GLACIAL-CONTROL THEORY OF CORAL REEFS, 177 



Glacial period, must have suffered peneplanation or great reduction 

 and then decomposition by down-seeping soil waters. The great 

 depth — 100 m. or more — for the shell of decayed rock is well illus- 

 trated in the southern Atlantic states of the North American Union, 

 in Brazil, and in many other tropical lands of the present day. 



Heights of flic Pleistocene Islands. The rate of wave-benching was 

 necessarily controlled in part by the ^'olume of rock to be removed by 

 the waves; hence the range of heights for the different kinds of islands 

 and coastal plains is an important element in the problem. For 

 convenience, the symbol d may be used to represent the vertical 

 (downward) shift of sea-level at a time of maximum glaciation. Simi- 

 larly, h may represent the height of the land l)efore glaciation set in; 

 and //, the height of the land above sea after the shift of sea-level. 



In the case of the young volcanic islands, h had values up to a limit 

 of about 4,000 m. At the other extreme were the very old, stable 

 islands, for which h approached zero in value; for them H and d were 

 nearly equal. Other ancient islands, which had been affected by 

 crustal uplift, may have passed through more than one erosion cycle, 

 with final peneplanation. Still others, once peneplained or greatly 

 reduced in volume, may have sunk well below sea, and then, in pre- 

 Glacial time, received a veneer of organic debris, whereby the surface 

 of each of these "banks" was brought close to the Pleistocene ocean 

 level. For such islands also, H and d were of the same order of 

 magnitude. The parts of the benches cut by pre-Glacial waves and 

 not veneered with coral or other growths, would furnish islands with 

 H less than d. For the continental shelves and purely fragmental 

 embankments about islands, H varied in value from zero to that of d. 

 For coasts surrounderl by pre-Glacial fringing reefs, the average value 

 of // was nearly equal to d. For offshore banks of mud, shells, or 

 ooze within the tropics, H was generally less than d, by at least 35 to 

 45 m., since, by hypothesis, they were covered in pre-Glacial tin\e by 

 water too deep for vigorous coral growth. 



Conclusions. Reviewing the facts and reasonable inferences, it 

 therefore appears that most oceanic islands, at the time of maximum 

 glaciation, were (a) low, with // ranging between zero and a value 

 little greater than d; and (h) composed of generaUij weal: material — 

 detrital embankments surrounding or covering an eroded central 

 mass of decayed volcanic rock, which itself was likely to be weak also 

 because of the presence of ash-be Is. The encircling talus embank- 

 ment carried veneers of coral-reef material of varying strength. 



The new coastal plains along the edges of continents and greater 



