266 CORALS AND CORAL ISLANDS. 
least the close relations of the two. Captain Beechey, in his 
“ Voyage in the Pacific,” implies this resemblance, when he 
says of the Gambier group, which he surveyed, “It consists 
of five large islands and several small ones, all situated in a 

GAMBIER ISLANDS. 
lagoon, formed by a reef of coral.” Balbi, the geographer, as 
Mr. Darwin remarks, describes those barrier reefs which 
encircle islands of moderate size, by calling them atolls with 
high lands rismg from their central expanse. 
The manner in which a further subsidence results in 
producing the atoll is illustrated in the following figures. 
Viewing V as the water line, the land is entirely sub- 
merged ; the barrier (b’” b””) then encloses a broad area of 
waters, or a lagoon, with a few island patches of reef over 
the peaks of the mountains. A continuation of the subsi- 
dence would probably sink beneath the waters some of the 
islets, because of their increasing in height less rapidly than 
the barrier; and this condition is represented along the upper 
line of the above Figure VI, subsidence having taken place 
to that level. The lagoon has all the characters of those of 
atoll reefs. 
Should subsidence now diminish greatly or cease, the 
