第31章

In this chapter I will give all the facts which I have collected, relating to the distribution of coral-reefs,--to the conditions favourable to their increase,--to the rate of their growth,--and to the depth at which they are formed.

These subjects have an important bearing on the theory of the origin of the different classes of coral-reefs.

SECTION 4.I.--ON THE DISTRIBUTION OF CORAL-REEFS, AND ON THE CONDITIONS FAVOURABLE TO THEIR INCREASE.

With regard to the limits of latitude, over which coral-reefs extend, I have nothing new to add. The Bermuda Islands, in 32 deg 15' N., is the point furthest removed from the equator, in which they appear to exist; and it has been suggested that their extension so far northward in this instance is owing to the warmth of the Gulf Stream. In the Pacific, the Loo Choo Islands, in latitude 27 deg N., have reefs on their shores, and there is an atoll in 28 deg 30', situated N.W. of the Sandwich Archipelago.

In the Red Sea there are coral-reefs in latitude 30 deg. In the southern hemisphere coral-reefs do not extend so far from the equatorial sea. In the Southern Pacific there are only a few reefs beyond the line of the tropics, but Houtmans Abrolhos, on the western shores of Australia in latitude 29 deg S., are of coral formation.

The proximity of volcanic land, owing to the lime generally evolved from it, has been thought to be favourable to the increase of coral-reefs.

There is, however, not much foundation for this view; for nowhere are coral-reefs more extensive than on the shores of New Caledonia, and of north-eastern Australia, which consist of primary formations; and in the largest groups of atolls, namely the Maldiva, Chagos, Marshall, Gilbert, and Low Archipelagoes, there is no volcanic or other kind of rock, excepting that formed of coral.

The entire absence of coral-reefs in certain large areas within the tropical seas, is a remarkable fact. Thus no coral-reefs were observed, during the surveying voyages of the "Beagle" and her tender on the west coast of South America south of the equator, or round the Galapagos Islands. It appears, also, that there are none (I have been informed that this is the case, by Lieutenant Ryder, R.N., and others who have had ample opportunities for observation.) north of the equator; Mr. Lloyd, who surveyed the Isthmus of Panama, remarked to me, that although he had seen corals living in the Bay of Panama, yet he had never observed any reefs formed by them. I at first attributed this absence of reefs on the coasts of Peru and of the Galapagos Islands (The mean temperature of the surface sea from observations made by the direction of Captain Fitzroy on the shores of the Galapagos Islands, between the 16th of September and the 20th of October, 1835, was 68 deg Fahr. The lowest temperature observed was 58.5 deg at the south-west end of Albemarle Island; and on the west coast of this island, it was several times 62 deg and 63 deg. The mean temperature of the sea in the Low Archipelago of atolls, and near Tahiti, from similar observations made on board the "Beagle", was (although further from the equator) 77.5 deg, the lowest any day being 76.5 deg. Therefore we have here a difference of 9.5 deg in mean temperature, and 18 deg in extremes; a difference doubtless quite sufficient to affect the distribution of organic beings in the two areas.), to the coldness of the currents from the south, but the Gulf of Panama is one of the hottest pelagic districts in the world. (Humboldt's "Personal Narrative," volume vii., page 434.) In the central parts of the Pacific there are islands entirely free from reefs; in some few of these cases I have thought that this was owing to recent volcanic action; but the existence of reefs round the greater part of Hawaii, one of the Sandwich Islands, shows that recent volcanic action does not necessarily prevent their growth.

In the last chapter I stated that the bottom of the sea round some islands is thickly coated with living corals, which nevertheless do not form reefs, either from insufficient growth, or from the species not being adapted to contend with the breaking waves.

I have been assured by several people, that there are no coral-reefs on the west coast of Africa (It might be concluded, from a paper by Captain Owen ("Geographical Journal", volume ii., page 89), that the reefs off Cape St.

Anne and the Sherboro' Islands were of coral, although the author states that they are not purely coralline. But I have been assured by Lieutenant Holland, R.N., that these reefs are not of coral, or at least that they do not at all resemble those in the West Indies.), or round the islands in the Gulf of Guinea. This perhaps may be attributed, in part, to the sediment brought down by the many rivers debouching on that coast, and to the extensive mud-banks, which line great part of it. But the islands of St.

Helena, Ascension, the Cape Verdes, St. Paul's, and Fernando Noronha, are, also, entirely without reefs, although they lie far out at sea, are composed of the same ancient volcanic rocks, and have the same general form, with those islands in the Pacific, the shores of which are surrounded by gigantic walls of coral-rock. With the exception of Bermuda, there is not a single coral-reef in the central expanse of the Atlantic Ocean. It will, perhaps, be suggested that the quantity of carbonate of lime in different parts of the sea, may regulate the presence of reefs. But this cannot be the case, for at Ascension, the waves charged to excess precipitate a thick layer of calcareous matter on the tidal rocks; and at St. Jago, in the Cape Verdes, carbonate of lime not only is abundant on the shores, but it forms the chief part of some upraised post-tertiary strata.