W. 31. Davis — Shaler Memorial Study of Coral Reefs. 239 



or perhaps 1000 feet. But before taking up that question, 

 further inquiry must be made into the possible origin of 

 lagoons by the abrasion of preglacial sea-level reefs. 



Lagoon Floors are not Wave-cut Platforms. — Consider the 

 case of a narrow-lagoon barrier reef, C, fig. 5. If the floor of 

 such a lagoon, half a mile or a mile wide, represents as much 

 work as marine abrasion can accomplish in reducing a dead, pre- 



Fig. 4. 



Fig. 4. Deduced stages of an atoll, according to the glacial-control 

 theory. 



glacial reef, A, to a smooth platform, B, during the lowered sea- 

 stand of the glacial period, it follows that the lagoon-floor of a 

 large reef, such as that of Hogoleu (or Truk) in the Carolines, 

 34 miles in diameter, cannot be of the same origin ; still less 

 could any large part of the vast lagoon of the Great Barrier reef 

 of Australia, sometimes 70 or more miles wide, be due to marine 

 abrasion during a lowered sea-stand, for it could have been 

 attacked by the waves only on one side. So large an atoll as 

 Hogoleu should preserve the central tabular part, B, fig. 4 (more 

 dissected than here drawn), of its preglacial reef-limestone 

 plain, A, standing practically at sea level and surrounded 

 by an abraded lagoon-fioor a mile or so wide, outside of which 

 a barrier reef, C, should rise to-day. No such central lime- 

 stone table is known in any atoll. If, on the other hand, the 

 floor, D, fig. 4, of a wide lagoon, E, like that of Hogoleu, or of 

 the Great Barrier reef of Australia, represents the abrasive 

 work of the lowered and chilled sea acting on a preglacial reef- 

 plain, A, during the glacial period, then not only the whole 

 width of a narrow preglacial encircling reef, A, fig. 5, ought 

 to be cut away, as at B, but cliffs, D, D, truncating the spur 

 ends, should have been cut all around the margin of the cen- 

 tral volcanic island as well, and after the barrier reef is built 

 up in the rising sea the spur-end cliffs should rise from the 

 postglacial lagoon, E. This would appear to be Daly's view 

 for many islands, for he writes : " The undefended islands, 



Am. Jour. Sci. 

 16 



-Fourth Series, Vol. XL, No. 237.— September, 1915. 



