THE TORTUGAS AND FLORIDA REEFS. 117 



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 other, once stretched 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 ridge has 

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

 from twenty to thirty miles from the coast, and running nearly at right angles to it. 

 Judging from its fossils and lithological chai-acters, the limestone of which this ridge 

 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^ 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 brought to it 



1 The map of Alacran on Plate V., copied from one of the Hydrographic Office charts, gives an excellent idea of this magnifi- 

 cent reef, its eastern face forming a great arc of a length of about twenty miles, exposed to the full sway of the easterly winds. 

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

 completely killed the corals on the eastern faces of the long, narrow sandy islands forming the western chord of the reef. Between 

 them and the walls of coral heads still runs an irregular channel, varying in depth from one to six fathoms, and connected 

 with the open water by narrow channels on the southern face of the reef. The section of the reef shows that, like the Tortugas, 

 it has been built up from the general level of the Yucatan Plateau, rising gradually from a depth of about thirty fathoms until 

 it reached the depth at which corals can flourish, when it shot up more vertically to form the present cone of the reef. The 

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

 than on the eastern face. 



