STRUCTURE OF CORAL REEFS. 127 



taking three degrees as the inclination, it gives 260 for the 

 thickness at the outer margin. The results are sufficiently- 

 accurate to satisfy us of the great thickness of many barrier 

 reefs. 



These calculations, however, are liable to error from many 

 sources. Very different results might generally be obtained 

 from different sides of the same island ; and the same group 

 often contains islands without reefs, and others with reefs one 

 or even several miles from the shores. But since we may show 

 that the absence of a reef, or its limited extent, may be traced 

 to some causes restricting or modifying its formation, it is ob- 

 vious that the error would probably be on the side of too low 

 an estimate. 



Adjacent to the larger islands, such as those of Vanua 

 Levu, and Australia, the error might be of the opposite kind ; 

 for the slopes of the land are of a more complex or irregular 

 character than on the smaller islands. In the latter, they may 

 be shown to belong generally to a single elevation of igneous 

 origin, or, at the most, to two or three combined ; while, in the 

 former, they may pertain to different ranges of hills or moun- 

 tains. For correct results in any instance, the land and its 

 declivities should be carefully studied beforehand, and the sys- 

 tem in its inclinations determined by observation. With re- 

 gard to Tahiti and Upolu, information bearing upon this point 

 was obtained, and the above conclusions may be received with 

 much confidence. Many of the Feejee reefs, on the same prin- 

 ciple, cannot be less than 2,000 feet in thickness. 



IX. A GOOD WORD FOR CORAL REEFS. 



All coral-bound coasts, and especially those of islands in mid 

 ocean, derive great benefit from their reefs. The wide coral banks 

 and the inclosed channels greatly enlarge the limits tributary 

 to the lands they incircle. Besides being barriers against the 

 ocean, they are dikes to detain the detritus of the hills. 

 They stop the waters of the streams, and cause it to drop the 



