112 TEE TORTUGAS AND FLORIDA REEFS. 



accretions of the solid parts of MoUusks, Echinoderms, Corals, Alcj^onoids, Annelids, 

 Crustacea, and the like, which have lived and died upon it, thus furnishing the lime- 

 stone for the gradual completion of the peninsula. No one who has not dredged near 

 the hundred-fathom line on the west coast of the great Florida Plateau can form any 

 idea of the amount of animal life which can be sustained upon a small area under 

 suitable conditions of existence. It w'as no uncommon thing for us to bring up in the 

 trawl or dredge large fragments of the modern limestone now in process of formation, 

 consisting of the dead carcasses of the very species now living on the top of this recent 

 limestone. To the westward of the western shore line, Florida now stretches out as 

 an immense submarine plateau, forming, as the sections show, a huge tongue coated 

 or veneered only by coral limestone over its very top. The whole of the peninsula 

 of Florida south of St. Augustine, as far as Tampa Bay, has probably been built up 

 in this way, from north to south, of limestone somewhat older than the reef limestone. 

 The plateau, judging from the inclination of the axis, has but a slight southward dip 

 until we reach the southern extremity of the peninsula, where the fall is more rapid 

 toward the outer reef. 



The whole of the eastern and western edges of Florida consist of recent Uraestones, 

 the immediate predecessors of the present limestone now forming on the western and 

 southern slopes of the great Florida Plateau. The early dredgings of Mr. Pourtal6s, 

 in 1867 and 1868, developed on the Gulf Stream slope off the Florida reefs an exten- 

 sive rocky plateau (Pourtales Plateau) from a depth of about ninety fathoms to about 

 two hundred and fifty fathoms. The rock of w^iich this plateau is built consists of the 

 same species of corals and shells as those now living upon it, to which it owes its 

 formation. A similar sea-bottom is found on the north side of Cuba, but with a much 

 steeper slope. These fringing limestones also formed the southern extremity of 

 Florida at a time when the northern part of the Everglades had perhaps been built up 

 to a level favorable for the growth of coral reef. In this northern portion of the Ever- 

 glades alone can we confidently speak of the first concentric reefs, which have little 

 by little built up Florida toward the south. It seems highly probable that on the 

 remainder of the peninsula north of the Everglades both the newer and older lime- 

 stones were built up b}' the same agencies as are now at work on the Florida Bank. 

 There are to-day other submarine banks which undoubtedly owe their origin to sim- 

 ilar agencies. The great bank to the east of the Mosquito coast, which practically 

 extends to Jamaica, has probably been formed in the same way as the Yucatan and 

 Florida Banks ; that is, by the gradual decay of the animals subsisting in great abun- 

 dance upon its slope, and fed by the pelagic materials which the currents and the 



