DARWIN ON CORAL REEFS. 383 



of some on the coast of Brazil), and has marked them on a map 

 with colours distinguishing the different kinds. He has thus 

 illustrated his own hypothesis on the subject, namely, that in both 

 atolls and barrier-reefs, the foundation on which the coral was 

 primarily attached has subsided, and that during this downward 

 movement the reefs have grown upwards. 



In order to give as distinct an idea as possible of the nature of 

 the various kinds of coral reefs, a detailed account is given of one 

 example characteristic of each class : the one selected as illus- 

 trating the subject of lagoon islands or " atolls," is Keeling Atoll, 

 of which a reduced chart and vertical section are here appended. 



The first thing to be observed with regard to the formation of 

 these reefs, is, that the living coral animal does not endure ex- 

 posure to the rays of the sun even for a very short time ; and that 

 consequently it is only at the outer edge, below the level of low- 

 water, that the reef increases, or at least that those species to 

 which the great mass of the reef is due can exist. The most 

 abundant coral on this outer edge was found to be the Porites, 

 which forms great irregularly rounded masses from four to eight 

 feet broad, and little less in thickness ; but a species of Millepora 

 (M. complanatd) is also able to resist the fury of the waves, and 

 grows in thick vertical plates, intersecting each other at various 

 angles, forming an extremely strong honey-combed mass, which 

 generally affects a circular form, the marginal plates only being 

 alive. The corals inhabiting the protected crevices on the outer 

 reef, and those in the interior of the lagoon, are quite distinct, and 

 belong to much more delicate varieties. 



From soundings taken outside the reef at Keeling Atoll, the 

 water was found to deepen gradually for a space between 100 and 

 200 yards wide, but beyond this, the sides plunge into the 

 unfathomable ocean at an angle of 45°. To the depth of 10 

 or 12 fathoms, the bottom seems to be, invariably, living coral; 

 between that depth and 20 fathoms, about an equal number of 

 times sand and coral ; but of twenty-five soundings at a greater 

 depth than 25 fathoms, every one showed the bottom covered with 

 sand, the sand consisting of finely pounded fragments of stony 

 corals. At a distance of a mile and a quarter from the breakers 

 no bottom was found with a line 7200 feet in length, proving that 

 the submarine slope was greater than that of any volcanic cone. 



The chart and section represent a ring partly formed of small 

 islets on a coral reef, but if the part above the level (c), which is 

 just covered at high water, were removed, there would be a simple 

 reef which forms what is essentially the " atoll," and it encloses 

 the lagoon on all sides, except at the northern end, where there 

 are two open spaces. The reef varies in width from 250 to 500 

 yards ; its surface is level, or very slightly inclined towards the 

 lagoon, and the islets are first formed between 200 and 300 yards 

 from its outer edge, through the accumulation of a pile of frag- 

 ments thrown together by some unusually strong gale. They are 

 afterwards increased by the addition of fragments on their outer 



