vaughan: geologic history of coral reefs 29 



the Florida counter-current, while its northern limb in each 

 instance trails with these currents. As many details of the ]Mar- 

 quesas and the Tortugas can not be considered in this summary, 

 the conclusion will at once be expressed that the atoll rims of 

 both the jMarquesas and the Tortugas are constructional phenom- 

 ena and owe their configuration to the prevailing winds and 

 currents. The detrital material on which these agencies have 

 worked in the Marquesas is mostly of other than coral origin; 

 while in the Tortugas, altho of complex composition, it consists 

 largely of coral debris. 



The Marquesas rim is geologically Recent, and evidently re- 

 quired no change of sea level for its formation. As the surficial 

 material of the Tortugas rests upon an older, harder, calcareous 

 basement, it appears that the Tortugas atoll with its inclosed 

 lagoon was outlined during a previous physiographic cycle. Fur- 

 ther evidence, later to be adduced, renders it reasonably certain 

 that the Tortugas were initially outlined during subsidence after 

 uplift following the close of the Pliocene, that they were then 

 elevated to an amount of perhaps 50 feet, that the last event 

 has been one of subsidence back nearly to the same level as that 

 preceding the elevation, and that the Recent corals have estab- 

 lished themselves on an old atoll basement already prepared for 

 them. 



OSCILLATIONS OF THE FLORIDA REEF TRACT 



A study of the Florida coast line shows that its last important 

 movement was downward. This deduction is based on the ex- 

 istence of submerged channels and submarine fresh-water springs 

 on the east coast, on the indented character of the coast line 

 with accompanying submerged channels on the west coast, and 

 on the presence of underground passages containing salt water 

 and free openings or cavities that extend to a depth as great as 

 30 feet below sea level on the southern keys. The evidence is 

 clear that the keys participated in the uplift and subsequent 

 depression that affected the mainland, and that at one time they 

 stood more than 30 feet higher with reference to sea level than 

 the}^ now do. This uplift and the subsequent depression, 

 according to all available evidence, extended to the Tortugas. 



