22 T raiisaefions. 



its margin and if tlie larger part of its discharged detritus is to be deposited 

 in the lagoon, G, of an upgrowing barrier reef, B ; but in this case the 

 early stages of subsidence must be so rapid, in order to provide sufficient 

 lagoon-space for the deposition of detritus, that the upgrowth of a reef 

 could hardly keep pace with it. It is not likely that the numerous barrier 

 reefs of to-day have all survived so threatening a danger : hence a slower 

 rate of early subsidence must be postulated. 



Let the island, therefore, stand almost or quite still during a con- 

 siderable period after its eruptive growth ceases. In this case the 

 detritus supplied by the erosion of deep valleys, CY (fig. 5), and by the 

 abrasion of high cliffs, CL, will be swept off shore in large amount, D, by 

 vigorous waves, unimjoeded by a barrier reef ; then, if intermittent sub- 

 sidence begin, placing sea-level at TE, the further discharge of detritus 

 will be detained in the embayed valleys, E, and reef-upgrowth may begin. 

 But, as under these conditions strong cliff-cutting will have accompanied 

 the erosion of deep valleys, a considerable measure of subsidence, placing 

 sea-level at UV, will be eventually necessary to submerge the cliff-toi^s, L, 

 if they are not seen to-day. Whether this supposition represents the actual 

 history of reef-encircled islands or not, it certainly provides a more reason- 

 able condition for reef-growth than any oth-er supposition here considered. 



W _ mrri nniiiiiiiniiiiliTTIliriiiiTiTlIi 



Fig. 6. 



Various combinations of diverse conditions may be imagined. For 

 example, the succession of events may be as follows : (1) Moderate cliff- 

 cutting during a still-stand period before reefs are developed ; (2) moderate 

 submergence and reef-upgrowth ; (3) a second still-stand period, resulting 

 in the smothering of reefs by outwashed detritus, and renewal of cliff- 

 cutting ; (4) further subsidence and renewed reef-growth. Tahiti seems 

 now to be approaching the third phase of this succession, for, if the present 

 still-stand that is attested by the alluvial lowland around the island border 

 endures as long as the earlier reefless period of valley- and cliff-cutting, the 

 lagoon will be overfilled, the smothered reefs will be abraded, and a new 

 attack will be made by the waves on the cliff's at a higher level than before. 



In any event, the only way of developing a barrier reef around a deeply 

 dissected and non-cliffed volcanic island seems to be either to allow it to 

 subside rapidly to a great depth while its reef is growing up, or to allow it 

 to subside to a less depth after strong cliff's have been cut around its 

 shore. And inasmuch as Reunion, Tutuila and the Marquesas, and Tahiti 

 exemplify the second of these alternatives, the first alternative is regarded 

 as the less probable of the two. 



Many more observations of reef-encircled islands are needed before the 

 questions here raised can be settled ; and the observations must evidently 

 be directed much more to the islands than to the reefs around them. The 

 various possibilities here outlined, and as many others as can be invented, 



