vaughan: geologic history of coral reefs 33 



conclusion, viz: (1) The platforms have an existence independ- 

 ent of coral reefs and were formed by other than coral reef agen- 

 cies; (2) the reefs exist only on those portions of the platforms 

 where the conditions requisite for the life of reef corals prevail. 



In this connection attention is especially directed to the per- 

 sistence along the margin of continental plateaus of a rapid de- 

 clivity from about the 30-fathom curve to the 50-fathom curve 

 as compared with the usually gentler slope from the shore to 

 between 30 and 40 fathoms. This relation is well exhibited along 

 both the Australian and the North American platforms. 



Having presented criteria for recognizing the relations of con- 

 tinental and large insular platforms supporting barrier reefs to 

 the presence of the reefs, islands such as those in the Society and 

 Fiji groups may be considered. Daly in 1910 pointed out that 

 the depth of the drowned valleys in these groups ''appears never 

 to exceed 45 fathoms." The maximum depth according to his 

 compilation is from 19 (Raiatea) to 41 fathoms (Murea), both 

 members of the Society group. There is wonderful accordance 

 between the maximum depths within the barriers of the Pacific 

 Islands and the depths on the Australian continental shelf both 

 within and without the Great Barrier Reef region. A study of 

 the charts of barrier reef islands, as Viti Levu, Fijis, and Tahiti, 

 Society Island, shows that the platforms are independent of the 

 presence of reefs, and therefore the relations in these islands are 

 similar to those indicated for barriers off continental shores, for 

 here the reefs are also superimposed on platforms antedating their 

 presence. The problem of the depth of barrier platforms is a 

 world-wide one for it is only an aspect of the general problem of 

 the history of continental shelves, 



ATOLLS 



The atolls previously discussed in this paper occur on plat- 

 forms, rise from comparatively shallow depths, and owe their 

 shapes to winds and currents. The atolls that margin flat sum- 

 mits of eminences rising from oceanic depth must be attributed 

 to somewhat different causes. The greater abundance and lux- 

 uriance of reef-forming organisms on the peripheries of atolls are 



