GLACIAL-CONTROL THEORY OF CORAL REEFS. 179 



tons of pelagic and shallow-water organisms. On account of the length 

 of time involved in the erosion (peneplanation aided by wave scour), 

 the organic increment to the terrace would be very great, perhaps 

 rivalling the inorganic material in bulk. If the island were circular, 

 with an original slope of 1 : 6 (nearly the average slope of young vol- 

 canic islands) above and below sea-level, the embankment would 

 probably have an average width at least one-third as great as the 

 initial radius of the island. The total area within the 50-meter 

 isobath, including the central outcrop of undisturbed rock, would be 

 nearly double the original area of the island. 



The result of the calculation suggests that a large percentage of the 

 area occupied by each of many Pleistocene islands would offer rela- 

 tively small resistance to the waves of that period. 



For the area of the initial island more than one possibility must be 

 weighed. If sea-level had remained constant throughout the preced- 

 ing erosion cycle, and if the island had escaped marine planation, the 

 volcanic rocks must have been more or less deeply weathered and 

 weakened. After the Pleistocene shift of sea-level, their surface would 

 be nearly at the height d above the new level. On the other hand, if 

 the island had been truncated by waves, because of a temporary failure 

 of reef protection during pre-Glacial times, the height of its central 

 lavas above the new Pleistocene sea-level would be less than d by 

 some tens of meters. 



Duration of Pleistocene Abrasion. As the facts concerning Pleisto- 

 cene glaciation become better known, geologists are becoming steadily 

 more convinced of the notable length of the Glacial period as a whole. 

 Because of their wide experience and deep study, the estimate of 

 Chamberlin and Salisbury is worthy of special emphasis. They regard 

 the time elapsing between the climax of the Kansan stage and the 

 climax of the Wisconsin stage as probably between 280,000 and 960,000 

 years. For the entire period other milleniums must be added, to 

 represent the time during which the Kansan ice grew to its full thick- 

 ness, and for the Sub-Aftonian stage, if the latter really was a time 

 of widespread glaciation. ^^ When Bain showed how thoroughly the 

 Kansan drift has been dissected by post-Kansan streams, he laid the 

 foundation for general belief that intense Pleistocene glaciation began 

 at least 300,000 years, if not at least 500,000 years, before the ice-caps 

 of the Wisconsin stage began to wane. 



Reliable estimates of the total duration of a greatly lowered sea- 



27 T. C. Chamberlin and R. D. Salisbury, Geology, New York, 3, 420 (1906). 



