Sect. II. 
ATOLLS. 
29 
oceans. The islets are first formed some way back 
either on the projecting points of the reef, especially if 
its form be angular, or on the sides of the main en- 
trances into the lagoon — that is in both cases, on points 
where the breakers can act during gales of wind in 
somewhat different directions, so that the matter 
thrown up from one side may accumulate against that 
before thrown up from another. In Lutke’s chart of 
the Caroline atolls, we see many instances of the former 
case ; and the occurrence of islets, as if placed for 
beacons, on the points where there is a gateway or 
breach through the reef, has been noticed by several 
authors. There are some atoll-formed reefs, rising to 
the surface of the sea and partly dry at low water, on 
which from some cause islets have never been formed ; 
and there are others, on which they have been formed, 
but have subsequently been worn away. In atolls of 
small dimensions the islets frequently become united 
into a single horse-shoe or ring- formed strip ; but 
Diego Garcia, although an atoll of considerable size, 
being thirteen miles and a half in length, has its 
lagoon entirely surrounded, except at the northern end, 
by a belt of land, on an average a third of a mile in 
width. To show how small the total area of the annu- 
lar reef and the land is in islands of this class, I may 
quote a remark from the voyage of Lutke, namely, that 
if the forty-three rings, or atolls, in the Caroline Archi- 
pelago were put one within another, and over a steeple 
in the centre of St. Petersburg, the whole would not 
cover that city and its suburbs. 
