126 CORALS AND CORAL LSLANDS. 



the land, and others incircling it Uke 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 Gambler group (p. 227) 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 thousa?id 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, on the 

 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 can- 

 not be shown to have affected the submarine slopes, except- 

 ing 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 Jii'e degrees, 460 feet. Adopting the 

 first estimate, the Gambler group would give for the outer 

 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 : 



