THE TORTUGAS AND FLORIDA REEFS. 11 



line of reef is indeed probably the only one which has played any important part 

 in the formation of land south of the line of the present southern extremity of the 

 peninsula of Florida. There seems, however, some reason to believe that a line of 



reefs, or perhaps two lines not very distant from each 



tretched along the 



southeastern end of the Everglades before the present reef began to extend west- 

 ward. Judging from the sections shown by the maps, the growth of the present 

 reef, as fast as the mud flats were formed to the south of it, has been altogether in 

 that direction (Plate VIII.). 



The Bahamas, the San Pedro and Yucatan Banks, have probably all been formed 

 by a similar process, — by the accumulation of limestone either upon an early fold of 

 the earth's crust, or upon a volcanic plateau, or upon a foundation of slower growth 

 from great depths. In Yucatan we can actually descend into the bank itself through 

 any one of the aguadas, or caverns, found everywhere in the northern part of that 

 country. Many of these caverns extend to a considerable depth. One of them, that 

 of Bolonchen, has a depth of seventy fathoms, the whole formation consisting of recent 

 limestone entirely composed of species of Invertebrates now living on the Yucatan 

 Bank. In Yucatan, as in Florida, we find a low ridge of limestone, somewhat older 

 than that of the coast, extending across the peninsula. The uplifting of this ri<ige ha- 

 caused the slight undulations of level traceable throughout Yucatan, at a distance of 



from twenty to thirty miles from the coast, and 



ly at right angles to it. 



Judging from its fossils and lithological characters, the limestone of which this rid 



is formed is identical with the so-called Vicksburg limestone of the central backbone 

 of Florida. I have already attempted to show, in my letter No. 1 (Bull. M. C. Z., V.. 

 No. 1), containing an account of the great Alacran 1 Reef on the Bank of Yucatan, 

 that we need not refer the atoll-shaped form of this reef (Plate V.) to the subsidence 

 of the Yucatan Bank as a whole, since the action of the prevailing winds and cur- 

 rents would account for all the existing phenomena. The decay of the animals living 

 upon the great plateau, added to the deposition of all the animal life brou 



C 



of Alacran 



forming 



The huge breakers pound incessantly upon this steep face of the reef, and drive all the silt to the westward. This silt lias already 



narrow sandy islands form 



varying 



Between 



channels on the southern face of the reef 



Tortugas, 



gradually from a depth of about thirty 



form the present cone of the reef. The 



western slope is not so steep as the eastern, and the silt below the twenty-fathom line is deposited on a much more gentle slope 

 than on the eastern face. 



