178 R. A. Daly — Problems* of the Pacific Islands. 



able to coral growth in the tropical seas. It also meant a 

 return of water to the sea, with an ultimate raising of its level 

 by about 180 feet. The positive shift of level must have been 

 slow, while colonization by coral larvse over large areas is 

 known to be rapid. In most cases the corals began vigorous 

 growth on the platforms, before the covering water reached 

 the depth of about 120 feet, at and below which corals cannot 

 thrive. The larvae would settle indiscriminately on the plat- 

 forms, but in general only those situated on the edges of the 

 platforms would form persistent reefs on the large scale. The 



Figs. 20-25. 



5. 1718 20 19 18 24 25 27 34 29 26 36 25 11 24 20 22 22 14 N. 



■ 



mm 

 20 



1 



S.E. 14 17 II 



21 



N. 6 12 8 5. 



w 



22 



I 



W. 10 6 23 31 33 32 32 32 32 32 31 35 14 CAY E. 



_? Sea Miles/ 



Figs. 20-25. Sections of small and middle-sized atolls, illustrating the 

 rule that small atolls are more filled with detritus than those of larger atolls. 

 Water is shown in solid black ; rocks, including the reefs, are lined. Uni- 

 form scales ; vertical scale is five times the horizontal. Depths in fathoms. 



20. Peros Banhos, Chagos group, Indian ocean. 



21. Salomon islands cluster, Chagos group. 



22. Six islands cluster, Chagos group. 



23. North Minerva atoll, at 23° 37' S. Lat. and 178° 56' W. Long. 



24. Wataru atoll, Maldive group. • 



25. Section through one of the rare cays of Loai ta "drowned atoll," 

 China Sea. «• 



colonies of the interior must be largely extinguished by the 

 platform mud, which is washed over them by waves coming in 

 on all sides. The border colonies, each subject to mud inva- 

 sions from only two quadrants instead of four, have a great 

 advantage. Once these rimming reefs have grown up to the 

 sea-surface, the danger of death from the platform mud is but 

 slight for the polyps growing on the outer face of the reef. 

 Accordingly, the reef growth is there specially rapid and con- 

 tinuous. Thus it follows that the platforms have generally 



