160 DALY. 



by the removal of thin veneers of relatively weak materials formed 

 on the oceanic plateaus in Tertiary and pre-Tertiary time. In this 

 respect the new theory is closely allied to that of Tyerman and Bennet, 

 who, nearly ninety years ago, suggested that the existing reefs have 

 grown on platforms cut by ocean waves and currents. Neither they 

 nor their successors holding the abrasion theory of reefs, like Wharton 

 and Agassiz, had explained how the abrasion could take place, for it 

 was apparently assumed by each of these authors that the defending 

 reef corals were living in the tropical seas continuousl}' and for an 

 indefinite period. The Glacial-control theory emphasizes the Pleisto- 

 cene as one period of inhibited coral growth, but the bulk of the ero- 

 sion which has affected the oceanic plateaus is clearly pre-Glacial in 

 date. The fuller statement of the theory permits an outlining of 

 the reasons for belief that marine abrasion has largely truncated 

 the older oceanic islands, long before the Glacial period. Because of 

 that preliminary truncation, very extensive smoothing by Pleistocene 

 waves and currents was possible. (See Figs. 1-4.) 



Further, the Glacial-control theory fully recognizes that there has 

 been Recent crustal warping in certain oceanic areas affected by coral 

 reefs. '^ Such local subsidence or elevation has influenced the growth 



Sections illustrating the development of barrier reefs and atolls. 



Figure 1. A normal volcanic island. 



Figure 2. The same island largely peneplained, with the necessary forma- 

 tion of an encircling embankment of detritus (stippled). It is here arbi- 

 trarily assumed that there has been no marine abrasion. 



Figure 3. The same island, extensively benched by the waves, involving 

 some increase of the embankment. Such benching is expected in very old 

 islands which have been exposed to active abrasion, either because of the 

 Pleistocene chilling of the ocean or because of temporary failure of reef pro- 

 tection in pre-Glacial time. 



Figure 4. Complete truncation of the same island by continued marine 

 abrasion, with a slight broadening of the embankment. This is a stage that, 

 in many instances, was possibly attained in pre-Glacial periods, as well as 

 during the Pleistocene. 



In Figures 2, 3, and 4 the size of the embankment, as drawn, corresponds 

 merely to the bulk of purely inorganic detritus. If intermixed reef and other 

 organic material were allowed for, the embankment must be represented as 

 broader. After the abrasion, fringing, barrier, and atoll reefs would be favor- 

 ably located at X, Y, Z, respectively. Shifts of sea-level are not shown. 



The sections are drawn to scale and are also intended to show the great 

 areal extent of the weak embankment materials, laid down around old oceanic 

 volcanoes in pre-Glacial time. About one-half of the platform represented 

 in Figure 4 is underlain by these materials, which must have offered little re- 

 sistance to the benching surf of the Pleistocene period. 



3 In this paper, "Recent" means "post-Glacial" and "recent" means "in 

 late geological time." 



