HO CORAL AND CORAL LSLANDS. 



shallow water for a distance of six or seven miles ; the water 

 deepened to ten or eleven fathoms the first mile, fifteen the 

 second, and at the last throw of the lead there were still but 

 twenty-five-fathoms. Christmas Island affords on its western 

 side another example of gradually deepening waters. Yet 

 these shallow waters terminate finally in a rapid declivity of 

 forty or fifty degrees. 



Off the prominent angles of an atoll, soundings generally 

 continue much beyond the distance elsewhere, as was first 

 observed by Beechey. At Washington Island, mostly abrupt 

 in its shores, there is a bank, according to the surveys of the 

 Expedition, extending from the east point to a distance of half 

 a mile, and another on the. west extending to a distance of 

 nearly two 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, though they are seldom 

 as remarkable ; yet nearly all islands, especially if the points 

 are much prominent, afford similar facts. The Florida reefs, 

 according to Prof. Agassiz, have a gradual slope to seaward 

 instead of the abruptness of the Bahamas. As corals may 

 grow on submerged land of any form, there is no reason why 

 the bottom around should not often deepen gradually. It has 

 been said that the reef to leeward is generally less abrupt than 

 that to windward, but facts thus far obtained are not sufficiently 

 definite or extensive to settle this question. It is probably true, 

 yet the difference, if any-, must be small. 



III. STRUCTURE OF CORAL ISLANDS. 



The descriptions of reefs and their islets already given apply 

 with equal force to coral islands. By transferring here the 

 statements respecting 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, and the action of the waves, of oceanic currents, 

 and of the winds. This resemblance will be rendered more 



