydrog. Descript.” p. 437) speaks of the low islets on the reef; coloured dark blue.

PELEW ISLANDS.—Krusenstern speaks of some of the islands being mountainous; the reefs are distant from the shore, and there are spaces within them, and not opposite valleys, with from ten to fifteen fathoms. According to a MS. chart of the group by Lieutenant Elmer in the Admiralty, there is a large space within the reef with deepish water; although the high land does not hold a central position with respect to the reefs, as is generally the case, I have little doubt that the reefs of the Pelew Islands ought to be ranked with the barrier class, and I have coloured them pale blue. In Lieutenant Elmer’s chart there is a horseshoe-formed shoal, laid down thirteen miles N.W. of Pelew, with fifteen fathoms within the reef, and some dry banks on it; coloured dark blue.—Spanish, Martires, Sanserot, Pulo Anna and Mariere Islands are not coloured, because I know nothing about them, excepting that according to Krusenstern, the second, third, and fourth mentioned, are low, placed on coral-reefs, and therefore, perhaps, contain lagoons; but Pulo Mariere is a little higher.