428 SCIENCE PROGRESS 



foundation. If conformable, their formation during pauses in 

 elevation is indicated ; but very few reefs of this kind are 

 known. If unconformable, as is usually the case, a period of 

 exposure to erosion, followed first by submergence, and later 

 by emergence, must be recognised ; and three alternative ex- 

 planations for the reefs are then open. First, the reefs may 

 have been formed during pauses in the final elevation following 

 a rapid submergence without reef growth ; second, the reefs 

 may have been formed during pauses in the submergence pre- 

 ceding a rapid and final elevation without reef growth ; third, 

 some reefs may have been formed during pauses in submergence, 

 and some during pauses in elevation. The only way to make 

 safe choice among these alternatives is to examine the detailed 

 structure of the reefs concerned. Observations of this kind 

 are so difficult that they have seldom been made. Certain 

 terraced reefs on Efate in the New Hebrides group, the structure 

 of which is fairly well exposed in a ravine, seemed to me to 

 accord better with the second or third explanation than with 

 the first. 



Inattention to Embayments and Unconformities. — It is not 

 my object to insist that all coral reefs have been formed in 

 one way ; still less that they have all been formed by upgrowth 

 during subsidence, although my personal belief is that that 

 method of formation best accounts for the great majority of 

 reef structures. The object of this article is to lead those 

 readers who are still interested in the old problem to consider 

 attentively both the qualitative and quantitative value of the 

 two lines of geological evidence favouring subsidence that are 

 provided by embayed shorelines and by unconformable contacts, 

 for sea-level as well as for elevated reefs ; also to examine not 

 only Darwin's theory and the Glacial-control theory, but also 

 the various still-stand theories of coral reefs, as set forth by 

 Rein, Semper, Murray, Wharton, and others, in which no 

 account whatever is taken of the two lines of geological evidence 

 here pointed out ; and then to select the theory that seems 

 best able to account for the facts. 



The result of such consideration and examination on my 

 own part is the conclusion that, famous as are the contributions 

 to the coral-reef problem by the above-named inventors of 

 various still-stand theories, their inventions are to-day of little 

 more than historic value, for their arguments are invalidated 



