MODE OF ORIGIN. 131 
of new principles, and the illustration afforded is highly satisfactory, 
we have given a sketch of the Gambier Group. The very features of 
the land, the deep indentations, are sufficient evidence of subsidence 
to one who has studied the character of the Pacific islands :* for 
these indentations correspond to valleys or gorges formed by denu- 
dation, during a long period, while the island stood above the sea. 
The manner in which a farther subsidence results in producing 
the atoll, may be illustrated by the following figure. Viewing V, 
as the water line, the land is entirely submerged; the barrier, 0”, 
Pigs 5: 
6’, is an annular reef, enclosing a broad area of waters, or 
lagoon, with a few island-patches of reef, over the peaks of the 
mountains.t At a still greater subsidence, (to the line VI,) the 
Fig. 6. 
islets, excepting one, have. disappeared, owing to their increasing 
less rapidly than the barrier. ‘The lagoon is in exact correspon- 
several small ones, all situated in a lagoon formed by a reef of coral.” —p. 120, Amer. ed. 
Balbi, the geographer, as Mr. Darwin remarks, describes those barrier reefs which 
encircle islands of moderate size, by calling them atolls, with high lands rising from their 
central expanse.—Darwin, op. cit. p. 41. 
* This subject is discussed in the chapter on the valleys of the Pacific islands. 
+ As the lagoon islets cover the summits of the subsided mountains, they afford the 
most favourable spots for reaching the rocks below by boring. In the figure above 
given, the depth required for this purpose on an islet in the lagoon weuld be hardly a 
fourth what would be necessary on the enclosing reef, 
