On Coral Reefs and Islands. - [ee 
Art. XIV.—On Coral Reefs and Islands; by James D. Dana. 
Part Sixth, 
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 
safely sent’ forth.’* ms 7 ag 
It has been a more popular theory that the coral structures 
were built upon the summits of volcanoes j—that the crater 01 
the voleano 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 
same seas.—But since a more: satisfactory explanation has 
been offered by Mr. Darwin, numerous objections to this hypoth- 
esis have become apparent. 9° ites et 
a. The’ volcanic cones must either have been subaerial and 
were afterwards sunk beneath the waters, or else they were su 
marine from the first. Inthe former ease 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. 
. The hypothesis, moreover, requires that the ocean’s bed 
should have been thickly planted with eraters—seventy in a sin- 
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 
mt ae 
SEOUL ae 
Srconp Srntes, Vol. XIII, No. 38.—March, 1852. 24 
