MODE OF ORIGIN. 133 
mountain range :—for example, the sickle-shape line of islets north 
of Nanuku. ‘Taritari and Makin, (Tarawan,) le together as if 
belonging to parts of one land. Menchicoff atoll, in the Caroline 
Archipelago, consists of three long loops or lagoon islands, united by 
their extremities, and further subsidence might reduce it to three 
islands. * 
The sizes of atolls offer no objection to these views, as they are no 
larger than many barrier reefs. Some of the larger Maldives, ac- 
cording to the crater theory, would require a crater seventy miles in 
diameter, with a rim made up of subordinate craters. No hypothesis 
of such extravagance is necessary. The facts all fall in with known 
principles, and are illustrated by known and established facts, reject- 
ing hypotheses of every kind. 
It is of some interest to follow still further the subsidence of a coral 
island, the earlier steps in which are illustrated in the preceding 
figures. One obvious result of its continuation is a gradual con- 
traction of the lagoon and diminution of the size of the atoll, owing 
to the fact already noted, that the detritus is mostly thrown inward 
by the sea. The lagoon will consequently become smaller and shal- 
lower, and the outline of the island in general more nearly circular. 
Finally the reefs of the different sides may so far approximate by 
this process, that the lagoon is gradually obliterated, and the large 
atoll is thus reduced to a small level islet, with only traces of a former 
depression about the centre. ‘Thus subsidence is connected with 
detritus accumulations in filling up the lagoon; and as filled lagoons 
are found only in the smallest islands, such as Swain’s and Jarvis, 
the two agencies have beyond doubt been generally united. 
This subsidence, if more rapid than the increase of the coral reef, 
becomes fatal to the atoll, by gradually sinking it beneath the sea. 
Of this character evidently is the Chagos Bank.t The southern 
* See Darwin on the probable disseverment of the Maldives, op. cit., p. 37, in which 
he points out indications of a breaking up of a large atoll into several smaller. A land 
with many summits or ranges of heights may at first have its single enclosing reef; but 
as it subsides, this reef contracting upon itself may encircle separately the several ranges 
of which the island consisted, and thus several atoll reefs may result in place of the large 
one; and further, each peak may finally become the basis of a separate lagoon island, 
under a certain rate of subsidence or variations in it, provided the outer reef is so broken 
as to admit the influence of waves and winds. The Maldives are a fine exemplification 
of this result. Some of the large atolls are properly atoll archipelagoes, 
+ For a detailed account of this and other submerged reefs, see Darwin, p. 106. 
34 
