GLACIAL-CONTROL THEORY OF CORAL REEFS. 211 



Special Development of Reefs at the Edges of Platforms. The location^ 

 of most barrier and atoll reefs on the peripheries of their platforms 

 has been explained by the greater abundance of food and oxygen 

 in the zone of breakers. This view still needs close scrutiny, not- 

 withstanding the high authority of the investigators who have adopted 

 it. Many reef-building species luxuriate also in lagoon areas, which 

 are removed from the influence of heavy surf and from the direct 

 impact of main ocean currents. Apparently, therefore, one or more 

 other factors must be considered. In the writer's opinion, a leading one 

 is the deleterious effect of bottom muds and sands on coral growth — 

 a principle well recognized by several authors but not sufficiently 

 emphasized in connection with the present problem. 



During the late Pleistocene, as sea-level was slowly rising, the mud, 

 sand, and organic ooze on the platforms were kept stirred by storm 

 waves. All coral colonies on open-sea platforms were liable to be 

 partially or totally killed off by the clouds of sediment so raised and 

 carried by currents. Colonies situated well inside a platform edge 

 must have suffered from sediment drifted, in turn, from all points of 

 the compass. Only a few of these plantations could continue to live 

 and thus follow up the rising sea-level. Those successful in growing 

 well above the general flat surface of the platform would henceforth 

 be less damaged by sediment, which, of course, most aft'ected the 

 wholesomeness of the water immediately overlying the deeper, flatter 

 parts of the platform surface. Hence the occasional knolls of thriving 

 coral in lagoons are explicable, though the average condition of the 

 central platform areas was unfavorable to the continued upgrowth of 

 reefs. 



On the other hand, a coral colony settled on the edge of a platfoi-m, 

 was subject to invasion by clouds of sediment, almost wholly from the 

 side of the platform itself. On the open-sea side, most of the finer 

 reef debris and plankton debris was persistently dragged out by the- 

 undertow and bottom currents into deeper water, where it was hence- 

 forth harmless. Throughout all post-Glacial time the water on the 

 platform edge was comparatively clean, except when the waves and 

 currents came from the side of the platform. Hence the coral planta- 

 tions on the edge had a decided advantage in their struggle for life. 

 As soon as they formed a surface reef (quickly accomplished), the 

 danger of fatal influxes of mud would be further greatly lessened for 

 those corals that grew on the outer face of the reef; since the reef 

 itself largely protected these corals from the platform sediments. 



"To him that hath shall be given." Once well established on the 



