W. M. Davis — Shaler Memorial Study of Coral Reefs. 231 



manifest origin by submergence, although " indications of 

 recent elevation '' in the form of high-water marks a few feet 

 above tide level are described.* Naturally enough, then, no 

 consideration of submergence is found in his theory of coral 

 reefs. 



This theory is inapplicable to any of the barrier reefs 

 that I have seen, (1) because the embayments of the central 

 island, as in sector O, fig. 2, prove that the relative level of 

 land and sea has changed, as already stated ; (2) because deltas 



Fig. 2. 



Fig. 2. Deduced stages, sectors G to L, of an outgrowing reef on a still- 

 standing island; sector 0, observed features of a barrier reef island. 



advancing beyond the outer margin of the non-embayed cen- 

 tral island into the lagoon, sectors H, J, K, as they should if 

 the island had stood still, are conspicuously absent in all but 

 two of the barrier-reef islands I have visited, although deltas 

 of moderate size are always to be found in the bay heads; (3) 

 because the lagoons, far from suffering excavation from solu- 

 tion, seem to be receiving new deposits by overwash from the 

 outer reef, by outgrowth of inner fringing reefs, by inwash 

 from stream deltas, and by organic growth on the lagoon floor ; 

 (4) because uplifted reefs, as far as known, do not show, as at 

 M, the steeply inclined talus-stratification resting on non-eroded 

 volcanic beds, as they should under this theory ; (5) because no 

 barrier reefs are known in which the central volcanic island is 

 worn down to a lowland within a delta plain, as in sector K, 

 and because no almost-atolls are known in which the residual 

 central islands are vanishing lowland remnants, as in sector L ; 

 although both these forms should occur if, as Murray briefly 



* Challenger Reports : Narrative, i, 508, 1 885. 



