182 CORALS AND CORAL ISLANDS. 



is another descent to the depth which prevails over the lagoon. 

 On some small lagoons the shore is a thick plastic mud, 

 either white or brownish, and forms a low flat which is very 

 gently sloping. On Henuake, these mud deposits are quite 

 extensive, and of a white color. At Enderby's Island, another 

 having a shallow lagoon, the mud was so deep and thick that 

 there was some difficulty in reaching the waters of the lagoon ; 

 the foot sunk in eight or ten inches and was not extricated 

 without some difficulty. It looked like a dirty brownish clay. 

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

 almost impalpable. 



The lagoons of the smaller islands are usually very shallow ; 

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

 existence of water. Instances of the latter kind are met with 

 only in islands less than three miles in diameter ; and those 

 with shallow lagoons are seldom much larger. These shallow 

 waters, when direct communication with the sea is cut off, be- 

 come, in some instances, very salt by evaporation, and contain 

 no growing coral, with few signs of life of any kind; and in 

 other cases, they, are made too fresh for marine life through 

 the rains. At Enderby's Island the water was not only ex- 

 tremely 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 obstruct- 

 ed 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 

 compared with their whole extent ; the greater part is an open 



