) 



-^ 



On Coral Reefs and Islands^ 31 



Ai 



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 he 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 coral;? are growing profusely among lifeless patches of coral 

 g sand and fragments. Often the dead areas much exceed those 



flourishing with zoophytes, and not unfrequenfly the clusters are 

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

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



?f- 



level. For ten to twenty yardc from the margin, the reef ^s ^ 

 usually very cavernous or pierced with holes or sinuous recesses, I 

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

 rias, the sea-anemones, and many a [iveily moWusc] and over 

 this portion, the gigantic Chama or Tridacna is generally found t 

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

 to gape a l^le its ponderous "shell, and expose to the waters a 

 gorgeously colored mantle. Farther in are occasional pools and ^ 

 basinSj alife with all that lives in these strange coral seas.jr 



^ , f V 



