158 COBALS AND CORAL ISLANDS. 



reef a thickness of at least 1,750 feet ; or with the second, 1, 150 

 feet. The island of Tahiti (taking the north side for data) 

 would give in the same manner 250 feet by the last estimate, 

 which we judge to be most correct; Upolu, by the same esti- 

 mate, 440 feet. The deduction for Upolu, may be too large : 

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

 thickness at the outer margin. The results are sufficiently ac- 

 curate 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. 



