Building of the Marquesas and Tortugas Atolls, etc. 63 



Most or all of the material above sea-level in the Tortugas is unconsolidated, 

 except an outer crust of some of the beach rock, as observed on Loggerhead 

 Key. Extending beneath the sea-level, however, to a depth of 8 feet or 

 more, is beach rock that appears more indurated. Harder rock was also 

 observed at a depth of 3 or 4 feet under water on Southwest and Northeast 

 keys, the subaerial portions of both have been cut away by wave action, 

 and in places aubmarine cliffs of hard rock, which has been undercut and 

 overhangs the sides of the channels, are evident at depths of (jver 20 feet. 

 The presence of an older formation for the living reef corals is clear. It 

 seems probable that this rock was subaerially indurated and then depressed. 

 The probability of this subsidence will be further discussed in a succeeding 

 paragraph, wherein the later movements of the Florida coast line will be 

 more fully considered. It appears that the Tortugas atoll with its inclosed 

 lagoon was outlined previous to the present development of the coral reefs 

 of that area, that it was elevated about 50 feet, and that there has been a 

 subsequent (the latest) depression; the land after the oscillation perhaps 

 lacked 10 or 15 feet of being depressed to its former level, thus leaving the 

 keys standing somewhat higher than before. 



It has already been stated that the skeletal remains of corals are not 

 only an important constituent of the Tortugas rocks, but are rather surely 

 the most important contributor to its composition. Whether or not the 

 various keys, banks, and isolated rocks were outlined by coral reefs may not 

 be ascertained, but as coral reefs obey the laws of current-moved detritus, 

 in many instances being the principal single source of the detritus, a positive 

 answer to this question is not essential for the validity of the explanation 

 here presented for the building of the Tortugas. Hedley and Taylor say 

 in their paper on the Great Barrier, referred to in the footnote on page 62 : 



The growth of an individual reef is shown to proceed in a regular cycle. If the reef 

 reaches the surface with its axis along the wind, then its shape endures; but if across the 

 wind, then its extremities are produced backward, forming first a crescent, later a horseshoe, 

 and lastly an oval, thus inclosing a lagoon. 



SKETCH OF THE GEOLOGIC HISTORY OF THE FLORIDA REEF TRACT. 

 The probability of elevation and subsidence in the Tortugas area leads 

 to a consideration of evidence from which the later movements of the 

 Florida coast line have been inferred and of conditions antecedent to the 

 development of the living Florida barrier reef. That the last important 

 movement of the Florida east coast was downward from Jacksonville to 

 San Augustine is attested by the submerged channel of the mouth of St. 

 Johns River and the submarine fresh-water springs off San Augustine. 

 Late depression of the west coast is positively shown not only by the forms 

 of Tampa Bay and Charlotte Harbor, but also by submerged channels off 

 shore in that region. As the channels off this coast, however, appear not 

 to affect the lo-fathom curve the depression was probably not so great as 

 60 feet. The very ragged character of the coast line from Cape Romano to 



