AfOLLS. 289 



extends upwards of 700 miles ; and Disappointment Island and 

 Duff's Group, are connected by a coral reef of 600 miles length, 

 over which the natives pass from one island to another. 



Coral reefs are divided into three great classes, namely Atolls, 

 Barrier, and Fringing reefs. The word atoll is the name given 

 by the natives to the circular islands enclosing a lagoon, or still 

 water in the centre. This is the most usual form of the coral 

 islands, and fails not to strike the attention of every one who has 

 crossed the Pacific. They occur of all s'izes ; of thirty-two ex- 

 amined by Capt. Beechy, the largest was thirty miles in diameter, 

 and the smallest less than a mile, they were of various shapes 

 and all but one, formed by living corals. This one had been 

 raised from the water about eighty feet, but was of coral forma- 

 tifen, and was encircled by a reef of living corals. All were slowly 

 increasing their size, and twenty-nine of them had lagoons in the 

 centre, which had probably existed in the others, until, in the 

 course of time, they were filled by the labors of the zoophytes, 

 and other substances. It was supposed by the earlier voyagers 

 that the coral-building animals instinctively built in the form of 

 great circles to protect themselves from the fury of the waters. 

 So far however, from this being the case, we have seen that those 

 massive kinds upon whose existence and increase, the reef de- 

 pends, flourish beat among the breakers on the outside of the 

 reef. Another and more probable theory, is that advocated by 

 Mr. Lyell, that they are based upon the crests of submarine cra- 

 ters, and this idea receives confirmation from the steep angle at 

 which the island plunges at all sides into the surrounding ocean, 

 and that every island yet examined in the immense region called 

 Eastern Oceanica, consists of volcanic rocks, or coral limestones. 

 In opposition to this opinion it is very plausibly argued by Mr. 

 Darwin, that the form and size of some, and the number, prox- 

 imity, and relative positions of others, are incompatible with this 

 theory. Thus, Suadiva atoll is 44 geographical miles in diame- 

 ter in one direction, and 34 in another; Rumsky atoll is 54 by 20 

 miles across; Bow atoll is 30 miles long, but only six in width. 

 Another theory, proposed by Chamisso, accounts for the circular 

 form of coral islands upon the well known fact, that the corals 



