158 CORALS 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. j 
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. 
