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 navigators, and though ill-defined at Cape Flo- 
rida, 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 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. 
