Coral Reefs and Islands. 275 



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 *. 



b. The broader conclusion Mr. Murray does not sustain by 

 a mention of special facts from the soundings, tending directly 

 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 

 elevation ; and in this he sustains the earlier statement of my 

 report, which says (p. 293) that the broad shore-plain of 

 Tahiti probably 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. 



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

 is illustrated by the soundings, as Dr. Greikie 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 report says 40° to 50°. I have assumed for the 

 slope of the bottom, outside of the reef-limit the same angle as 

 for the surface-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 Upoluf ; and the assumption as regards Tahiti 

 is sustained by the i 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 indefinite drifting of coral-sands into 

 the deep ocean % ; and this argument the Tahiti soundings 

 sustain. 



With reference to the occurrence off some shores of precipi- 



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

 giTen illustrating the effect of widening, 

 t Page 47. % Page 154. 



