534 W. M. DAVIS SUBSIDEXCE OF EEEF-EXCIRCLED ISLANDS 



fringing-reef patches are formed here and there on lava points, detritus 

 will be so abundantly supplied by actively outflowing streams that the 

 incipient reefs will be smothered. Wave action will thereupon set in, and 

 the island will be benched and clift as long as no change of level occurs. 

 It is thus that the scarcity of reefs is explained around the immaturely 

 dissected volcanic island of Reunion, where the retrograded cliffs have 

 well developed beaches along their base and where the non-embayed val- 

 leys are fronted by still wider beaches. The former absence of reefs and 

 the cutting of high cliffs around the immaturely dissected island of Tahiti 

 must be similarly explained ; the fringing and barrier reefs that now im- 

 perfectly encircle that island must have been formed after submergence, 

 presumably due to subsidence, had drowned the cliff-base and embayed 

 the valleys to a depth of 600 or 800 feet, as I have briefly stated elsewhere 

 (1916, a), and as is much more fully set forth in the forthcoming article 

 in the Annales de Geograpliie, above mentioned. The cliffs and reefs of 

 Tutuila may be provisionally explained in the same way, as noted above. 



REEFS AROUND DEEPLY ERODED ISLANDS 



Many other volcanic islands, much more deeply and maturely dissected 

 than Eeunion and Tahiti, are surrounded by barrier reefs; and it has 

 usually been assumed that such reefs began their growth when volcanic 

 action ceased, and continued their growth, according to Darwin's theory, 

 as the island subsided. Other theories, however, assume that reef-encir- 

 cled islands have not subsided, and that their barrier reefs have been 

 formed by the outgrowth of fringing reefs. If the physiographic devel- 

 opment of the deeply dissected islands be attentively considered, both of 

 these explanations for their reefs become untenable. Not only are the 

 embayed shorelines and the unconformable reef contacts of such islands 

 beyond explanation by all theories which assume still-standing islands, 

 but in view of the enormous volume of detritus that has been discharged 

 from the islands in the course of their deep dissection, the reefs are also 

 beyond explanation as continuous upgrowths on slowly subsiding founda- 

 tions since eruptions ceased. 



In the maturely dissected island of Huaheine, in the Society group, the 

 volume of detritus discharged during the erosion of the former volcanic 

 cone is from 50 to 70 times the volume of the lagoon inclosed by the 

 present barrier reef; in the skeleton island of Borabora of the same 

 group the ratio of discharged detritus to lagoon volume is probably greater 

 still. In Murea, near Tahiti, in the Society group, the enormous volume 

 of discharged detritus may be inferred from the depth and width of its 

 valleys, as sketched in the upper part of figure 12; the narrow lagoon. 



