ORIGIN OF CHANNELS WITHIN BARRIERS. 87 



4. Origin of the Channels within Barriers, and of the Atoll 

 Form of Coral Islands. 



In the review of causes modifying the forms of reefs, no rea- 

 son was assigned for the most striking, we may say the most sur- 

 prising, of all their features, — that they so frequently take a belt- 

 like form, and enclose a wide lagoon ; or in other cases, range 

 along, at a distance of some miles, it may be, from the land they 

 protect, with a deep sea separating them from the shores. 



This peculiar character of the coral island was naturally the 

 wonder of early voyagers, and the source of many speculations. 

 The instinct of the polyp was made by some the subject of spe- 

 cial admiration ; for the "helpless animalcules" were supposed to 

 have selected the very form best calculated to withstand the vio- 

 lence 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 work- 

 ing ground under its lee, "where their infant colonies might be 

 safely sent forth."* 



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 appar- 

 ently supported by the volcanic character of the high islands in 

 the same seas. — But since a more satisfactory explanation has 

 been offered by Mr. Darwin, numerous objections to this hypoth- 

 esis have become apparent. 



a. The volcanic cones must either have been subaerial and 

 were afterwards sunk beneath the waters, or else they were sub- 

 marine from the first. In the former case the crater 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 sin- 

 gle archipelago, — and 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 volca- 

 noes of the Andes differ from one to ten thousand feet in altitude, 

 and scarcely two cones throughout the world are as nearly of the 



* Flinders. 



