ORIGIN OF THE BARRIER REEF 259 
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. Mount 
