GEOLOGY AND PALEONTOLOGY OF THE CANAL ZONE. 321 



The living coral rims on the banks enumerated have formed during 

 and subsequent to Recent submergence. 



A review of the conditions under which the principal barrier reefs 

 in the Pacific Ocean were formed leads to essentially identical con- 

 clusions. Those of the Australian Great Barrier, of New Cale- 

 donia, the Fiji Islands, and Tahiti are superposed on antecedent 

 platforms that have been submerged in Recent geologic time. The 

 submergence of the Australian continental shelf apparently can be 

 assigned to Recent rise of sea level because of deglaciation, as it 

 seems that most of the surface of the platform was exposed as a dry- 

 land area by withdrawal of water from the ocean during at least a 

 part of Pleistocene time. The submergence of the Fijian platforms 

 is concomitant with, if not entirely due to, differential crustal move- 

 ment. The superposition of the barrier reefs off the shores of the 

 smaller Society Islands on antecedent platforms is not proved. 

 Evidence sufficient for the basis of an opinion is not available. The 

 absence of reefs around Mehetia, where there is no shore platform, 

 is significant. That the barriers off the other smaller islands were 

 formed after the submergence of their basements is clear. The small 

 cliffs at the spurs ends, in my opinion, do not constitute evidence 

 against the presence of shore platforms, flats, or lowlands, ante- 

 cedent to submergence. That ocean level in the Indo-Pacific, 

 because of deglaciation, in Recent time has risen to an amount of 

 about 60 meters (about 33 fathoms) as postulated by Humphreys 

 and Daly, and that this rise of ocean level had influenced the 

 development of living coral reefs, is, I believe, so well established 

 as to be almost if not quite incontrovertible. 



The rims of the large atolls, and perhaps of the smaller ones also, 

 are growing, in my opinion, on the surfaces of, mostly the edges of, 

 flat summit areas that have undergone geologically Recent submerg- 

 ence. These flats, I believe, were mostly formed in pre-Pleisto- 

 cene time, and it is my opinion that they were largely out of water, 

 or were very near the surface of the water, during Pleistocene time. 

 If they projected above the water for an appreciable time, they should 

 have been wave cut around their edges by the lowered Pleistocene 

 sea, and evidence of such benching should be sought. I believe the 

 evidence will not be found on the hydrographic charts at present 

 available, for the object of the published charts is to guide navigators 

 rather than to serve as a basis for physiographic studies of the sea 

 bottom in depths where navigation is safe. 



From what precedes I believe it is clear that I consider that there 

 are two factors that determine the vigorous development of offshore 

 reefs, which under the most favorable conditions form barriers or 

 atoll rims, the other proper ecologic conditions also being present. 

 The first factor is the existence of an offshore flat, which may have 



