230 ANNUAL REFOET SMITHSONIAN INSTITUTION, 1917. 



postulates of the Darwin-Dana hypothesis ; or are the flats in origin 

 independent of the existence of the reefs and are the reefs merely 

 superposed on flattish areas that antedate their presence? 



There are at least three criteria that are applicable in deciding be- 

 tween the^e two interpretations. The first of these is the relation of 

 the width and depth of the flat, or platform, to the presence or ab- 

 sence of barrier reefs. If the flat is dependent on the presence of 

 the reef, where a break in the reef occurs there should be a landward 

 projecting reentrant in which the seabottom is deeper than behind 

 the reef. The second criterion consists in the position of the barrier 

 on the surface of the flat. If the flat is due to infilling behind the 

 reef, the reef should stand on its outer edge, not back from the edge 

 with the flat projecting seaward beyond the reef. The third criterion 

 concerns the composition and geologic history of the flat landward 

 of the reef. In many places it is possible to ascertain the nature of 

 the rock forming the sea floor between a barrier and the shore. Such 

 a floor,- if formed by agencies associated with the presence of the 

 reef, will not exhibit geologic phenomena that in age antedate 

 the reef; but, on the other hand, if the floor can be shown 

 to be composed of rock older than the reef, or to have had 

 any kind of geologic history antecedent to the presence of the reef, it 

 is demonstrated that the reef is merely growing on the surface of 

 a flat whose formation is independent of the reef development. 



The Great Barrier Eeef of Australia is definite in its testimony. 

 Text figure 12 presents cross sections south of the reef limits and 

 across the reef tract. Profiles 1, 2, and 3, which are south of the 

 southern end of the reef, show the continuity of the platfonn south- 

 ward beyond the end of the reef; while profiles 4 and 5 show the 

 platform projecting some miles beyond the reef. At its northern 

 end the reef appears usually to stand on the seaward edge of the plat- 

 form or shelf. The continuity of barrier platforms irrespective of 

 the presence or absence of reefs is general off the shores of large 

 land areas. Plate 38 i^ from a photograph of a model of the 

 Gulf of Mexico and the Carribbean Sea. There are offshore reefs 

 on the Floridian Plateau, and on both Campeche and Mosquito 

 banks, but a person would indeed be bold to contend that these fea- 

 tures of the earth's crust are due to infilling behind reefs, especially 

 when some additional facts presented in the next paragraphs are 

 considered. 



The geologic succession of the reef-coral faunas of Georgia and 

 Florida is given in the table on page 229. The geographic extent and 

 composition of the Ocala limestone, of late Eocene age, which 

 forms the basement of the Floridian Plateau, have been ascertained 



