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



be investigated ; and fourth, that it is in connection with 

 barrier reefs, B, B, B, fig. 1, that the desired witnesses to 

 the facts of the past can be most readily found ; for the central 

 island, rising from the lagoon within a barrier reef, was surely 

 there while the reef was forming around it, and the features 

 of the island 'shoreline will, as Darwin long ago showed for 

 still-standing islands and as Dana a few years later pointed 

 out for subsiding islands, afford critical evidence regarding the 

 changes which the island suffered contemporaneously with the 

 formation of the encircling reef. By means of such evidence 



Fig. 1. 



Fig. 1. Fringing and barrier reefs. 



it should be possible to exclude certain theories with which 

 the changes suffered by the central island are inconsistent, and 

 to substantiate any theory, the logical consequences of which 

 include the changes suffered by the central island quite as 

 necessarily as the formation of the reef itself. These consider- 

 ations may make it clear why my work has been almost wholly 

 confined to the central islands of barrier reefs, though several 

 examples of uplifted reefs were not neglected. 



Coral Reefs on still-standing Islands. — The various theo- 

 ries of coral reefs may be divided into two groups. Those of 

 the first group, some six or seven in number, postulate a fixed 

 relation between land and sea level during the development of 

 the reefs : these will be called the still-stand theories. The two 

 theories of the second group postulate a change in the relative 

 level of land and sea during the development of the reefs. As 

 far as the barrier reefs that I have visited are concerned all the 

 theories of the first group must be rejected, because within 

 every one of these barrier reefs the embayments or drowned 

 vallej's, C, C, C, fig. 1, by which the shoreline of the central 

 volcanic island is indented, give indisputable evidence of 

 recent change of level, of such a kind that the sea has gained 

 on the land. This conclusion evidently postulates that the 



