RECENT ORIGIN OF GREAT OCEAN DEPTHS 533 



remarkable that the best explanation for tlie fringing reefs so prevalent 

 around the shores of the Philippines is to be found in the brief passage 

 above quoted from young Darwin^s book, in which their origin was clearly 

 stated though the unconformable nature of their contact with the shore 

 rocks was not mentioned. 



In this connection it is desirable to point out that certain ocean-bottom 

 troughs of exceptionally great depth occur in close association with islands 

 of more or less disturbed history, from, which the recent deepening of the 

 troughs may be inferred. Thus a long deep trough closely adjoins the 

 Philippines on the east. Another passes next to the east of the Pelew 

 Islands, where recent tilting is indicated. A third lies next west of the 

 Solomon Islands, and a fourth lies west of the New Hebrides, in both 

 of which groups many and diverse modern changes of level are inferred. 

 A fifth lies east of the Tonga Islands and banks, where recent tilting has 

 occurred. Whatever degree of quietude may be inferred for other parts 

 of the Pacific, these areas can hardly be regarded as having long enjoyed 

 a stationary condition. 



Disappearance of Detritus from deeply eroded Islands 



conditions of reef establishment 



Eeef-building corals need a firm rock foundation for their attachment 

 and clear water free from v^ave-shifted detritus for their growth. Eeefs 

 are therefore wanting on beached shores, from which a sheet of detritus, 

 supplied by streams and waves, usually extends offshore into deep water. 

 Incipient reefs formed on a rocky shore will be destroyed if detritus is 

 spread over them by wave or current action. In the absence of reefs on 

 shores thus characterized, a cliff-backed platform will be abraded along 

 them ; and as long as no change of level takes place, the sheet of detritus, 

 which ordinarily lies on such a platform and which is shifted about at 

 time of storms, will be maintained and reef growth will be excluded. 

 Only when subsidence takes place rapidly enough to drown the valleys 

 and pocket their stream-washed waste in embayments, and to submerge 

 the cliff base so that waves beat ineffectively on the cliff face, may reef 

 growth begin, either as a fringe on the cliff face or as an offshore reef 

 which may grow up as a barrier from ledges laid bare on the margin of 

 the submerged platform, where detritus, no longer supplied, has been 

 drifted away. 



CLIFT ISLANDS IN THE CORAL SEAS 



If these principles are correct, it follows that no large reefs can be 

 formed around the shore of a lofty young volcanic island; for even if 



