194 DALY. 



of the same order as the depths computed for the Pleistocene, wave- 

 formed benches. ^^ The agreement is visible in spite of possible, 

 though necessarily slight, uplift or subsidence in the areas listed. 



The tables indicate that the new theory withstands a statistical test, 

 which is as plainly damaging to the subsidence theory. Neither 

 maximum nor general depths in atoll and barrier-reef lagoons of larger 

 size should so nearly agree if subsidence has been the essential control 

 in forming coral reefs. (See page 235 ff .) Concerning this subject a 

 few statements from those who have had special experience with reefs 

 are of moment. 



Darwin himself wrote: "The greater part of the bottom in most 

 lagoons, is formed of sediment; large spaces have exactly the same 

 depth, or the depth varies so insensibly, that it is evident that no 

 other means except aqueous deposition, could have levelled the sur- 

 face so equally." ^^ Dana remarked: "The bottom of these large 

 lagoons is very nearly uniform, varying but little except from the 

 occasional abrupt shallowing produced by growing patches of reef." ^^ 



After making many soundings in five different atolls of the Maldive 

 group, Gardiner reported that the bottom of each lagoon "was 

 found to be of an almost uniform dead-level between the reefs and 



Sections showing general flatness of lagoon floors, and their topographic 

 unconformity with the reefs; also proflles of a bank and a "drowned atoll." 



Figure 16. Suvadiva atoll, Maldive group. 



Figure 17. Kolumadulu atoll, Maldive group. 



Figure 18. Western end of Seychelles bank, Indian ocean. 



Figure 19. Macclesfield bank, China Sea, where rimless. 



Figure 20. Macclesfield bank, showing the main reef and coral knolls of 

 this "drowned atoll." 



Uniform scales; vertical scale 7 times the horizontal. Depths in fathoms. 

 Water is shown in black; rocks, including reefs and coral knolls, are lined. 



31 A rough idea of the average thickness of the clastic-organic-chemical 

 veneer may be obtained by first calculating the maximum amount of calcium 

 carbonate which has been formed in the ocean during post-Glacial time, 

 estimated as from 20,000 to 50,000 years. This maximum would include 

 calcium oxide imported by the rivers, as well as any initial excess of that oxide 

 above the total now in the ocean. That excess can only be found in the cal- 

 cium sulphate content, since the ocean is now nearly saturated with the car- 

 bonate. Considering the needs of organisms outside of the reef areas, and 

 also the small proportion of reef material that can enter a lagoon, the calcu- 

 lation shows that the calcareous veneer in each of the larger lagoons can 

 average only a few meters in thickness. A similar result is obtained by com- 

 puting the maximum annual increment of calcium carbonate on a platform, 

 assuming an extreme speed of growth for the whole living part of its reef, e. g., 

 3 cm. per annum. 



32 C. Darwin, Coral Reefs, London, 3rd ed., p. 36 (18S9). 



33 J. D. Dana, Corals and Coral Islands, New York, 3rd. ed., p. 183 (1890). 



