ORIGIN OF THE BARRIER REEF. 



219 



It has been a more popular theory that the coral structures 

 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 satisfactory expla- 

 nation has been offered by Mr, Darwin, numerous objections 

 to this hypothesis have become apparent, such as the follow- 

 ing : — 



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 crater would 

 have been destroyed, wdth rare exceptions, during the subsi- 

 dence; and in the latter there is reason to beUeve 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 nearl\^ 

 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 Loa 

 and Mount Kea, of Hawaii, present a remarkable instance of 

 approximation, as they differ but two hundred feet ; but the 

 two sides of the crater of Mount Loa differ three hundred 

 and fourteen feet in height. Mount Kea, though of volcanic 

 character, has no large crater at top. Hualalai, the third 

 mountain of Hawaii, is 4,000 feet lower than Mount Loa. 

 The volcanic summit of East Maui is 10,000 feet high, and 

 contains a large crater ; but the wall of the crater on one side 

 is '700 feet lower than the highest point of the mountain ; and 

 the bottom of the crater is 2,000 feet below the rim of the 

 crater. Similar facts are presented by all volcanic regions. 



c. It further requires that there should be craters over fifty 



