306 Daly — Pleistocene Glaciation and Coral lieef Problem. 



sidence hypothesis, though by tlie present writer's liypothesis 

 moat of the higoons wouUl tend to enlarge, rather tlian climinish 

 in area, with the process of time. 



Drowned Valleys of Oceanic Islands. — The Pleistocene 

 deepening of the inter-tropical seas is precisely of the amount 

 required to explain the drowned valleys of the volcanic islands 

 Avhich are now surrounded by barrier reefs. The following 

 list is representative of the maximum depths recorded for the 

 drowned portions of these valleys : 



Maximum depth 



charted in vjiliey 



(fathoms) 



Taliaa, Society Islands 22 



Raiatea, " " '. 19 



Murea, " " 26-41 ' 



Huaheine, " " _ 25 



Yavau, Tonga archipelago (two valleys, each) . . 30 



Mbengba, Fiji group 19 



Moala, '■' " 16 



The maximum depth charted for these drowned portions of 

 the valleys appears never to exceed 45 fathoms. 



Dana's contention that the drowned valleys indicate crustal 

 subsidence evidently needs revision, since a i-ecent swelling of 

 the equatorial waters without crustal movement gives the same 

 result. By tlie hypothesis here suggested, these valleys must 

 have been deepened during the time of heavy glaciation, when 

 their base-level was lower than now and that for a total time 

 long enough for the deepening of each valley to the extent 

 I'eqnired by the hypothesis. 



Composite Atolls of the Maldives. — Late Pleistocene drown- 

 ing will account for the existence of the many small atolls or 

 "faros"* which make up the emerged rims of the great atolls 

 of the Maldive ai'chipelago. The typical form and relation of 

 each "faro" to its main atoll fail of explanation on the Darwin- 

 Dana hypothesis. Neither can the lagoon of the main atoll 

 nor that of the bordering "faro" be explained by the doctrine 

 of an unvarying sea-leveL A study of the charts of the Mal- 

 dives shows that Murray's hypothesis of solution by sea-water 

 can hardly account for these lagoon basins. It may be recalled 

 that, at Funafuti, the lagoon water is not sensibly richer in 

 carbon dioxide (the special solvent invoked by Murray) than is 

 the water of the open ocean ; and, secondly, that the observers 

 on Funafuti have decided that its lagoon is being slowly filled 

 with shells, skeletons, and detritus, and is not increasing in 

 deptli through solution. 



* Cf . A. Agassiz, Memoirs Museum of Comp. Zoology at Harvard College, 

 vol. xxix, 1903, p. xii. 



