Chapter 4

CHAPTER IV.--ON THE DISTRIBUTION AND GROWTH OF CORAL-REEFS.

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