THE TORTUGAS AND FLORIDA REEFS. 



but which have gradually diminished in force from the Pine Key C 

 northward. The depth of the passage between the Tortugas and 



allows a larger and stronger body of water to pass through that 

 through any other. 



It has been clearly proved by Hunt that the extensive 





111 



Channel to the 



Rebecca 



Shoal 



channel 



than 



the extensive flats to the northward of 

 the keys have been formed by the agency of the tides, the whole triangular space 

 between the Rebecca Shoal and Cape Sable being filled up with silt. The flood 

 running in a northerly and the ebb in a southerly direction, the tides in their 

 alternation hold in suspense the silt which they wear away from the reef or from 

 the shores of the keys. During storms this floating silt is driven either on to the 

 flats to the north of the keys, or on to the slope of the reef toward the Gulf Stream. 

 An examination of the present condition of the Tortugas, and of the mud Hats 



beyond the Marquesas, gives us a very simple explanation of the formation, and 



gradual extension westward to its present limits, of the small reef origin illy existing 

 only as a diminutive spit, but gradually spreading to the southwest from ( ip. 

 Florida until it reached its present gigantic proportions. 



The Tortugas show us, as will be seen, how the reef was actually formed, whili 

 the extension of the mud flats beyond the Marquesas explains how the bottom is 

 prepared and gradually raised to a level at which corals will flourish. One other 



condition was, how 



to the development of the coral reef, that of 



existence of a powerful current, such as the Gulf Stream, bringing an immense 

 quantity of pelagic animals to serve as food for the corals found along its path. There 

 is practically no evidence that the Florida Reef, or any part of the southern peninsula 



the effect of 



of Florida which has been formed by corals, owes its existence to 



tion ; or that the atolls of this district, such as those of the Marquesas or of the great 



Alacran Reef, owe their peculiar structure to subsidence. 



It cannot be denied that the backbone of the Florida peninsula was first produced 



by a fold of the earth 



geological period. Smith and Ililgard 



also shown that such a fold or folds formed the axis which has raised a part of the 

 northern base of the peninsula to a height of something less than two hundred feet, and 

 that this axis, which has still, at the latitude of Lake Okeechobee, an elevation of about 

 forty feet, but sinks gradually as we o-o south, was formed before the Vicksburg lime 



& , & 



stone age, while on either side of it are deposited the more recent limestones which 

 have given Florida its present width. They have pointed out, moreover, as a secondary 

 result of this folding, the formation of an immense submarine plateau directly in the 

 track of the Gulf Stream, which has been gradually built up since that time by the 



