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 ex- 
tensive, and of a white color. At Enderbury’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 Enderbury’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. Ee 
In the larger islands, the lagoons contain but small reefs 
compared with their whole extent; the greater part is an open 
