GLACIAL-CONTROL THEORY OF CORAL REEFS. 229 



warped up or down, and the subaerial and submarine topography of 

 the islands in warped areas have been accordingly explained. More- 

 over, it will be noted that a moderate sinking of the surface of a 

 central island does not necessarily imply subsidence of the earth's 

 crust beneath. (See page 233.) 



Hence, several lines of reasoning show the grave danger of error in 

 regarding drowned valleys as proofs of the general crustal subsidence 

 postulated by Darwin and Dana for all barrier reefs and atolls. Ex- 

 cept in regions showing deformation in Tertiary time, the present 

 depths of the corresponding bays are no greater than those expected 

 as the result of erosion in the Pleistocene time of lowered sea-level. 

 Therewith a measure of positive support is given to the Glacial-control 

 theory. Vastly more compelling is the assemblage of facts regarding 

 the bathometry of barrier lagoons and of atolls. The whole physiog- 

 raphy of reef-covered areas, both below and above sea, must be con- 

 sidered in attaining a just estimate of the problem. Ninety-nine per 

 cent of that physiography is submarine and it does not accord with 

 the Darwin-Dana theory. In the writer's opinion, a few scores of 

 drowned valleys in an admittedly unstable part of the western Pacific 

 are far less worthy of emphasis; they can hardly be held to prove 

 general crustal subsidence in the coral seas. 



Pleistocene Cliffing of Oceanic Islands. Objection has been raised to 

 the new theory, on the ground that it does not agree with the observed 

 topography of the shores between the bays of central islands. If 

 platforms as extensive as the Maldive or Macclesfield banks were 

 planed off by Pleistocene waves and currents, it is held that central 

 islands generally should show strong cliffs at the shore ends of the 

 erosion spurs. As a matter of fact, many of these islands are more or 

 less cliffed. Sometimes the cliffs reach scores of meters in height, 

 though usually the cliffs at the ends of spurs are only a few meters 

 high.®^ That they are not often much higher would be a strong 

 argument against one phase of the Glacial-control theory, if the 

 Pleistocene islands were all of the same height and rock-strength. 

 Their great variability in each of these respects has been described. 

 The Pleistocene waves must have cut wide benches in the pre-Glacial 

 mud banks, shell banks, and banks of loose volcanic debris. Just as 

 clearly they have made little impression on the lavas of Hawaii or 

 Tahiti. 



61 For many examples see the splendid series of photogravures illustrating 

 .Agassiz's various expeditions to the Pacific archipelagoes. 



