108 THE TORTUGAS AND FLORIDA REEFS. 



which is now going on between the Florida keys and reefs from Cape Florida to the 

 Tortugas, and must end in transforming them, in like manner, into a continuous tract, 

 to be connected eventually with the mainland. 



In Agassiz's report no attempt was made to explain the substructure of the penin- 

 sula upon which the reef corals groAv. Le Conte, however, attributed this substratum 

 to the mass of material brought along by the Gulf Stream. He believed that the Gulf 

 Stream then ran parallel with the line of the present peninsula, and that the sub- 

 stratum was formed by the heaping up of these loose materials along that line. All 

 the later investigations show, however, that the Gulf Stream never followed this 

 course. Then, as now, it swept across, and not parallel with, the line of the peninsula, 

 and though it undoubtedly assisted in the building up of Forida, it simply brought 

 then, as it does to-day, the food, or the greater part of the food, consumed by the 

 animals living on the Bank of Florida. These animals supply, by their growth and 

 decay, the building material for the great Florida Bank. No doubt, the floating 

 animals brought by the Gulf Stream add something beside to the mass of the bank 

 itself; but they are chiefly consumed by the animals living upon it. 



The curve of the Florida Reef (Plate VI.) 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 navigators, and, though ill-defined at Cape Florida, it 

 becomes stronger and wider as it goes west. It has a width 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 channels between the keys, 

 the flood running north and the ebb soutli. 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 char- 

 acter. The line of keys seems to be formed by the waste of the exterior present reef, 



