Chapter 4 - Page 2

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. The apparently capricious distribution, therefore, of