STRUCTURE OF CORAL REEFS. 19 



'C. Thickness of reefs. 



We have considered in the preceding pages the peculiarities of 

 form and structure characterizing the reef formations bordering 

 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. 



It will be shown in another part of this volume, that the moun- 

 tain slopes of the islands of the Pacific, except when increase 

 ed by degrading agents, cannot be assumed to exceed 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 de- 

 grees. Throughout the Pacific, the steeper slopes of the moun- 

 tains 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 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 Jive 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. 



