On Coral Reefs and Islands. — 371 
e. Thickness of reefs. : 
We have considered in the preceding pages the peculiarities of 
form and structure characterizing the regs formations borderin 
islands and continents, and their influence upon the enclosed land. 
Could we raise one of these coral-bound islands from the waves, 
we should find that the reefs stand upon the submarine slopes, 
like massy structures of artificial masonry ; some forming a broad 
flat platform or shelf ranging around the land, and others encircling 
it like vast ramparts, perhaps a hundred miles or more in circuit. 
The reefs that were near the water-line of the coast would be 
seen to have stood in the shallowest water, while the outer ram- 
parts rested on the more deeply submerged slopes. Indeed, it is 
obvious that with a given slope to 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, therefore, 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 observation, 
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 isl- 
" ands, and he arrives at the conclusion, as his figures indicate, 
.. that some coral reefs, at their outer limits, are at least two thou- 
sand feet in thickness. 
Assuming eight 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 
nelination, it gives 260 for the thickness at the outer.margin. 
results are sufficiently accurate to satisfy us of the great 
of many barrier reels. ‘ae bee te, 
4 eae Ee - 2 : 
