506 W. M. DAVIS SUBSIDENCE OF REEF-ENCIRCLED ISLANDS 



of reef-encircled islands have not ordinarily been given greater weight by 

 students of the coral-reef problem in the Pacific. As far as I can judge, 

 after an extended review of many articles on the subject, the answer to 

 this inquiry is that both the inductive and the deductive sides of the 

 problem have generally been neglected, in so far as they concerned the 

 physiographic features of reef-bordered coasts. The deductive phase of 

 physiographic investigation in particular has been overlooked: how can 

 the prevalent inattention to the embayments of reef -encircled islands be 

 explained otherwise ! A simple matter of observational record, such as 

 the depth of a lagoon, is treated with due respect; but the equally perti- 

 nent matter of physiographic inference touching the submarine prolonga- 

 tion of an island slope has usually been mistrusted or disregarded by the 

 objectors to Darwin's theory. Of course, an inference of this kind is not 

 so well assured as an observed fact; but when it comes to estimating the 

 thickness of barrier reefs and the conditions and processes of their forma- 

 tion, not only every recorded fact, but every pertinent inference, such as 

 that above presented concerning the relation of supermarine and sub- 

 marine slopes, should be given due weight. 



Unconformable Elevated Eeefs 



neglect of geological evidence 



Since the contact of reef limestones on their foundation of volcanic 

 rocks is a geological rather than a physiographic matter, it may be said 

 that the geological as well as the physiographic aspects of the coral-reef 

 problem have been too generally neglected; for the prevailingly uncon- 

 formable nature of reef-limestone contacts is discovered by observation 

 of reef-encircled islands as easily as the prevailing inattention to the 

 nature of such contacts is discovered by inspection of standard works and 

 articles on the coral-reef problem. It would not seem to require a lofty 

 flight of scientific imagination to deduce the unlike consequences as to 

 reef-limestone contacts that are involved in the theories of outgrowing 

 reefs, as shown in sectors A, B, C, figure 5, on still-standing, down-wear- 

 ing volcanic islands, and of upgrowing reefs, as shown in sectors X> Y, Z, 

 on intermittently down-sinking volcanic islands. In the first case, if the 

 reefs are not smothered by outwashed detritus, as will be shown on a later 

 page to be probable, all the reef limestones must lie conformably, as is 

 shown in the section of sector B, on a non-eroded submarine slope of 

 constructional origin. In the second case, all the limestones that are 

 deposited, as the island subsides, above the level of the original reef at- 

 tachment must lie, as is shown in the section of sector Y, more or less 



