210 DALY. 



Upward Groivth of the Reefs. x\s compared with other constructive 

 processes in the ocean, coral growth is very rapid. Gardiner con- 

 cluded that, under normal conditions, reefs can grow upwards at the 

 rate of from 27 m. to 45 m. in 1,000 years. ^^ According to Sluiter, 

 a young reef at the Black Cliff of Krakatoa grew to a thickness of 0.2 m. 

 in a period of not more than 5 years — a rate of 40 m. per thousand 

 years. '^^ Wood-Jones found certain branching corals to grow at an 

 average rate of 9.4 cm. per year, while massive coral increased its 

 circumference at the rate of 9.2 cm. per year.*^ 



Growth rates of such magnitude amply suffice to account for the 

 continued success of most of the early coral plantations, in spite of 

 the gradual rise of sea-level due to melting of the ice-caps. The 

 glaciers were not likely to have lost more than 1 m. in average thick- 

 ness by a year's melting. Melting even at that high rate would cause 

 the general sea-level to rise only 4 cm. per year or 40 m. in 1,000 years, 

 at the end of which time the ice-caps would have approximated their 

 present size. More probably the North American sheet, at least, 

 melted away in a period of more than 2,000 years.** The drowning 

 may have been temporarily too rapid for the vigorous growth of some 

 coral colonies, which, however, have finally succeeded in forming 

 surface reefs during the many thousands of years represented in post- 

 Glacial time. Other platforms were so situated that early coloniza- 

 tion was forbidden and to-day they carry no reefs, probably because 

 of post-Glacial drowning. Isolated corals do grow at depths equal to 

 those of the original platforms below present sea-level, but, because 

 of their sparseness, they have been able to shoal those plateaus only 

 with extreme slowness.*^ (See Fig. 37.) 



41 J. S. Gardiner, The Fauna and Geography of the Maldive and Laccadive 

 Archipelagoes, Cambridge, England, p. 333 (1903); Amer. Jour. Science, 16, 

 209 (1903). 



42 C. P. Sluiter, Natuurkundig Tijdschr. v. Nederl. Indie, 49, 375 (1889). 



43 F. Wood-Jones, Coral and Atolls, London, p. 71 (1910). 



44 See W. Upham, Geol. Mag., 1, 344 (1894). De Geer has lately shown the 

 duration of melting for the Scandinavian sheet to be much more than 2,000 

 years. 



45 J. S. Gardiner [Proc. Cambridge Phil. Soc, 9, 482 (1898); Proc 4th Inter- 

 nat. Cong. Zoology, 120 (1898); The Fauna and Geography of the Maldive 

 and Laccadive Archipelagoes, Cambridge, 1, Part 1, p. 177 (1903)] has made 

 the valuable suggestion that reef corals are restricted to depths generally 

 less than 55 m., not because of a direct need of the corals themselves for light, 

 but because their chief food consists of chlorophyll-bearing algae. Light 

 at depths greater than 55 m. is too feeble for the thriving of green algae; 

 hence it becomes an indirect control over coral growth. The objection to 

 Gardiner's view by F. Wood-Jones (Coral and Atolls, London, p. 241 (1910)), 

 is not convincing, and his own suggestion, that the depth limit is rather 

 due to mud-control, is not sufficiently general in its application. 



