73 Couthouy on Coital Formations 



of the saxigenous polypes, reefs of this nature were supposed 

 to be raised by tliem from a depth Uke that found outside. 

 But later and more careful investigations of their habits, have 

 undeniably proved the incorrectness of this opinion. By the 

 concurrent testimony of all recent observers, it is now shown, 

 that instead of inhabiting such profound depths, the reef- 

 building polypes require for their development and support, a 

 certain degree of light and heat, not penetrating lower than 

 one hundred and twenty, or at the utmost, one hundred and 

 thirty feet in any part of the ocean. Some indeed have as- 

 serted less than half that depth to be the limit of growth, but 

 this can only be true of particular tracts, as I shall have oc- 

 casion to show in another place. 



Another theory, and one obtaining the sanction of some 

 distinguished names among the geologists of Europe, was 

 suggested by the circumstance of nearly all the coral islands 

 having a lagoon of variable depth in their centre. From this 

 peculiarity it was conjectured that the reefs rested upon the 

 summit of extinct sub -marine volcanoes, whose craters were 

 represented by the lagoon. 



It cannot be denied that this hypothesis presents many 

 plausible features, but still there are some knotty and stubborn 

 facts for which it fails satisfactorily to account. It is true, 

 that a knowledge of such enormous craters as those on the 

 summit of Mauna Loa in Hawaii, and Haleakala in Maui,* 

 which are estimated at twentyfour and twenty seven miles in 

 circuit, might in a measure quiet the doubts of those to whom 

 the great extent of some of these lagoons, appeared the chief 

 obstacle in the way of assuming their crateric basis. Yet 

 although it may be possible that some of the lagoon islands 



* Hawaii and iMaul are the two principal islands of the Hawaiian Group. The 

 great f rater on Mauna Loa, here epoken of, is on the very summit of the moun- 

 tain, which is little less than fourteen thousand feet high. Jt must not be con- 

 founded with that of Ka lua Pele, (.r Kilauea. spoken of by Lord Byron, Ellis, 

 t^lewart, and others. This latter is on the S. E. flank of the mountain, about four 

 thousand feet above sea level, and is at present in full activity. No signs of 

 action, other than a faint smoke, have been perceived in the terminal crater for 

 about fifteen years. The great crater of Haleakala, or " The House of the 

 Sun," also a terminal one, at an elevation of nearly eleven thousand feet, has 

 been extinct from a period beyond that reached by the traditious of the islanders. 



