STRUCTURE OF CORAL ISLANDS. 31 



form of reef-rock at the same elevation as the shore platform 

 sometimes extends out into the lagoon ; but it is more common 

 to find it a little submerged and covered for the most part with 

 growing corals: and in either case, the bank terminates outward 

 in an abrupt descent of a few yards or fathoms, to a lower area 

 of growing corals, or a bottom of sand. Still more commonly, 

 we meet with a sandy bottom gradually deepening from the 

 shores without growing coral. These three varieties of condi- 

 tion are generally found in the same lagoon r characterizing its- 

 different parts. The lower area of growing corals slopes out- 

 ward, and usually ceases where the depth is 10 to 12 fathoms ; 

 from this there is another descent to the depth which prevails- 

 over the lagoon. 



Ou some small lagoons the shore is a thick plastic mud, either 

 white or like clay, and forms a low flat which is very gently slop- 

 ing. On Henuake, these mud deposits are quite extensive, and 

 of a white color. At Enderby's Island, another having a shal- 

 low lagoon, the mud was so deep and thick that there was 

 some difficulty in reaching the waters of the lagoon ; the foot 

 sunk in 8 or 10 inches and was not extricated without some dif- 

 ficulty. The color at this island was a dirty brownish clay. 

 This mud is nothing but comminuted coral, so fine as to be al- 

 most impalpable. 



The lagoons of the smaller islands are usually very shallow ; 

 and in some, merely a dry bed remains, indicating the former exis- 

 tence of water. Instances of the latter kind are met with only 

 in islands less than three miles in diameter ; and those with shal- 

 low lagoons are seldom much larger. These shallow waters, 

 when direct communication with the sea is cut off, become, in 

 some instances, very salt by evaporation, and contain no growing 

 coral, with few signs of life of any kind : and in other cases 7 

 they are made too fresh for marine life, through the rains. At 

 Enderby's Island the water was not only extremely saline, but 

 the shores of the lagoon were in some places incrusted with salt. 

 But when there is an open channel, or the tides gain access over 

 a bare reef, corals continue to grow, and a considerable portion 

 of the lagoon may be obstructed by them. At Henuake, the 

 sea is shut out except at high water, and there were consequently 

 but few species of corals, and those of small size. At Ahii 

 (Peacock's Island) there was a small entrance to the lagoon, and 

 though comparatively shallow, corals were growing over a large 

 part of it. 



In the larger islands, the lagoons contain but small reefs com- 

 pared with their whole extent ; the greater part is an open sea, 

 with deep waters and a sandy bottom. There are instances, as 

 at the southern Maldives, of a depth of 50 and 60 fathoms. 

 Twenty to thirty-five fathoms is the usual depth in the Paumo- 



