GLACIAL-CONTROL THEORY OF CORAL REEFS. 245 



Glacial-control theory imposes little strain on the logical faculty. 

 The series of topographic forms, from fringing reefs through barrier 

 reefs to atoll reefs, look as simple as a natural sequence can well he, 

 but it may be wholly deceptive. 



Psychological Influence of Classic Diagrams. The text diagrams 

 ordinarily used to illustrate the Darwin-Dana theory are subject to 

 serious criticism. In Darwin's book the sinking island is represented 

 as a single, somewhat eroded, volcanic cone, the slopes above sea-level 

 (at the beginning of the subsidence) having angles of 16° to 70° (Fig. 

 44). The legends imply that no exaggeration of slope is intended in 

 this part of each section. The initial submarine slope is shown for a 

 short distance, with a rapid flattening to less than 5°, in the direction 

 of the open sea. The initial breadth of the island is only four times 

 its height. In Dana's diagrams, published in " Corals and Coral 

 Islands," the original island is likewise represented as a single volcanic 

 cone, its subaerial slopes ranging from 16° to 45°; no submarine slopes 

 are indicated, nor does legend or text suggest any exaggeration of the 

 natural slopes. The initial breadth of the island is less than six times 

 its height. In the last edition of his "Manual of Geology," Dana 

 gives sections essentially like those of Darwin. Nearly all the sections 

 published by other writers to illustrate the theory are, in principle, 

 similar. 



Original islands of such proportions could, by subsidence, produce 

 only very small atolls. To explain any of the greater atolls in the 

 same way, the original island must be assumed to have had very 

 different proportions, even if it culminated in a point 4,000 m. above 

 sea. Its average slope must be much less than that shown in the 

 diagrams mentioned. With gentler initial slopes, the increase in the 

 lagoon area during sinking must be more rapid than in the case of a 

 steep-sided island. The more rapidly the lagoon area expands, the 

 slower must be the aggradation of its floor by detritus from the outer 

 reef or from the central island. As already observed, sections drawn 

 to scale for such a large island would indicate the difficulty of explain- 

 ing why many of the world's lagoons are not more than about 90 m. 

 in depth. Undoul^tedly the subsidence theory has too long enjoyed 

 the fictitious aid of imperfect diagrams, which have been studied in, 

 or copied from, the classic works. 



Assuming for the original island a more probable average value for 

 the subaerial slope, one not exceeding 10°, and assuming the sub- 

 marine slope as not more than about 10°, Dietrich's mean \alue for 

 volcanic islands to a depth of 2,000 m., the sections illustrating the 



