GLACIAL-CONTROL THEORY OF CORAL REEFS. 209 



submarine shelves with depths of the order expected if the shelves 

 represent Pleistocene, wave-formed benches, veneered with post- 

 Glacial sediments, (b) If the islands have been uplifted in the post- 

 Glacial period, they either lack marked submarine shelves entirely, 

 or have shelves at depths differing from the inferred depth of a Glacial 

 wave-formed bench, by the amount of the uplift, plus the thickness of 

 Recent sediment deposited on the platforms, (c) Where there has 

 been post-Glacial subsidence, the existing shelf is at a depth corre- 

 spondingly deeper than the depth computed for the Pleistocene, 

 wave-formed bench, (d) The depths of the lagoons of Recently 

 uplifted atolls are never greater than those expected if the lagoon 

 floors are situated on platforms cut by the waves of the Glacial period. 

 Conclusions. Before passing on to a discussion of the existing reefs 

 themselves, a few words summarizing the problem as to the origin of 

 the underljdng platforms may not be amiss. According to several 

 distinct lines of evidence, the present reefs are shallow veneers on 

 benches formed by Pleistocene waves at levels averaging about 70 m. 

 below present sea-level. The platforms on which the coral crowns 

 rest are, in general, not too large to be so explained, except for a 

 few areas of the continental shelf. The widths of most platforms are 

 closely related to the strength of the materials composing the Pleisto- 

 cene islands. x'Vllowing for post-Glacial sedimentation and organic 

 growths, the depths of the shelves along stable coasts, whether within 

 the coral seas or outside of them, correspond well with the inferred 

 depth of the typical Pleistocene, wave-cut bench below the existing 

 sea-level. The Glacial-control theory withstands a further, and 

 specially severe, test, as the subaerial and submarine topography of 

 Recently elevated or sunken islands is compared with the topography 

 of stable islands and continental shores. 



Origin of the Existing Reefs. 



Colonization of the Platforms. According to paleontological evi- 

 dence the last Glacial climax was speedily followed by a period of 

 average air temperature at least as high as the present average. 

 Warming of the surface water in tropical seas must have occurred, 

 even before the Pleistocene ice-caps were essentially diminished; 

 large areas were thus soon restored to the temperature where reef 

 corals could thrive. Since their larvae can survive drifting for several 

 weeks, if not months, there can be little doubt that widespread and 

 rapid colonization of the wave-formed plateaus would take place. 



