SWALLOW SHOAL, according to Horsburgh (volume ii., page 431) "is formed, LIKE MOST of the shoals hereabouts, of a belt of coral-rocks, "with a basin of deep water within."--HALF-MOON SHOAL has a similar structure; Captain D.
Ross describes it, as a narrow belt of coral-rock, "with a basin of deep water in the centre," and deep sea close outside.--BOMBAY SHOAL appears (Horsburgh, volume ii., page 432) "to be a basin of smooth water surrounded by breakers." These three shoals I have coloured blue.--The PARAQUAS SHOALS are of a circular form, with deep gaps running through them; not coloured.--A bank gradually shoaling to the depth of thirty fathoms, extends to a distance of about twenty miles from the northern part of BORNEO, and to thirty miles from the northern part of PALAWAN. Near the land this bank appears tolerably free from danger, but a little further out it is thickly studded with coral-shoals, which do not generally rise quite to the surface; some of them are very steep-to, and others have a fringe of shoal-water round them. I should have thought that these shoals had level surfaces, had it not been for the statement made by Horsburgh "that most of the shoals hereabouts are formed of a belt of coral." But, perhaps that expression was more particularly applied to the shoals further in the offing. If these reefs of coral have a lagoon-like structure, they should have been coloured blue, and they would have formed an imperfect barrier in front of Palawan and the northern part of Borneo. But, as the water is not very deep, these reefs may have grown up from inequalities on the bank: I have not coloured them.--The coast of CHINA, TONQUIN, and COCHIN-CHINA, forming the western boundary of the China Sea, appear to be without reefs: with regard to the two last-mentioned coasts, I speak after examining the charts on a large scale in the "Atlas of the Voyage of the 'Favourite'."
INDIAN OCEAN.
SOUTH KEELING atoll has been specially described. Nine miles north of it lies North Keeling, a very small atoll, surveyed by the "Beagle," the lagoon of which is dry at low water.--CHRISTMAS Island, lying to the east, is a high island, without, as I have been informed by a person who passed it, any reefs at all.--CEYLON: a space about eighty miles in length of the south-western and southern shores of these islands has been described by Mr. Twynam ("Naut. Mag." 1836, pages 365 and 518); parts of this space appear to be very regularly fringed by coral-reefs, which extend from a quarter to half a mile from the shore. These reefs are in places breached, and afford safe anchorage for the small trading craft. Outside, the sea gradually deepens; there is forty fathoms about six miles off shore: this part I have coloured red. In the published charts of Ceylon there appear to be fringing-reefs in several parts of the south-eastern shores, which I have also coloured red.--At Venloos Bay the shore is likewise fringed.
North of Trincomalee there are also reefs of the same kind. The sea off the northern part of Ceylon is exceedingly shallow; and therefore I have not coloured the reefs which fringe portions of its 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.
