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



produced being modified only in a minor degree by changes of 

 sea level due to causes outside of the coral-reef region. 



The Merits of Darwin's Theory. — The chief merits of 

 Darwin's theory of subsidence are its simplicity, its breadth, 

 its capacity to explain critical facts — the drowned valleys of 

 barrier-reef islands, as in sectors J, K, fig. 9, and the uncon- 

 formable contacts of elevated reefs on eroded foundations, as 

 in section M — that were not known when it was invented ; also 

 the ease with which it may be modified by the addition of 

 supplementary processes without invalidating its own essential 

 process. It contains no postulate so arbitrary as that of a still- 

 standing reef foundation on a fixed ocean bottom, of which the 

 rigidity is relaxed only to permit elevation but not subsidence ; 

 it avoids the assumption of great uplifts in other parts of the 

 ocean to produce less great submergence in coral-reef regions, 

 and explains local submergence by local subsidence of the same 

 amount ; but it easily accepts the possibility of some submer- 

 gence or emergence being caused at any time by a general 

 change of ocean level from whatever cause ; it understands 

 that other organisms than corals contribute to the formation of 

 coral reefs ; it freely recognizes that subsidence may be inter- 

 rupted by still-standing pauses or reversed by elevation ; it 

 considers and accepts the possibility of the outward growth of 

 reefs during still-stands, but regards such outward growth as 

 subordinate to upward growth, because existing sea-level reefs 

 are generally of moderate breadth and because the deltas which 

 occupy the bay-heads of central islands within barrier reefs are 

 generally too small to project into the lagoon ; it considers axid 

 rejects the explanation of barrier reefs as veneers on abraded 

 rock platforms, because the central islands are not cliffed as 

 they should be if this theory were correct ; it perceives the 

 possibility of reefs being established on submarine banks, built 

 up to the shallowness required for coral growth by other kinds 

 of organic deposits, but it accepts this idea, to which Murray 

 later gave wide application, only for rapidly-growing banks 

 near continental borders and rejects it for pelagic banks, where 

 the very slow up-building would be easily overcome by a less 

 slow submergence or outstripped by a less slow upheaval, and 

 where wave-action might sweep away fine deposits in greater 

 depths than twenty fathoms. 



Dana's Confirmation of Darwin's Theory. — The chief 

 deficiencies in Darwin's statement of his theory are : the fail- 

 ure to deduce the embayment of central islands within barrier 

 reefs and the unconformable contact of the reef mass with its 

 volcanic foundation as essential consequences of subsidence, 

 and the omission of the possibility of submergence, due to a 

 rising ocean surface, as an alternative to submergence due to a 

 subsiding ocean bottom. The theory as first stated involved a 



