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 may 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 Plawaii, 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 



