ECOLOGY OF THE MURRAY ISLAND CORAL REEF. 



13 



As Daly states, the Darwin-Dana subsidence theory fails to explain the 

 fact that throughout the Pacific and Indian Oceans the bottoms of the lagoons 

 of barrier reefs and atolls are extraordinarily flat plains sunken to a nearly 

 uniform depth of 20 fathoms; and, as pointed out by Andrews and Vaughan, it 

 does not explain the extension of the platforms into cold regions where corals 

 cease to grow. The coral reefs rise abruptly as patches above or as mere walls 

 along the seaward edges of these platforms. In fact, as recently pointed out 

 by Vaughan, theGreat Barrier Reef of Australia does not throughout its length 

 always margin the Australian continental shelf, for near its southern end the 

 reef stands back some distance from the seaward edge of the shelf. If various 

 more or less local submergencies caused atoll lagoons they ought to range 

 considerably in depth, but as Daly shows, the maximum depth in any coral 

 lagoon is only 49 fathoms (91 meters) in Budd-Iambu, Fiji. The Daly theory 

 readily explains the flat bottoms and uniform depths of lagoons, but the sub- 

 marine platforms appear in places to be too wide to have been formed during 

 the glacial epoch, and indeed, while Daly points this out, Vaughan has shown 

 that some platforms date from old Tertiary times (Eocene and Oligocene). 



The Murray-Agassiz solution theory is weak in that tropical (Tortugas) 

 sea-water dissolves limestone at so slow a rate that the lagoons could not 

 have been appreciably deepened in this manner. There are, however, other 

 agencies which dissolve limestone, and these may be more or less effective, 

 such as holothurians and fish which swallow large quantities of sand, but 

 most if not all atoll lagoons are filling up more rapidly than they are dissolv- 

 ing out. Moreover, it is difficult to explain why, if solution is so effective 

 in shallow water, it should suddenly decline at depths below 20 fathoms. 

 Agassiz's idea of scouring due to attrition over the bottoms of lagoons is a 

 factor worthy of consideration, but it has not been quantitatively evaluated. 

 The average upward rate of coral growth, being from 6 to 20 mm. per 

 year, is possibly more rapid than the average rate of sinking of oceanic islands 

 which are subsiding; and if this be true the encircling reefs of most subsiding 

 islands might be expected to maintain themselves at the surface in accord- 

 ance with Darwin's hypothesis. Indeed, Davis (1915, American Journal of 

 Science, vol. 40, p. 250) presents evidence tending to show that this occurred 

 at Vanua Mbalavu, Fiji, and elsewhere, and it would explain the great thick- 

 ness of many elevated coral reefs. Perhaps most volcanic islands tend tem- 

 porarily to sink after they cease to be active, and, indeed, Daly's theory is 

 not adverse to that of Darwin, but merely emphasizes the control which a 

 temporarily lowered sea-level may have exercised over the modern reefs, and it 

 seems not impossible that some of the pre-glacial reefs were built up upon sub- 

 siding foundations, whereas most of the modern reefs have grown upward 

 upon drowned platforms which have for the most part remained stationary 

 since Pleistocene times, the modern period having been too short to permit 

 of many significant depressions or elevations of the reef regions. 



