148 Dr. J. D. Dana on the Origin of 



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 smallness 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 to- 

 ward either condition, and the seas bej^ond 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 funda- 

 mental 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 widening, the upper part of the submarine slopes; (3) to 

 accumulate, 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) re- 

 main 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 carried over the narrow reef to the lagoon by the 

 waves whose throw on all sides is shoreward ; (3) lose in 

 irregularity of outline, and thus approximate toward an an- 

 nular 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 Pau- 

 motri 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 



