GLACIAL-CONTROL THEORY OF CORAL REEFS. 



247 



subsidence theory would be very different from those of Darwin and 

 Dana.'^^ When one remembers that most of the detritus abraded from 

 the main reef goes to form talus on the outer submarine slope; and, 

 secondly, that the growth of new coral is much faster on that side, he 

 cannot fail to expect a centrifugal tendency for the encircling reef, as 

 the island sinks (Fig. 48). Von Lendenfeld seems to have been the 

 only author who has hitherto recognized this tendency and published 

 diagrams to accord with it (Figs. 45-47)." On the other hand, the 

 sections of both Darwin and Dana indicate a centripetal tendency 

 for the reef as sinking progresses, so that the area enclosed by the reef 

 continually diminishes. 



The Test bij Boring Through Reefs. If von Lendenfeld's view is 



Figure 48. Diagrammatic section showing the great amount of a reef's 

 centrifugal displacement which is necessary if the reef continued growth during 

 the subsidence of a normal volcanic island (vertically lined), and if the "moat" 

 were always filled with detritus, so as to show depths no greater than those 

 of actual lagoons. The line 1-2-3 is arbitrarily drawn straight. If the rate 

 of subsidence were uniform, this line should be concave upwards. At 1, 2, and 

 3 are the respective sea-levels at fringing, barrier, and atoll stages {Fr, Ba, and 

 At) of the reef. 



correct, the massive reef of a large atoll must lie unconformably upon 

 talus of indefinite depth. Hence the Funafuti borings could not, in 

 any case, have penetrated massive reef material in situ to a depth 

 greater than about 45 m. That the actual site of the borings was 

 unwisely chosen is apparent. The theory of subsidence itself, fairly 

 developed, should have indicated as much. It needs testing by bore- 

 holes sunk well inside the reefs of atolls and barrier-encircled islands. 

 The Bermuda boring gives the nearest approach to a vital test. Its 

 result does not favor the subsidence theory but its value must remain 



76 F. Dietrich, Untersuchungen iiber die Boschungsverhaltnisse der Sockel 

 ozeanischer Inseln, Greifswald, 1892; also A. Supan, Grundziige der physischen 

 Erdkunde, Leipzig, 3te Aufl., p. 690 (1903). 



77 R. von Lendenfeld, Gaea, Jahrg. 26, 196 (1890); Westermann's Monats- 

 hefte, Jan., 1896, p. 505. 



