IV. 



THE BUILDING OF THE MARQUESAS AND TORTUGAS ATOLLS 



AND A SKETCH OF THE GEOLOGIC HISTORY OF THE 



FLORIDA REEF TRACT. 



By Thomas Waylaxd Vaughak. 



INTRODUCTION. 



The following brief article is supplemen'ar>' to my paper "A contri- 

 bution to the geologic history of the Floridian Plateau,"^ in which it is 

 stated that "the geologic history of the Marquesas and Tortugas is reserved 

 for future consideration." The study of the geology and the geologic 

 processes of the Florida reef tract, and especially of the Tortugas and the 

 Marquesas, has been continued since 1910. During the field season of 

 1913 Mr. R. B. Dole, of the U. S. Geological Survey, undertook a special 

 investigation of the solvent effect, by virtue of the content of carbon dioxide, 

 of sea-water flowing into and out of Tortugas Lagoon. As the various lines 

 of investigation have given definite results, the principal factors in the 

 formation of the Marquesas and Tortugas atolls and their inclosed lagoons 

 now seem clear, although some details still require elucidation. It is now 

 also possible to outline with a feeling of confidence the salient geologic 

 episodes in the history of the entire Florida reef tract and to institute 

 comparisons with other coral-reef areas. As this paper is only a preliminary 

 statement, it is intended subsequently to publish a more comprehensive 

 discussion of the different problems here treated in a cursory manner. 



BUILDING OF THE MARQUESAS AND TORTUGAS ATOLLS. 

 An inspection of the charts of the U. S. Coast and Geodetic Survey, 

 No. 1252 for the Marquesas and No. 471a for the Tortugas, will show a 

 striking similarity in the configuration of both, for in each banks or keys 

 surround a central depression and in each there are major entrances in the 

 southeast, southwest, and northwest quadrants. The Tortugas, however, 

 are of a more elliptical form than the Marquesas, the axis of the ellipse extend- 

 ing from northeast to southwest. The Marquesas lagoon is only i to 15 

 feet in depth, while that of the Tortugas is from 7 to 13 fathoms. In both 

 the principal key or bank slightly under water extends from the southeast 



1 Camegie Institution of Washington, Pub. 133. PP- 99-i8s. IS pIs. 1910. 



57 



