242 James D. Dana, Esq.. on 



or the other, as is clone by Darwin, from the faet that the 

 reefs are small or wholly wanting, until the possible opera- 

 tion of the several causes limiting their distribution has been 

 duly considered. 



The influence of volcanoes in preventing the growth of 

 zoophytes extends only so far as the submarine action may 

 heat the water ; and it may therefore be confined within a few 

 miles of a volcanic island, or to certain parts only of its shores. 



There are three epochs of changes in elevation which m ay 

 be distinguished and separately considered : 1. The subsi- 

 dence indicated by atolls and barrier reefs ; 2. Elevations 

 during more recent periods, and also during the same epoch 

 of subsidence ; 3. Changes of level anterior to the atoll sub- 

 sidence, and the growth of recent corals. On this last point 

 we have few facts. 



t. Subsidence indicated by atolls and barrier reefs. 



In a survey of the ocean, the eye observing its numerous 

 atolls, sees in each, literally as well as poetically, a coral urn 

 upon a rocky island that lies buried beneath the waves. 

 Through the equatorial latitudes such marks of subsidence 

 abound, from the eastern Paumotu to the western Carolines, 

 a distance of about 6000 geographical miles. In the Pau- 

 motu Archipelago there are about eighty of these atolls. 

 Going westward, a little to the north of west, they are 

 found to dot the ocean at irregular intervals ; and at the 

 Tarawan Group the Carolines commence, which consist of 

 seventy or eighty atolls. 



If a line be drawn from Pitcairn's Island, the southern- 

 most of the Paumotus, by the Gambier Group, the north of 

 the Society Group, Samoa, and the Salomon Islands, to the 

 Pelews, it will form nearly a straight boundary trending N. 

 70° W., running between the atolls on one side, and the 

 high islands of the Pacific on the other, the former lying to 

 the north of the line, and the latter to the south. 



Between this boundary line and the Hawaiian Islands, an 

 area nearly two thousand miles wide and six thousand long, 

 there are two hundred and four islands, of which only three, 

 are Mgh, exclusive of the eight Marquesas. These three 



