GLACIAL-CONTROL THEORY OF CORAL REEFS. 235 



per cent, of recorded geological time. Clearly, similar sinking could 

 not have occurred often in the same area, during the remaining 95 

 per cent of that record. No reason for specially great and rapid 

 subsidence since the beginning of the "coral-reef period "' (early 

 Miocene?) has yet been given. If this late sinking were actually 

 preceded by many similar ones in the same area, during pre-Cambrian 

 and later periods, one must assume intervening epochs when the sea- 

 bottom was upheaved ; so that the final depth of the ocean should be 

 no greater than it actually is. Rhythmic diastrophism of the kind 

 and scale demanded is improbable. It should have left traces in the 

 bottom topography of the Pacific, which, however, seems to be lacking 

 in such evidences. 



The antiquity of a deep Pacific basin may be false doctrine, but, 

 so long as manj^ facts continue to require its assumption, the subsi- 

 dence theory has to bear the heavy burden of explaining the recent 

 character of the postulated sinking of the Pacific bottom. In less 

 measure the difficulty also applies to the Indian ocean area of reefs. 



High antiquity for the basin does not exclude its progressive deep- 

 ening, but the relative stability of most of its floor in the later Tertiary 

 seems proved by the size of the larger reef platforms and other 

 banks. In whatever way these plateaus have been formed, the process 

 must have taken very much time, even measured by the geological 

 scale. The Glacial-control theory holds that only the final touches 

 in fashioning the platforms were applied during the Pleistocene. 

 Banks of sand and mud and low islands of similar material or of weak 

 rocks were then truncated by the waves. During the relatively brief 

 Glacial period, the sea bottom was so nearly stable as to permit of 

 the wide benching of such banks and islands. The whole period was 

 but a minute fraction of recorded geological time, and therefore is 

 likely to have been one of general sea-floor stability, if the ocean basin 

 dates from an early stage in the earth's history. 



The problem is baffling because of insufficient data, but the conclu- 

 sion remains that the subsidence theor}^ is at a disadvantage because 

 of the difficulty of reconciling it with facts, independent of coral reefs 

 and suggesting an immense age for most of the ocean basin. 



Small Maximum Depth of Lagoons. Dana remarks that his theory 

 " explains all the varying depths of lagoons, from the condition of near 

 obliteration to that of a basin one to three hundred feet deep." ^^ A 

 few paragraphs be^^ond, he adds: "The coral-growing areas over the 



67 J. D. Dana, Corals and Coral Islands, New York, 3rd ed., p. 272 (1890). 



