THE FLORIDA REEFS. 61 



and Cape Sable being filled up with silt. Since the flood runs 

 in a northerly and the ebb in a southerly dii-ection, the tides in 

 their alternation hold in suspension 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 graduaUy spreading to the southwest 

 from Cape Florida until it has reached its present gigantic pro- 

 portions. 



The Tortugas show us, as wiU be seen, how the reef was 

 actually formed, while the extension of the mud flats beyond 

 the Marquesas explains how the bottom is prepared and grad- 

 ually raised to a level at which corals will floui'ish. 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 elevation ; 

 or that the atolls of this district, such as those of the Marque- 

 sas or of the great Alacran Reef, owe their pecuhar structure 

 to subsidence. 



It cannot be denied that the backbone of the Florida penin- 

 sula was first produced by a fold of tlie 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 something less 

 than two hundred feet ; and that this axis, which has stiU, at 

 the latitude of Lake Okeechobee, an elevation of about forty 

 feet, but sinks gradually as we go south, was formed before the 

 Vicksburg limestone age, while on either side of it are deposited 

 the more recent Hmestones which have given Florida its present 

 width. They have pointed out, moreover, as a secondary result 



