THE FLORIDA REEFS. 89 



below six fathoms by the ooze, and their place is taken by gor- 

 gonians. 



AU 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 growi;h 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 surface, forming at first a 

 barrier somewhat less compact than the present line 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 strictly 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 dif&cult 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 gi-eat 

 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 



