THE TORTUGAS AXD FLORIDA REEFS. 129 



Still found at a somewhat greater depth. This bank is an excellent specimen of an 

 isolated coral patch, such as must have formed the basis of all the ke3's and reefs. 

 This patch will undoubtedly in time form a new reef flush with the surface to the 

 westward of the Tortugas. The greatest depth at Avhich reef-building corals were 

 observed to grow was in the Southwest Channel on the steep banks of White Shoal, 

 and in the channel to the southwest of Bird Key, wdiere Madrepores grow to a 

 depth of about ten fathoms. As a general rule, however, the corals were generally 

 choked below six fathoms by the ooze, and their place was taken by Gorgoniaj. 



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 is 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 surface, form- 

 ing 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, one can only say that in Florida, limiting the estimate strictly to the depth at 

 which corals grow, it Avould probably take one thousand to twelve hundred years 

 for corals to rise from the seven-fathom line to the surface. This would give lis 

 no clue wdiatever to the actual age of the reef, because it is difficidt to determine 

 how far the width of any coral reef is due to the growth of coral. But supposing 

 the reef to have an average width of half a mile, and their lateral growth to be 

 say four or five times more rapid than their 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 great 

 rapidity if the plateau upon which it grows is of the right depth. Take, for 

 instance, the width of flats upon which Madrepores flourish. A plateau at faA^orable 

 depth would \ery soon be covered by them ; they would spread rapidly until they 

 reached the edge beyond which no corals could thrive on account of the depth. 



Thu.s we see from the sections and a study of the distribution of the coi'als, 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 the channels, and that 

 we can find nowhere any trace of elevation. Here the calcareous material has evi- 



VOL. XI. 17 



