GLACIAL-CONTROL THEORY OF CORAL REEFS. 213 



On the Great Chagos bank the peripheral reef crown, now covered 

 with 7 m. to 20 m. of water, carries only a small proportion of living 

 coral. Darwin suggested that this bank was formerly a normal atoll 

 and he attributed the killing of most of its corals to a very rapid 

 subsidence. To this view there are several direct objections. The 

 assumption that the species of the once living reef would be killed by a 



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Figure 33. Chart of North Danger "drowned atoll," China Sea. Scale, 

 1:144,000. Depths in fathoms. 



subsidence, even one nearly instantaneous, is not proved. If the 

 sinking took place at a rate comparable to the average rate of obserA^ed 

 epeirogenic movements elsewhere, the coral species could easily keep 

 pace with the deepening and retain their favored depth below sea-level. 

 Moreover, the subsidence would have to be incredibly uniform over a 

 vast area, to explain the nearly uniform depth of the present rim on all 

 sides of the bank. 



