134 T. W. Vaughan — Origin qf Barrier Coral Reefs. 



The detailed investigations so far made have substantiated 

 two of the Darwin-Dana contentions, which are: (1) that off- 

 shore reefs have formed either after or during submergence ; 

 (2) that the lagoon channels inside barrier reefs and atoll 

 lagoons owe their origin to constructional geologic agents 

 which have built up more or less interrupted inclosing walls. 

 In a third relation, however, the results are not in accord with 

 their hypothesis, for the platforms which the reefs margin or 

 on which they grow, wherever adequately investigated, have 

 been shown to antedate the development of the reefs, and that 

 reefs have formed on such basements, during or after sub- 

 mergence, where the necessary ecologic conditions prevail. 

 The Grand Banks of Newfoundland, the banks off the coast of 

 Nova Scotia, and Georges Bank off Cape Cod, are instances of 

 recently submerged platforms which would furnish proper 

 habitats for reef-building corals did they not lie outside the 

 life zone of such organisms ; while the Floridian Plateau, 

 Campeche Bank off Yucatan, and Mosquito Bank off Nicaragua 

 and Honduras, which lie within the life zone of such organisms, 

 support many reefs. 



The origin of the different platforms and the cause of their 

 submergence must now engage the attention of the students of 

 these problems. Andrews has made two notable contributions 

 to our knowledge of the origin of the platforms in his work on 

 the Great Barrier Reef and in that on the Fijis ; and Profes- 

 sor Pirsson has made one of great value in his account of the 

 geology of Bermuda Island. Much attention has been paid to 

 these relations in Florida and in some of the West Indian 

 islands. The relations in the last mentioned islands are com- 

 plex and can not be expressed in a single formula. The base- 

 ments, which are fundamentally due to structural and construc- 

 tional geologic processes, have been subjected to the action of 

 subaerial and submarine planation processes during many 

 changes in the position of the strand line. Some reefs, as 

 those off the eastern end of Porto Rico and those off Virgin 

 Gorda, are growing on what w r as a land surface only a short 

 time ago, while others, as the one off the south side of St. 

 John Island, appear to have grown upward on a submarine 

 plain which has recently undergone deeper submergence. 



The investigation of the causes producing submergence is 

 only in a preliminary stage. That local differential crustal 

 movement has been important in producing Recent change in 

 the position of strand line in coral reef areas is certain. Daly 

 has ably presented the evidence in favor of another factor, 

 which is that through deglaciation there has been increase in 

 volume of oceanic water and a consequent rise of sea level in 

 the tropics ; and Barrell has helped in the formulation of the 



