240 ANNUAL OF SCIENTIFIC DISCOVERY. 



inner reef of Tongatabu were 25 feet in diameter ; and Astreas and 

 Meandririas, both there and in the Feejees, measured 12 to 15 feet." 



Sometimes the barrier reef recedes from the shore, and forms wide 

 channels or inland seas where ships find ample room and depth of 

 water, exposed, however, to the danger of hidden reefs. The reef on 

 the northeast coast of New Holland and New Caledonia extends 400 

 miles, at a distance varying from 30 to 60 miles from shore, and hav- 

 ing as many fathoms of depth in the channel. West of the large 

 Feejee Islands the channel is in some parts 25 miles wide, and 12 to 

 40 fathoms in depth. The sloop-of-war Peacock sailed along the 

 west coast of both Viti Lebu and Vanua Lebu, within the inner reefs, 

 a distance exceeding 200 miles. 



A barrier reef, inclosing a lagoon, is the general formation of the 

 coral islands, though there are some of small size in which the lagoon 

 is wanting. These are found in all stages of development ; in some 

 the reef is narrow and broken, forming a succession of narrow islets 

 with openings into the lagoon ; in others there only remains a depres- 

 sion of surface in the centre to indicate where the lagoon originally 

 was. The most beautiful are those where the lagoon is completely 

 inclosed, and a quiet lake rests within. 



These islands evidently rise abruptly out of the unfathomable depths 

 of the ocean, for, in speaking of one of them, Mr. Dana says, " Seven 

 miles east of Clermont Tonnere, the lead ran out to 1,145 fathoms 

 (6,870 feet) without reaching bottom. Within three quarters of a 

 mile of the southern point of this island, the lead, at another throw, 

 after running out for a while, brought up an instant at 350 fathoms, 

 and then dropped off again and descended to 600 fathoms without 

 reaching bottom." Several similar soundings are recorded by Mr. 

 Dana. 



Another peculiarity of them is the small amount they present of 

 habitable surface. They are but narrow and often interrupted borders, 

 just cutting out a certain part of the ocean. In the Marshall Islands 

 the dry land is not more than the one hundredth part of the whole ; 

 and in the Pescadores the proportion of land to the whole area is about 

 1 to 200. The lagoons are generally shallow, though in the larger 

 islands soundings gave 20 to 35, and even 50 and 60 fathoms. 



Mr. Dana gives full descriptions of the various species of coral 

 zoophytes, their mode and probable time of growth, &c., most of 

 which present few facts susceptible of condensation. One error, how- 

 ever, it may be well to insert his correction of. The coral is not 

 built by the polypi, but is simply the natural secretion which belongs 

 to them, as the shell of the oyster does to it. It is not, however, a 

 shell for defence into which the animal withdraws itself, it being 

 formed entirely within its living and fleshy part. There are polypi 

 which secrete no lime or coral, in every other respect similar to those 

 which do. They grow upon rocks, and are provided with tentacula to 

 secure their food. They increase by buds which shoot out from their 

 sides. In coral formations the buds spread out so thickly as to stop 

 the life within, and hence, as the process goes on, all is dead mass, 

 except just at the surface. The most extensive family of these zoo- 



