THE TOETUGAS AND FLORIDA KEEFS. Ill 



but which have gradually diminished in force from the Pine Key Channel to the 

 northward. The depth of the passage between the Tortugas and Rebecca Shoal 

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

 through any other. 



It has been clearly proved by Hunt that 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 flats 

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

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

 only as a diminutive spit, but gradually spreading to the southwest from Cape 

 Florida until it reached its present gigantic proportions. 



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

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

 prepai'ed and gradually raised to a level at which corals will flourish. One other 

 condition was, however, essential to the development of the coral reef, that of the 

 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 

 of Florida which has been formed by corals, owes its existence to the effect of eleva- 

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

 Alacran Reef, owe their peculiar structui'e to subsidence. 



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

 by a fold of the earth crust in an earlier geological period. Smith and Hilgard have 

 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 somethingl^^s 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 go south, was formed before the Vicksburg hme- 

 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 



