228 CORALS AND CORAL ISLANDS. 



the philosophical mind of Mr. Darwin the first hint with regard 

 to the origin of the atoll ; the contrast, and, at the same time, 

 the resemblance, were striking ; the conclusion was natural 

 and most happy. 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 lagooji fo?'med by a reef 

 of coral'' Balbi, the geographer, as Mr. Darwin remarks 

 (p. 41), describes those barrier reefs which encircle islands of 

 moderate size by calling them atolls, with high lands rising 

 from their central expanse. 



As some interest is connected with the history of new prin- 

 ciples, and the illustration afforded is highly satisfactory, we 

 give here a sketch of the Gambier Group. The very features 

 of the coast of the included islands, — the deep indentations, — 

 are sufficient evidence of subsidence to one who has studied 

 the character of the Pacific islands ; for these indentations cor- 

 respond to valleys or gorges formed by denudation during a 

 long period while the island stood above the sea. 



The manner in which a further subsidence results in pro- 

 ducing the atoll is illustrated in the upper of the following 

 figures. Viewing V as the water line, the land is entirely 

 submerged ; the barrier, b"'\ b"", then incloses a broad area of 

 waters, or a lagoon, with a few island patches of reef over the 

 peaks of the mountains. A cpntinuation of the subsidence 

 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 following figure, VL, subsidence having taken place to 

 that level. The lagoon has all the characters of those of 

 atoll reefs. 



Should subsidence now cease, the reefs, no longer increasing 

 in height, would go on to widen, and the accumulations pro- 

 duced by the sea would commence the formation of dry land, 

 as exhibited in figure 2. Verdure may soon after appear, and 

 the coral island will finally be completed. It is not impossible 

 that dry land might form in certain favourable spots on the reef 



