14 
ATOLLS. 
Cu. L 
cumstanced. Hence the layer composed by their growth, 
merely fringes the reef for a space of about 20 yards in 
width, either under the form of separate mammillated 
projections, where the outer masses of coral are separate, 
or more commonly, where the corals are united into a 
solid margin, as a continuous smooth convex mound 
(B in wood-cut) like an artificial breakwater. Both 
the mound and mammillated projections stand about 
three feet higher than any other part of the reef, by 
which term I do not include the islets, formed by the 
accumulation of rolled fragments. We shall hereafter 
see that other coral reefs are protected by a similar 
thick growth of Nulliporse on the outer margin, the part 
most exposed to the breakers, and this must effectually 
aid in preserving it from being worn down. 
The wood-cut (at p. 8) represents a section across 
one of the islets on the reef, but if all that part which 
is above the level of G were removed, the section 
would be that of the reef, as it occurs where islets 
have not been formed. It is this reef which essen- 
tially forms the atoll. In Keeling atoll the ring 
encloses the lagoon on all sides except at the northern 
end, where there are two open spaces, through one 
of which ships can enter. The reef varies in width 
from 250 to 500 yards ; its surface is level, or very 
slightly inclined towards the lagoon, and at high-tide 
the sea breaks entirely over it : the water at low tide 
thrown by the breakers on the reef, is carried by the 
many narrow and shoal gullies or channels on its sur- 
face, into the lagoon : a return stream sets out of the 
