FRINGING AND BARRIER REEFS. A7 
the declivity of the land, the thickness of the reef resting upon it may 
be directly determined, as it would be twice as great two hundred 
feet from the shore as at one hundred feet. The only difficulty, there- 
fore, in correctly determining the depth or thickness of any given 
reef, arises from the uncertainty with regard to the submarine slope 
of the land. It is, however, admitted as the result of extensive ob- 
servation, that in general, these slopes correspond nearly with those of 
the land above water. Mr. Darwin has thus estimated the thickness 
of the reefs of the Gambier Group and some other Pacific islands, 
and he arrives at the conclusion, as his figures indicate, that some 
coral reefs, at their outer limits, are at least tivo thousand feet in 
thickness. 
It will be shown in another part of this volume, that the mountain 
declivities of the islands of the Pacific, except when increased by 
degrading agents, cannot be assumed as above twelve or fourteen 
degrees, and they are often but half this amount. The slopes of 
Mauna Kea and Mauna Loa, island of Hawaii, do not average over 
eight degrees. On the north side of Upolu, where the reefs are wide, 
the inclination is from three to six degrees. ‘Throughout the Pacific, 
the steeper slopes of the mountains are due to agencies which cannot 
be shown to have affected the submarine slopes, excepting in cases of 
disruption of islands by forces below. 
Assuming ezght degrees as the mean inclination, we should have 
for the depth of reef, (or water,) one mile from the shore, 740 feet ; or 
assuming five degrees, 460 feet. Adopting the first estimate, the 
Gambier Group would give for the outer reef a thickness of at least 
1750 feet; or with the second, 1150 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 estimate, 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 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 obvious that the errors would be probably on the side of too low 
