16 
ATOLLS. 
Ch. I. 
mulation of a pile of fragments, thrown together by 
some unusually strong gale. Their ordinary width is 
under a quarter of a mile, and then' length varies from 
a few yards to several miles. Those on the S.E. and 
windward side of the atoll, increase solely by the addi- 
tion of fragments on their outer side ; hence the loose 
blocks of coral, of which their surface is composed, as 
w'ell as the shells mingled with them, almost exclu- 
sively consist of those kinds which live on the outer 
coast. The highest part of the islets (excepting hil- 
locks of blown sand, some of which are 30 feet high), is 
close to the outer beach (E of the wood-cut) and aver- 
ages from six to ten feet above ordinary high-water 
mark. From the outer beach the surface slopes gently 
to the shores of the lagoon ; and this slope no doubt 
is due to the breakers, the further they have rolled 
over the reef, having had less power to throw up 
fragments. The little waves of the lagoon heap up 
sand and fragments of thinly-branched corals on the 
inner side of the islets on the leeward side of the atoll ; 
and these islets are broader than those to windward, 
some being even 800 } r ards in width ; but the land thus 
added is very low. The fragments beneath the surface 
are cemented into a solid mass, which is exposed as a 
ledge (D of the wood-cut), projecting some yards in 
front of the outer shore, and from two to four feet high. 
This ledge is just reached by the waves at ordinary 
high-water : it extends in front of all the islets, and 
everywhere has a water-worn and scooped appearance. 
The fragments of coral which are occasionally cast on 
