GEOLOGY AND PALEONTOLOGY OF THE CANAL ZONE. 319 



time. It is reasonable to ascribe this submergence to rise in ocean 

 level because of deglaciation, because the order of magnitude of the 

 submergence is the same as the order of magnitude expected from 

 deglaciation. Marginal wave-cut benches should exist, or should 

 have existed around the atoll banks. Perhaps more accurate hydro- 

 graphic surveys and more detailed studies of the submarine profiles 

 will discover them. 



Conclusions. 



The results of an examination of the Tertiary, Pleistocene, and 

 living coral reefs and reef corals of the West Indies, Cenljral America, 

 and the Southeastern United States are as follows: 



1. The fringing reefs have form.ed usually, if not invariably, during 

 periods of intermittent uplift, following considerable submergence. 



2. All the important offshore reefs, both fossil and living, possibly 

 except the reefs off the southeast coast of Barbados, have devel- 

 oped during or following submergence after the subaerial erosion of 

 their basements. 



3. Most of the fossil offshore reefs, all of those on which informa- 

 tion has been obtained, and all of these living reefs are superposed on 

 antecedent flattish basements or platforms. Where there are no 

 platforms, as off fault shore lines and young volcanic islands, there 

 are no offshore reefs. 



4. Although corals are constructional geologic agents, they are 

 subordinate to other limestone forming agencies, and none of the 

 American platforms were formed by infilling behind a barrier. 



5. Submarine flats and plateaus at proper depths below sea level 

 to have furnished basements for offshore reefs are not confined to 

 the temperature zone suitable for coral growth. Such extralimital 

 banks are Georges Bank, the banks off the coast of Nova Scotia, and 

 the Grand Banks of Newfoundland. Reefs form on such banks 

 where the proper ecologic conditions for the life of reef building 

 corals prevail. 



6. The submergences during and after which the fossil reefs were 

 formed were almost certainly due to differential crustal movement; 

 the submergence of the basement of the living reefs is probably due 

 to complex causes, for there was differential crustal movement in 

 the area under consideration during Pleistocene time, also at some 

 places within it during Recent time, and, in addition to these more 

 or less local movements, there seems to have been during Recent 

 time a general submergence of the eastern coast of America from 

 Argentina to New England. The amount of the general Recent sub- 

 mergence lies between 40 and slightly more than 20 fathoms; an 

 amount of the order of magnitude that would be expected to result 

 from the effect of deglaciation in raising sea level. The principal 

 wave-formed Pleistocene plain now lies between 26 and 36 fathoms 



