Coral Reefs [108]
shores, and the adjoining islets, as well as the Indian promontory of MADURA.
CHAGOS, MALDIVA, AND LACCADIVE ARCHIPELAGOES.
These three great groups which have already been often noticed, are now well-known from the admirable surveys of Captain Moresby and Lieutenant Powell. The published charts, which are worthy of the most attentive examination, at once show that the CHAGOS and MALDIVA groups are entirely formed of great atolls, or lagoon-formed reefs, surmounted by islets. In the LACCADIVE group, this structure is less evident; the islets are low, not exceeding the usual height of coral-formations (see Lieutenant Wood's account, "Geographical Journal", volume vi., page 29), and most of the reefs are circular, as may be seen in the published charts; and within several of them, as I am informed by Captain Moresby, there is deepish water; these, therefore, have been coloured blue. Directly north, and almost forming part of this group, there is a long, narrow, slightly curved bank, rising out of the depths of the ocean, composed of sand, shells, and decayed coral, with from twenty-three to thirty fathoms on it. I have no doubt that it has had the same origin with the other Laccadive banks; but as it does not deepen towards the centre I have not coloured it. I might have referred to other authorities regarding these three archipelagoes; but after the publication of the charts by Captain Moresby, to whose personal kindness in giving me much information I am exceedingly indebted, it would have been superfluous.
SAHIA DE MALHA bank consists of a series of narrow banks, with from eight to sixteen fathoms on them; they are arranged in a semicircular manner, round a space about forty fathoms deep, which slopes on the S.E. quarter to unfathomable depths; they are steep-to on both sides, but more especially on the ocean-side. Hence this bank closely resembles in structure, and I may add from Captain Moresby's information in composition, the Pitt's Bank in the Chagos group; and the Pitt's Bank, must, after what has been shown of the Great Chagos Bank, be considered as a sunken, half-destroyed atoll; hence coloured blue.--CARGADOS CARAJOS BANK. Its southern portion consists of a large, curved, coral-shoal, with some low islets on its eastern edge, and likewise some on the western side, between which there is a depth of about twelve fathoms. Northward, a great bank extends. I cannot (probably owing to the want of perfect charts) refer this reef and bank to any class;--therefore not coloured.--ILE DE SABLE is a little island, lying west of C. Carajos, only some toises in height ("Voyage of the 'Favourite'," volume i., page 130); it is surrounded by reefs; but its structure is unintelligible to me. There are some small banks north of it, of which I can find no clear account.--MAURITIUS. The reefs round this island have been described in the chapter on fringing-reefs; coloured red. --RODRIGUEZ. The coral-reefs here are exceedingly extensive; in one part they project even five miles from the shore. As far as I can make out, there is no deep-water moat within them; and the sea outside does not deepen very suddenly. The outline, however, of the land appears to be ("Life of Sir J. Makintosh," volume ii., page 165) hilly and rugged. I am unable to decide whether these reefs belong to the barrier class; as seems probable from their great extension, or to the fringing class; uncoloured. --BOURBON. The greater part of the shores of this island are without reefs; but Captain Carmichael (Hooker's "Bot. Misc.") states that a portion, fifteen miles in length, on the S.E. side, is imperfectly fringed with coral reefs: I have not thought this sufficient to colour the island.
SEYCHELLES.
The rocky islands of primary formation, composing this group, rise from a very extensive and tolerably level bank, having a depth between twenty and forty fathoms. In Captain Owen's chart, and in that in the "Atlas of the Voyage of the 'Favourite'," it appears that the east side of MAHE and the adjoining islands of ST. ANNE and CERF, are
CHAGOS, MALDIVA, AND LACCADIVE ARCHIPELAGOES.
These three great groups which have already been often noticed, are now well-known from the admirable surveys of Captain Moresby and Lieutenant Powell. The published charts, which are worthy of the most attentive examination, at once show that the CHAGOS and MALDIVA groups are entirely formed of great atolls, or lagoon-formed reefs, surmounted by islets. In the LACCADIVE group, this structure is less evident; the islets are low, not exceeding the usual height of coral-formations (see Lieutenant Wood's account, "Geographical Journal", volume vi., page 29), and most of the reefs are circular, as may be seen in the published charts; and within several of them, as I am informed by Captain Moresby, there is deepish water; these, therefore, have been coloured blue. Directly north, and almost forming part of this group, there is a long, narrow, slightly curved bank, rising out of the depths of the ocean, composed of sand, shells, and decayed coral, with from twenty-three to thirty fathoms on it. I have no doubt that it has had the same origin with the other Laccadive banks; but as it does not deepen towards the centre I have not coloured it. I might have referred to other authorities regarding these three archipelagoes; but after the publication of the charts by Captain Moresby, to whose personal kindness in giving me much information I am exceedingly indebted, it would have been superfluous.
SAHIA DE MALHA bank consists of a series of narrow banks, with from eight to sixteen fathoms on them; they are arranged in a semicircular manner, round a space about forty fathoms deep, which slopes on the S.E. quarter to unfathomable depths; they are steep-to on both sides, but more especially on the ocean-side. Hence this bank closely resembles in structure, and I may add from Captain Moresby's information in composition, the Pitt's Bank in the Chagos group; and the Pitt's Bank, must, after what has been shown of the Great Chagos Bank, be considered as a sunken, half-destroyed atoll; hence coloured blue.--CARGADOS CARAJOS BANK. Its southern portion consists of a large, curved, coral-shoal, with some low islets on its eastern edge, and likewise some on the western side, between which there is a depth of about twelve fathoms. Northward, a great bank extends. I cannot (probably owing to the want of perfect charts) refer this reef and bank to any class;--therefore not coloured.--ILE DE SABLE is a little island, lying west of C. Carajos, only some toises in height ("Voyage of the 'Favourite'," volume i., page 130); it is surrounded by reefs; but its structure is unintelligible to me. There are some small banks north of it, of which I can find no clear account.--MAURITIUS. The reefs round this island have been described in the chapter on fringing-reefs; coloured red. --RODRIGUEZ. The coral-reefs here are exceedingly extensive; in one part they project even five miles from the shore. As far as I can make out, there is no deep-water moat within them; and the sea outside does not deepen very suddenly. The outline, however, of the land appears to be ("Life of Sir J. Makintosh," volume ii., page 165) hilly and rugged. I am unable to decide whether these reefs belong to the barrier class; as seems probable from their great extension, or to the fringing class; uncoloured. --BOURBON. The greater part of the shores of this island are without reefs; but Captain Carmichael (Hooker's "Bot. Misc.") states that a portion, fifteen miles in length, on the S.E. side, is imperfectly fringed with coral reefs: I have not thought this sufficient to colour the island.
SEYCHELLES.
The rocky islands of primary formation, composing this group, rise from a very extensive and tolerably level bank, having a depth between twenty and forty fathoms. In Captain Owen's chart, and in that in the "Atlas of the Voyage of the 'Favourite'," it appears that the east side of MAHE and the adjoining islands of ST. ANNE and CERF, are