STRUCTURE OF CORAL ISLANDS. 157 
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 ramparts 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 nay be directly determined, as it would be 
twice as great two hundred feet from the shore as at one hun- 
dred feet. The only difficulty, therefore, in correctly determin- 
ing 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 thick- 
ness of the reefs of the Gambier Group (p. 265) 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 
two thousand feet in thickness. 
The mountain slopes of the islands of the Pacific, except 
when increased by degrading agents, do not exceed in angle 
twelve or fourteen degrees, and they are often but half this 
amount. The slopes of Mauna Kea and Mauna Loa, isl- 
and 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 can- 
not be shown to have affected the submarine slopes, excepting 
in cases of disruption of islands by forces below. 
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 
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