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 direction, 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 gradually spreading to the southwest 
from Cape Florida until it has reached its present gigantic pro- 
portions. 
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 prepared and grad- 
ually 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 elevation ; 
or that the atolls of this district, such as those of the Marque- 
sas or of the great Alacran Reef, owe their peculiar structure 
to subsidence. 
It cannot be denied that the backbone of the Florida penin- 
sula 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 something less 
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 limestone 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 
