146 Daly— The C oral-Reef Zone. 



factors were introduced with the first intergiacial period ; 

 to a brief consideration of these we may now turn. 



It is obvious that warming of air and ocean began 

 immediately after, or soon after, the glacial maximum, 

 that is, during a time of lowered sea-level. The condi- 

 tions of reef growth were established by a bettered 

 temperature and also by the incipient rise of sea-level 

 through melting of the ice. A slight deepening of water 

 on the new shelves and benches decreased the turbidity, 

 which had already been somewhat diminished through 

 gradation of the shelves during the first glacial stage. 

 Hence, well before the end of that stage, corals were able 

 to colonize the outer edges of the shelves, where, on 

 account of "mud-control" (see the 1915 paper), barrier 

 reefs might be expected to develop with special success. 

 In a similar way the outer edges of detached banks would 

 be early colonized, giving atoll reefs. 



Whatever flaws there may be in the reasoning so far, 

 one is probably justified in assuming the development of 

 barrier, atoll, and fringing reefs during the first inter- 

 glacial stage ; and also in assuming the usual, partial fill- 

 ing of lagoons with detritus, coral heads, algal growths, 

 plankton remains, etc. 12 



During the second glacial stage the corals may have 

 been injured or killed by oceanic chilling. If so, the 

 reefs and lagoon deposits of the first intergiacial stage 

 may have been cut away by the waves at the new sea- 

 level. Even if the corals were uninjured by chilling, 

 they would not necessarily continue to grow on the sea- 

 ward faces of the young atoll and barrier reefs. Judg- 

 ing from present types, the seaward slopes of the 

 emerged Pleistocene reef were too steep or too sandy to 

 permit coral growth which would be rapid enough to pre- 

 vent breaching of the reef by the surf. If the reef were 



12 In the writer's 1915 paper (page 180) it was stated that "the sea 

 was actively attacking the islands and continental coasts throughout nearly 

 the whole Glacial period. The reef -building corals were largely killed off 

 long before the ice-caps of the first Glacial stage reached their full size. 

 The succeeding Intergiacial stage may have witnessed a partial re-establish- 

 ment of reefs in the open ocean, but, if so, such reefs must have been 

 relatively feeble and short-lived defenders of the islands. Similar reasoning 

 applies to the other recognized stages of the Glacial period. Hence, though 

 sea-level swung down and up several times, lively wave abrasion must have 

 been almost continuous." Further consideration, stimulated by Davis's 

 criticism, has led the writer now to lay more stress on the likelihood of 

 vigorous reef growth during one or more of the intergiacial stages. Thus, 

 the degree of continuity for the .Pleistocene abrasion seems to be somewhat 

 overstated in the passage quoted. 



