GLACIAL-CONTROL THEORY OF CORAL REEFS. 159 



publication was to offer the hypothesis for discussion, especially by 

 those who have a closer personal acquaintance with coral reefs. The 

 problem is importan,t, as it vitally affects the physiography, geological 

 history, and geological dynamics of about one-eighth of the earth's 

 surface, as Avell as the recent history of the ocean as a unit. 



The available data seem to show that the whole of the ocean was 

 chilled during the Pleistocene Glacial period. Over wide stretches of 

 the tropical seas the reef corals were exterminated or greatly weakened 

 in their reef-building power. The land-masses or shoals, which had 

 been defended by the pre-Glacial living reefs in those regions, were now 

 successfully attacked by the waves of the open ocean, and benched. 

 At the climax of glaciation, the waves of the tropical seas ran over a 

 surface lower than now: first, because w^ater had been removed from 

 the ocean to form the ice-caps (located chiefly on the continents); 

 secondly, because each ice-cap attracted the remaining ocean water 

 to itself and thus lowered the level of the seas within the tropics. The 

 depth of the platforms, cut-and-built by the waves during maximum 

 glaciation, was estimated to be from 30 to 50 fathoms, or 55 to 90 m., 

 below present sea-level. 



With the late-Pleistocene warming of the air, the surface water of 

 the tropical seas grew rapidly warmer, the ice-caps were slowly melted, 

 and general sea-level was correspondingly raised again. The warming 

 of the tropical seas allowed the coral larvae, emanating from the limited 

 reefs not entirely killed in spite of the Pleistocene chilling, to colonize 

 the new, wave-cut platforms. Since reef corals thrive best on the 

 outer edges of such benches, the new colonies there specially formed 

 reefs, which grew upward as sea-level rose. Of course many larvae 

 would settle elsewhere on the platforms, as well as in the shore breakers. 

 In general, however, the colonies on or near the outer rims of the wave- 

 formed platforms would thrive better than the inside colonies and the 

 dominant reef would be linear, following the edges of the platforms. 

 The fringing, barrier, and atoll reefs are thus explained as shallow 

 crowTis recently built up on wave-formed platforms. The hypothesis 

 implies that barrier reefs and atolls have not necessarily characterized 

 the warm seas of the pre-Pleistocene periods but represent physio- 

 graphic forms due to the highly specialized effects of a Glacial period. 



The offered explanation does not involve any vertical movements of 

 the earth's crust and thus contrasts with the famous Darwin-Dana 

 theory. It does not imply that any large proportion of the total 

 erosion suffered by oceanic islands was accomplished during the Glacial 

 period, but merely that the reef platforms were then finally smoothed 



