GLACIAL-CONTROL THEORY OF CORAL REEFS. 175 



Regarding some of these shore-line materials special consideration 

 is necessary. Numbers 4, 5, and 6 refer to continental shelves and to 

 submarine embankments encircling islands, and were of breadth 

 depending on the antiquity of the land masses concerned. The 

 breadth of the continental shelf, such as that off eastern Australia, 

 measured scores of kilometers; the embankments surrounding large, 

 very old islands were generally narrower but doubtless often measured 

 many kilometers in width. Initially, the plains underlain by the 

 marine sediments, laid bare during the first Glacial maximum, would 

 vary in elevation, the highest parts being above the new sea-level by" 

 just the amount of the negative shift from the pre-Glacial sea-level. 



The coral reefs of pre-Glacial age formed steep initial bluffs over- 

 looking the new sea-level. The average massive coral is more re- 

 sistant to wave abrasion than the loose deposits just mentioned but is 

 much less resistant than an unweathered lava flow. Observation 

 shows an ordinary reef to be greatly weakened by lenses of sand and 

 poorly cemented coral breccia. If the sea-level fell as much as 60 m., 

 the massive coral, only 35 to 50 m. thick and resting on talus sand and 

 blocks, would be liable to undermining, and therefore quick destruc- 

 tion, by the waves. The friable nature of the talus and other material 

 underlying the existing reefs is suggested by the logs of borings in 

 Florida, Funafuti, Bermuda, Sumatra, and the Hawaiian islands. ^^ 



How wide the pre-Glacial reefs were, it is impossible to say. Among 

 other things, their width depended on the antiquity of the present 

 coral-reef fauna as a whole. There is no evidence that it dates back 

 of the Jurassic period, nor, indeed, is it proved that the present coopera- 

 tive habit of these species was well established before the Miocene. 

 Hence one cannot assume the pre-Glacial islands or continents to have 

 been fringed with indefinitely wide reefs. 



Paleozoic and younger organic growths of all kinds were liable to 

 wave-benching until protected, apparently in late geological time, by 

 the "invention" of strong coral reefs. Even now, many reefs are 

 just able to hold their own against the breakers; others have been 

 completely truncated during recent years; and still others, the so- 

 called "drowned atolls," have long failed to reach the surface at all. 



23 E. O. Hovey, Bull. Mus. Comp. Zool., 28, No. 3 (1896); The Atoll of 

 Funafuti, published by the Royal Society of London, 1904; L. V. Pirsson, 

 Amer. Jour. Science, 38, 191 (1914); C. P. Sluiter, Petermann's Geog. Mitt., 

 1891, Lit. Ber., p. 46; A. Agassiz, Bull. Mus. Comp. Zool., 17, 121 (1889). 



The figures stating the range of thickness for the exposed reefs are deduced 

 from the well-known depth limits of vigorous growth of reef corals. 



