152 Daly — The C oral-Reef Zone. 



of the island's radius, while the length of the shore-line 

 increases directly as the radius. 



d. Exceptionally heavy rainfall, inducing rapid 

 stream erosion, accelerates the delivery of detritus to the 

 shore belt. 



e. The members of the Siboga expedition found that 

 coral growth was prevented on a large bank, south of 

 Saleyer island, by the smothering action of loose debris 

 of algae, especially Halimeda. 16 Possibly the same kind 

 of inhibition affects some tropical islands. 



/. Other, as yet unsuspected reasons for the local 

 failure of coral growth may work singly or in cooperation 

 with the known causes. 



New Caledonia, Hawaii, Tahiti, Reunion, and Tutuila — 

 examples of strongly clifTed islands — show a combination 

 of factors noted in the foregoing list. In particular, 

 they are characterized by high altitude; long, steep 

 slopes; and relatively large areas. 17 These features 

 doubtless characterized all five islands since the begin- 

 ning of the Glacial period. Only recently were more 

 favorable conditions established, with the last rise of sea- 

 level, drowning Pleistocene valleys, re-drowning some 

 pre- Glacial valleys, and trapping much land detritus in 

 the bay deltas. As already observed, Pleistocene grada- 

 tion of the offshore shelves had also prepared the condi- 

 tion for special purity of the shore waters at present 

 sea-level. Moreover, the shelves around Tahiti and 

 Tutuila could not become the sites of stable barrier reefs 

 until the shelves had been sufficiently broadened. They 

 were broader at the close of the last glacial stage than 

 ever before. 



Thus, there seem to be good grounds for interpreting 

 the exceptional, clifTed islands of the tropical zone with- 

 out abandoning the hypothesis of some oceanic chilling 

 and inhibition of reef growth during the Glacial period. 

 Nor is it necessary to assume, in each case, a rapid, 

 strong, Recent subsidence of the island. On the other 

 hand, the boring made under Mayer's direction on the 

 Tutuila reef discovered volcanic rock at a depth appro- 

 priate to the Glacial-control theory. 



w A. Weber and M. Foslie, Siboga-Expeditie, Corallinaceae, Leiden, p. 6, 

 1904. 



17 Obviously the argument does not imply that all small islands should be 

 reefed, nor that all large islands should be reefless. 



