J. D, Dana — Origin of Coral Reefs and Islands. 183 



food, and therefore less favorable to the growth of the more 

 massive kinds of corals, partly from the injurious effects of cal- 

 careous sediment upon coral growth there, and partly also from 

 the solvent action of the carbonic acid of the sea-water upon 

 the dead coral." 



Mr. Semper gives examples of the effects of currents at the 

 Pelew Islands, stating that by striking against or flowing by 

 the living corals they make the reef grow with steeper sides 

 and determine it's direction, and urging that abrasion and solu- 

 tion have made not only the deep lagoon-like channels, but the 

 deeper channels between the islands. He holds that in Kri- 

 angle, which he describes as a true atoll with no channel leading 

 into the lagoon from the sea, that the lagoon may have been 

 " the result of the action of currents on the porous soil 

 during a period of slow upheaval.* He says, further, that 

 the large channel in the main island of the group "forty 

 fathoms deep and many miles wide," " finds an easy explana- 

 tion on the assumption of an upheaval;" it became " wider in 

 proportion as the enclosed island consisting of soft stone [tufa] 

 was gradually eaten away, and during slow upheaval it would 

 continue to grow deeper in proportion as the old porous por- 

 tions of the reef and the rock in which it was forming were 

 more and more worn down by the combined action of boring 

 animals and plants, and of the currents produced by the tides 

 and by rain." Mr. Semper refers to the dead depressed tops 

 of some masses of Pontes near tide-level as the effects of the 

 deposit of sediment over the top of the living coral and of 

 erosion by the waves and exposure to rains while the sides 

 continued to grow ; and the fact is made an example on a very 

 small scale of atoll-making. Other examples of the action of 

 currents, sediment, boring species, and the solvent action of 

 carbonic acid in the waters, are mentioned by Mr. Agassiz, 

 in his excellent account of the " Tortugas and Florida reefs." 



a. The theory, if satisfactory, accounts not only for the 

 origin of an atoll, but for the origin of atolls of all sizes, shapes 

 and conditions, and for great numbers of them in archipelagos 

 and chains ; not only for channels through fringing reefs, like 

 those that abrasion in other cases makes, but for all the irreg- 

 ular outlines of barriers, for the great barriers reaching far 

 away from any land, and for the positions and indented coasts 

 of the small included lands. Is it a sufficient explanation of 

 the facts ? 



1. The currents that influence the structure of reefs are : (1), 

 the general movement or drift of the ocean, in some parts 

 varying with seasonal variations in the winds ; (2), the currents 



* Animal Life," pp. 269, 270. 



