324 BULLETIN 103, UNITED STATES NATIONAL MUSEUM. 



manufacture; there is controversy as to the origin of the Sunderland 

 terrace in Maryland and Virginia, but no geologist will deny that 

 certain houses have been built on the surface of the Sunderland 

 terrace flat; although the geologic history of the pre-Cambrian 

 formations in Michigan and in other areas adjacent to the Great 

 Lakes may be inadequately known, no one is justified in denying for 

 such a reason that glacial deposits overlie the geologically old rocks, 

 as it is obvious that the overlying material has in some way been 

 placed on the underlying. The superposition of a geologic formation 

 on another may be recognized without knowing the complete history 

 of either the upper or the lower. The oligocene coral reef along 

 Flint River near Bainbridge, Georgia, rests on the eroded surface of an 

 upper Eocene limestone now designated the Ocala limestone. That 

 knowledge of the Ocala limestone may not be adequate does not 

 invalidate the recognition of the facts that the fossil reef overlies it 

 and that an erosion period intervened between its deposition and the 

 growth of the reef, which obviously formed during or after the sub- 

 mergence of its basement. 



To ascertain the origin of the submarine flats on which offshore 

 reefs stand is important in the advancement of our knowledge of 

 geologic history, and I have acquired as much information on the 

 subject as I could. I am completely convinced that there is no one 

 explanation that can be applied to all of them. The following kinds 

 have already been recognized: (1) Slightly tilted bedded tuff, as in 

 the fossil reefs of Antigua; (2) slightly tilted bed of limestone, as off 

 the south coast of St. Croix and Cuba; (3) submerged coastal flats, 

 as in the Fiji Islands; (4) submerged peneplained surfaces, as in the 

 fossil reefs of Porto Rico; (5) submarine plains due to uplift of con- 

 siderable areas of the ocean bottom and to the deposition of organic 

 deposits on such a surface, as the Floridian Plateau previous to the 

 formation of the middle and upper Oligocene reefs of Florida and 

 southern Georgia ; (6) flats of complex and not definitely known origin, 

 such as those of the Antigua-Barbuda Bank, the Virgin Bank, and 

 the continental shelves of tropical America and Australia. Plains 

 suitable for the growth of corals have been formed by subaerial and 

 submarine deposition, and by both subaerial base-leveling and sub- 

 marine plantation. Nearly every, if not every, plain-producing 

 process operative in tropical and subtropical regions has taken part 

 in the formation of plains on which corals have grown or are growing 

 where the plains have been brought below sea level and where the 

 other ecologic conditions for offshore reef formation obtain. 



I will revert to this subject in discussing the Glacial-control theory 

 and in making suggestions as to future research. 



