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



a far greater degree than the observation of visible structures 

 and processes, although the latter must always remain the 

 essential prerequisite of the former. It is for this reason 

 that so much attention is given in the present article to 

 inferences regarding unseen things. 



Darwin's original theory of subsidence, supported by Dana's 

 principle of shore -line development, gives by far the most 

 satisfactory explanation of all the barrier reefs that I have 

 visited in the Pacific or studied on large-scale charts ; and as 

 atolls often occur in association with barrier reefs, Darwin's 

 theory of subsidence appears to give the best explanation of 

 such atolls also. Atolls that are not associated with barrier 

 reefs may be of some other origin, but this seems very 

 improbable. 



Changes of ocean level, resulting from movements of the 

 ocean bottom and causing emergence or submergence of still- 

 standing coasts, are undeniable ; but they seem subordinate to 

 the effects of local uplift and subsidence. Hence in the fol- 

 lowing paragraphs the term submergence will be replaced by 

 its apparent cause, subsidence. 



The elevated reef along the south coast of Oahu, Hawaii, 

 was formed during or after a sub-recent period of subsidence, 

 for its limestones enter well-defined valleys of erosion. The 

 sea-level reef of Oahu was formed during a later and smaller 

 subsidence, by which valleys eroded in the uplifted reef were 

 partly drowned. 



The Fiji group has suffered various movements of subsidence 

 and elevation by which its many islands were affected in unlike 

 ways. Elevation has occurred in different islands at different 

 times, for some of the elevated reefs are elaborately dissected, 

 and others are very little dissected ; still others remain at sea 

 level. The embayments on the larger islands, Viti Levn and 

 Vanua Levu, are largely filled with delta plains. All the reefs, 

 those now elevated as well as those at sea level, appear to have 

 been formed during periods of subsidence, the evidence afforded 

 by the elevated reef of Vanua Mbalavu being especially signifi- 

 cant on this point. The medium-sized island of Taviuni has few 

 visible reefs, becanse its flanks and shores are flooded by sheets 

 of recent lava. The small island of Wakaya seems to be a 

 tilted block of lava beds, not a dissected volcano. 



The extensive barrier reef of New Caledonia has grown up 

 during a recent subsidence by which that long and maturely 

 dissected island has been much reduced in size and elaborately 

 embayed ; but unlike most encircled islands this one was 

 strongly cliffed around its southeastern end and along much of 

 its northeastern side, before the recent subsidence took place. 



The two southeastern members, Mare and Lifu, of the 

 Loyalty group are former atolls, evenly uplifted about 300 feet. 



