A Shaler Memorial study of coral reefs

Abstract

The general result of my voyage already announced above, as well as several special results, may now be concisely stated: The origin of coral reefs cannot be determined by a study of the visible features of sea-level reefs, for they can be explained by any one of the eight or nine theorized that have been proposed to account for them, provided that the postulated conditions and processes of the theories are accepted. Apart from research by deep borings, the true theory can be detected only by the study of associated problems, such as the form of the central islands within barrier reefs, or the structure of uplifted reefs. The origin of coral reefs, like many other geological problems, involves the discussion of invisible structures and processes to 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. Darwin9s original theory of subsidence, supported by Dana9s 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 atolls often occur in association with barrier reefs. Darwin9s 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 following 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 subsidences, by which valleys eroded in the uplifted reef were partly drowned.