1T0 B. A. Daly— Problem's of the Pacific Islands. 



The same process is imagined for islands, great and small, 

 which represent the high parts of non-volcanic mountains, 

 recently sunk beneath the sea. 



As in Dana's corresponding diagram, Darwin's initial island 

 is drawn small and very steep-sided. In order to produce one 

 of the larger atolls by subsidence, a large island with more 

 normal slopes must be assumed. A good type is Kauai of the 

 Hawaiian group. Fig. 3a is a north-south section of this 

 eroded mass of volcanic rock. The section is about twenty 

 miles long. The highest point is nearly one mile above sea- 

 level. The positions of fringing reefs are shown. If Kauai 

 slowly sinks, the reef grows up and outward, on its own talus 

 (marked T), passing through the barrier stage of fig. 3b into 



Fig. 4. 



w. 



I 



^^^^m 



44 45 43 45 



L±^ 



Fig. 4. East-west section of Suvadiva atoll (southern Maldives of the 

 Indian ocean). Water shown in solid black. The whole section is 33 nau- 

 tical miles in length ; vertical scale is seven times the horizontal. Depths 

 in fathoms. 



the atoll stage of fig. 3c. The enormous amount of reef debris 

 which must be washed into the " moat," so as nearly to till it, 

 should be noted ; for no atoll has lagoon water essentially 

 deeper than is shown, to scale, by the thickness of the upper 

 black lines at the lagoons of figs. 3b and 3c. 



Many difficulties have been urged against this apparently 

 simple theory. The gravest of them has just been suggested. 

 To explain the larger atolls, the subsidence must be reckoned 

 in thousands of feet. As it progresses, reef detritus and other 

 materials are washed into the lagoon, tending to till the " moat." 

 Neither the rate of sinking nor the supply of filling material 

 for each square mile of the lagoons can be constant for all bar- 

 riers and atolls. Hence great differences should characterize 

 the depths of reef lagoons. The fact is, however, that each of 

 the world's larger lagoons regularly has an almost constant 

 depth, except in the immediate vicinity of the main reef and 

 of the occasional coral knolls of the lagoon interior. More- 

 over, the general lagoon depth is almost always between 150 

 and 250 feet and the maximum depth is nowhere significantly 

 more than 300 feet. Figure 4 is a section of Suvadiva atoll in 

 the Indian ocean. The section is atypical one and it illustrates 



