26 ON CORAL REEFS ANI> ISLANDS. 



miles. At Kuria, one of the Kingsmills, soundings continue for 

 three miles from the north extremity, along a bank stretching off 

 from this point to the north-northwest. Many other instances 

 might be cited, but they are seldom as remarkable ; yet nearly all 

 islands, especially if the points are much prominent, afford simi- 

 lar facts. It has been said that the reef to leeward is generally 

 less abrupt than that to windward, but no facts were obtained by 

 the Expedition sufficiently definite or extensive to settle this 

 question. It is probably true, yet the difference if any must be 

 slight. 



B. Structure of Coral Islands. 



The descriptions of reefs and their islets apply with equal 

 force to coral islands. By transferring here the statements re- 

 specting the former, we should have a nearly complete account 

 of the latter. The same causes, with scarcely an exception, are 

 at work : — the growing of coral-zoophytes, the action of the 

 waves, oceanic currents, and the winds. This resemblance will 

 be rendered more apparent by a review of their characters ; the 

 description will be found to be a simple recapitulation of a for- 

 mer paragraph. 



The reef of the coral atoll, as it lies at the surface still uncov- 

 ered with vegetation, is a platform of coral rock, usually two to 

 four hundred yards wide, and situated so low as to be swept by 

 the waves at high tide. The outer edge, directly exposed to the 

 surf, is generally broken into channels and jagged indentations, 

 along which the waters of the resurging wave drive with great 

 force. Though in the midst of the breakers, the edge stands a 

 few inches, and sometimes a foot, above other parts of the plat- 

 form ; the incrusting Nullipores cover it with varied tints, and 

 afford protection from the abrading action of the waves. There 

 are usually three to five fathoms water near the margin ; and 

 below, over the bottom which gradually deepens outward, beds 

 of corals are growing profusely among lifeless patches of coral 

 sand and fragments. Often the dead areas much exceed those 

 flourishing with zoophytes, and not unfrequently the clusters are 

 scattered like tufts of vegetation in a sandy plain. The growing 

 corals extend up the sloping edge of the reef, nearly to low tide 

 level. For ten to twenty yards, from the margin, the reef is 

 usually very cavernous or pierced with holes or sinuous recesses, 

 a hiding-place for various crabs, or a retreat for the echini, aste- 

 rias, the sea-anemones, and many a pretty mollusc ; and over 

 this portion, the gigantic Chama or Tridacna is generally found 

 lying more than half buried in the solid rock, with barely room 

 to gape a little its ponderous shell, and expose to the waters a 

 gorgeously colored mantle. Farther in are occasional pools and 

 basins, alive with all that lives in these strange coral seas. 



