MODE OF FORMATION. 123 
that only an inch or two in breadth of their ponderous shells are ex- 
posed to view. Without some means like this of securing their habi- 
tations, these molluscs would be destroyed by the waves. A tuft of 
byssus, however strong, which answers for some small bivalves, would 
be an imperfect security against the force of the sea for shells weigh- 
ing one to five hundred pounds. 
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 reason was 
assigned for the most striking, we may say the most surprising, 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 won- 
der of early voyagers, and the source of many speculations. The 
instinct of the polyp was made by some the subject of special admira- 
tion; for the “ helpless animalcules” 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 
their infant colonies might be safely sent forth.’’* 
It has been a more popular theory that the coral structures were 
built upon the summit 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 re- 
sult of an eruption. ‘This view was apparently supported by the vol- 
canic character of the high islands in the same seas.—But since a 
more satisfactory explanation has been offered by Mr. Darwin, nume- 
rous objections to this hypothesis have become apparent. 
a. The volcanic cones must either have been subaerial and were 
afterwards sunk beneath the waters, or else they were submarine 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 
* Flinders. 
