THE FLORIDA REEFS. 89 
below six fathoms by the ooze, and their place is taken by gox- 
gonians. 
All estimates of the age of the southern extremity of Florida, 
or of the reef alone, must necessarily be very defective. The 
great age assigned by Professor Agassiz to the northern part of 
the peninsula may not be exaggerated, if it be understood as 
including the time at which the Vicksburg limestone forming 
its backbone was deposited. But the extension of the coral 
reefs proper so far north in Florida has never been proved. 
The rate of growth of the reef-builders is very rapid, and it is 
quite possible: that the reef-builders of the Florida Reef began 
at once all along the line extending from Key West to Cape 
Florida, and quickly reached the shires forming at first a 
barrier somewhat less compact than the present lino of reef. 
Uncertain as we are respecting the time at which the various 
parts of the reef reached the surface, we can only say that in 
Florida, limiting the estimate strietly to the depth at which 
corals grow, it would probably take from one thousand to twelve 
hundred years for corals to rise from the seven-fathom line to 
the surface. "This would give us no clue whatever to the actual 
age of the reef, because it is difficult to determine how far the 
width of any coral reef is due to the growth of coral. But sup- 
posing the reef to have.an average width of half a mile, and its 
lateral growth to be say four or five times more rapid than its 
vertical increase, we should get at least twenty thousand years 
as the age of the outer reef. It is quite possible for a great 
width of reef to be forming at one time, and to spread laterally 
with rapidity if the plateau upon which it grows is of the right 
depth. Take, for instance, the width of flats upon which madre- 
pores flourish. A plateau at favorable depth would very soon 
be covered by them; they would spread rapidly until they 
reached the edge beyond which no corals could thrive, on ac- 
count of the depth. 
Thus we see, from the sections and a study of the distribution 
of the corals, that at the present day material is constantly 
added to the knoll forming the Tortugas; that this material is 
derived either from the animals and plants living upon the reef, 
or from the pelagic animals which die while passing through 
