THE FLORIDA REEFS. 57 



The curve of the Florida Reef (Fig. 34) along the Gulf 

 Stream is due in great measure, as Hunt shows, to a counter 

 current along the reef, running westward. This current is 

 known to all na\dgators, and though ill-defined at Cape Flo- 

 rida, becomes stronger and wider as it goes west. It has a 

 Avidth of at least ten miles at Key West, and of twenty miles 

 at the Tortugas. This is clearly shown by the mass of surface 

 animals driven along upon this westerly counter current by the 

 south-easterly winds. 



The tides set strongly across the reefs, and through the chan- 

 nels between the keys, the flood running north and the ebb 

 south. When storms occur, the fine silt of the bank, made up 

 of coral sand from the reefs, is taken into the bay back of the 

 keys and deposited there. The counter current then carries this 

 to the westward, and thus material has gradually been added to 

 the flats. As Hunt has already noticed, tides and currents have 

 undoubtedly been the principal agents here. That this material 

 has not been brought by the Gulf Stream from the mouth of 

 the Mississippi, is shown by the fact that no trace of Mississippi 

 mud has ever been found in any of the innumerable soundings 

 taken to the eastward of the Mississippi, or more than a hundred 

 miles from its mouth. It is also probable that the action of the 

 waves from the southeast, in forming a talus of coarser material, 

 does not penetrate below one hundred fathoms, and everything 

 once fixed below that depth has its final character. The line 

 of keys seems to be formed by the waste of the exterior present 

 reef, rather than by the remains of an older anterior reef. At 

 the Tortugas, the contrary seems to be the case ; but this per- 

 haps is due to the fact that the strong currents which sweep 

 over the reefs, and have excavated the Southwest Channel, 

 have also established conditions favorable to the growth of 

 corals on both sides of this channel, and that the two lines of 

 keys are due to this cause. Had the currents run only from the 

 southeast through the Northwest Passage, larger keys, separated 

 by channels running north and south, would then have been 

 formed. 



I shall first show by an examination of the Tortugas ^ how 



1 A. Agassiz, The Florida Reefs ; Mem. Am. Acad. xi. 1883. 



