J. D. Dana — Origin of Coral Reefs and Islands. 93 



being in many cases two to three times greater than the limit- 

 ing depth (120 feet) of living reef-making corals. 



8. The great depth of the ocean in near vicinity to the atolls 

 is another source of evidence added. 



9. He urged also, in supporting his views, the non-existence 

 in the ocean now, and the extreme improbability of existence 

 at any time, of submarine volcanoes or chains of mountains 

 having their numerous summits within a hundred feet of the 

 surface. 



10. Darwin speaks of small ness of size in coral islands as 

 a result of continued subsidence. In my Report I base an 

 argument for subsidence on smallness in the proportion of dry 

 land, and on smallness of size, when there is gradation toward 

 either condition, and the seas beyond are free of islands. The 

 facts bear on the general conclusion with regard to a Central- 

 Pacific area of subsidence as well as on the fundamental point 

 in the theory. 



If an atoll-reef is not undergoing subsidence the coral and 

 shell material produced that is not lost by currents serves: (1) to 

 widen the reef ; (2) to steepen, as a consequence of the widen- 

 ing, the upper part of the submarine slopes ; (3) to accumu- 

 late, on the reef, material for beaches and dry land ; and (4) to 

 fill the lagoon. 



But if, while subsidence is in progress, the contributions 

 from corals and shells exceed not greatly or feebly the loss by 

 subsidence and current waste, the atoll-reef, unable to supply 

 sufficient debris to raise the reef above tide level by making 

 beaches and dry land accumulations, would (1) remain mostly 

 a bare tide- washed reef; (2) lose in diameter or size because 

 the debris that is not used to keep the reef at tide-level is car- 

 ried over the narrow reef to the lagoon by the waves whose 

 throw on all sides is shoreward ; (3) lose in irregularity of out- 

 line and thus approximate toward an annular form ; (4) lose 

 the channels through the reef into the lagoon by the growth of 

 corals and by consolidating debris; and (5) become at last a 

 small bank of reef-rock with a half obliterated lagoon-basin. 



The Pacific contains reefs of the three kinds : (1) atolls with 

 much of the reef under trees and shrubbery ; (2) others, of 

 large and small size, with the reefs mostly or wholly tide- 

 washed ; (3) others only two or three square miles in area, 

 without lagoons. Further, the kinds are generally grouped 

 separately and gradationally. (1) The islands of the Paumotu 

 and Gilbert archipelagos have usually half or more of the 

 reef dry and green ; (2) the northern Carolines and the 

 northern Marshall Islands, and the eastern Feejees, although 

 in part of large size, are mostly bare reefs ; while (3) the islands 

 of the Phoenix Group, of the equatorial Pacific east of the line 



