322 BULLETIN 103, UNITED STATES NATIONAL MUSEUM. 



a land area on one side and open ocean on the other or which may 

 be the top of an oceanic eminence. The second factor is gradual sub- 

 mergence. The vigor of offshore reefs where these conditions pre- 

 vail can be correlated with certain ecologic demands of reef-forming 

 corals. 



Reef corals thrive on offshore flats, near or against ocean water, 

 because they are there removed from the deleterious effects 

 of both land-derived and other sediment. Some of these rela- 

 tions are well exemplified in the barrier reef off the east side of 

 Andros Island, Bahamas. This reef grows on the outer, windward, 

 edge of a small shallow flat, against the deep water of the Tongue of 

 the Ocean. As the winds set landward across the reef no oceanic 

 or land-derived sediment is deposited on the reef, it is bathed by 

 the purest ocean water, and receives the largest amount of animal 

 plankton that that part of the sea can supply. On the great shoals 

 of the Bahama Banks and in the shoal waters of Florida behind the 

 reefs the winds stir up the mud on the bottom; the sediment while 

 in suspension kills the plankton ; when it settles it kills those bottom- 

 living organisms that can not endure being covered by mud. On such 

 flats reef-forming corals can not live. On shallow banks coral 

 reefs therefore thrive best on the windward sides. However, if the 

 flat extends far enough offshore for land-derived sediment not to 

 reach the reef and if the depth is sufficient for waves under ordinary 

 conditions not to stir up the mud on the bottom, but not too deep 

 for the growth of reef corals, barriers may develop on the leeward 

 sides of islands. A land area to the windward may actually 

 favor coral growth, as it breaks the force of the winds. A position 

 on an offshore flat, particularly on the windward edge of a flat, 

 insures a supply of the purest ocean water and an abundance of 

 animal plankton. 



The gradual submergence of an offshore flat perpetuates the favor- 

 able conditions for the life of reef-building corals, and gives an 

 opportunity for continual growth upward. With upward growth 

 during slow submergence of the basement the ecologic conditions 

 for the life of reef -forming corals are made better, for the deleterious 

 effects of sediment are minimized. 



As regards the life of corals, the method of bringing about these 

 conditions is of no importance. Whether the flat was formed by 

 marine planation, by alluviation and the building of a coastal flat, 

 by base-leveling through subaerial erosion, by the formation of a 

 submarine plain of deposition, or by any other special process, is 

 unimportant, provided the flat be formed. Whether the submerg- 

 ence be caused by differential crustal movement, local or remote, or 

 by rise in ocean level due to the melting of glaciers, is unim- 

 portant, provided there be gradual submergence of the basement. 



