46 
ATOLLS. 
Ch. I. 
stand ; their outer margins are invariably bordered by 
living coral , 1 within which there is a flat surface of 
coral rock ; on this flat, sand and fragments have in 
many cases accumulated and been converted into islet s 
clothed with vegetation. They are indeed larger, and 
contain deeper lagoons than many true atolls standing 
in the open sea ; and I can point out no essential 
difference between these little ring-formed reefs and 
the most perfectly characterised atolls, excepting that 
they are based on a shallow foundation, instead of on 
the floor of the ocean, and that instead of being 
scattered irregularly, they are grouped closely together 
with the marginal rings arranged in a rudely-formed 
circle. 
The perfect series which can be traced from a linear 
reef like that surrounding an ordinary atoll, to others 
which are ring-formed and much elongated but con- 
taining only a very narrow lagoon, and to others which 
are oval or almost circular, renders it probable that the 
latter are merely modifications of a linear and normal 
reef. The fact that the marginal annular reefs 
generally have their longest axes directed in the line 
which the exterior linear reef would have held, agrees 
with this view. We may also infer that the central 
annular reefs are modifications of those irregular ones, 
which are found in the lagoons of all common atolls. 
It appears from the charts on a large scale, that the 
1 Captain Moresby informs me that Millepora complanata is one 
of the commonest kinds on the outer margin, as it is at Keeling 
atoll. 
