J. D. Dana — Origin of Coral Reefs and Islands. 175 



ously been a part of the Darwinian theory ; for, as stated in 

 §10 (Part I), a fringing reef, where no subsidence is going on, 

 widens above and steepens its seaward-slope, and it could do 

 this only by the process described : that is, by building out upon 

 a base of debris, or, more correctly, upon true coral-reef rock 

 made by the gradual consolidation of the debris.* 



h. The broader conclusion Mr. Murray does not sustain by a 

 mention of special facts from the soundings, tending directl/ to 

 meet the question of change of level, but by attempting to show 

 that through the eroding action of currents and other means 

 (as had been argued by Prof. Semper), in connection with the 

 process already explained, reefs of" all kinds can be made from 

 submarine banks without aid from subsidence. 



In this place I confine myself to the question as to the fact 

 of subsidence. The only direct argument presented against 

 subsidence is contained in the statement, that the very broad 

 shore-plain of Tahiti shows that " the island has not in recent 

 times undergone subsidence," and may indicate a slight eleva- 

 tion ; and in this he sustains the earlier statement of my Eeport, 

 which says (p. 293) that the broad shore-plain of Tahiti prob- 

 ably overlies in some parts the fringing reef, and (p. 300) the 

 shore-plain, if built upon reefs, as I was assured, may afford 

 proof of a rise of one or two feet." But this admission, as I 

 have explained for other cases of local elevation, is in no way 

 opposed to the theory of subsidence. 



c. The kind of submarine slopes to be looked for off reefs is 

 illustrated by the soundings, as Dr. Geikie indicates. But it is 

 interesting to note that the facts, while very important, sustain 

 instead of correcting those announced by earlier observers. 

 Beechey and Darwin make the mean slope about 45°, and my 

 Eeport says 40° to 50°. I have assumed for the slope of the 

 bottom outside of: the reef-limit the same angle as for the sur- 

 face-slope of the island just above the water level : 5° to 8° off 

 Tahiti, of which 5° is accepted as most correct, and 3° to 5° off 

 Upolu ;f and the assumption as regards Tahiti is sustained by the 

 Challenger soundings. My Report states (from the Expedition 

 surveys) that off Upolu, the bottom " loses more and more in 

 the proportion of coral sand till we finally reach a bottom of 

 earth," and introduces this as an argument against the indefi- 

 nite drifting of coral sands into the deep ocean ;^ and this argu- 

 ment the Tahiti soundings sustain. 



With reference to the occurrence off some shores of precipi- 

 tous submarine slopes, the Challenger soundings give definite 

 facts as to one case. It leaves undisturbed the previously re. 



* My Expedition Geological Report, pp. 131, 132, where figures are given illus- 

 trating the effect of widening. 



f Ibid., page 47. % Ibid., page 154. 



