ORIGIN OF THE BARRIER REEF. 255 



were supposed to have selected the very form best calculated 

 to withstand the violence of the waves, and apparently with 

 direct reference to the mighty forces which were to attack the 

 rising battlements. They had thrown up a breastwork as a 

 shelter to an extensive working ground under its lee, " where," 

 as Flinders observes, "their infant colonies might be safely 

 sent forth." 



It has been a more popular theory that the coral struc- 

 tures were built upon the summits of volcanoes ; — that the 

 crater of the volcano corresponded to the lagoon, and the rim 

 to the belt of land ; that the entrance to the lagoon was over 

 a break in the crater, a common result of an eruption. This 

 view was apparently supported by the volcanic character of 

 the high islands in the same seas. But since a more satisfac- 

 tory explanation has been offered by Mr. Darwin, numerous 

 objections to this hypothesis have become apparent, such as 

 the following : 



a. The volcanic cones must either have been subaerial and 

 then have afterward sunk beneath the waters, or else they 

 were submarine from the first. In the former case the cra- 

 ter would have been destroyed, with rare exceptions, during 

 the subsidence ; and in the latter there is reason to believe 

 that a distinct crater would seldom, if ever, be formed. 



b. The hypothesis, moreover, requires that the ocean's bed 

 should have been thickly planted with craters — seventy in a 

 single archipelago, — and that they should have been of nearly 

 the same elevation ; for if more than twenty fathoms below the 

 surface, corals could not grow upon them. But no records 

 warrant the supposition that such a volcanic area ever existed. 

 The volcanoes of the Andes differ from one to ten thousand 

 feet in altitude, and scarcely two cones throughout the world 

 are as nearly of the same height as here supposed. Moun 



